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ADS8681, ADS8685, ADS8689
SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
ADS868x 16-Bit, High-Speed, Single-Supply, SAR ADC Data Acquisition System
With Programmable, Bipolar Input Ranges
1 Features
3 Description
•
•
The ADS8681, ADS8685, and ADS8689 belong to a
family of integrated data acquisition system based on
a successive approximation (SAR) analog-to-digital
converter (ADC). The devices feature a high-speed,
high-precision SAR ADC, integrated analog front-end
(AFE) input driver circuit, overvoltage protection
circuit up to ±20 V, and an on-chip 4.096-V reference
with extremely low temperature drift.
1
•
•
•
•
•
•
•
•
•
16-Bit ADC With Integrated Analog Front-End
High Speed:
– ADS8681: 1 MSPS
– ADS8685: 500 kSPS
– ADS8689: 100 kSPS
Software Programmable Input Ranges:
– Bipolar Ranges: ±12.288 V, ±10.24 V,
±6.144 V, ±5.12 V, and ±2.56 V
– Unipolar Ranges: 0 V–12.288 V, 0 V–10.24 V,
0 V–6.144 V, and 0 V–5.12 V
5-V Analog Supply: 1.65-V to 5-V I/O Supply
Constant Resistive Input Impedance ≥ 1 MΩ
Input Overvoltage Protection: Up to ±20 V
On-Chip, 4.096-V Reference With Low Drift
Excellent Performance:
– DNL: ±0.4 LSB; INL: ±0.5 LSB
– SNR: 92 dB; THD: –107 dB
ALARM → High, Low Threshold
multiSPI™ Interface With Daisy-Chain
Extended Industrial Temperature Range:
–40°C to +125°C
The devices operate on a single 5-V analog supply,
but support true bipolar input ranges of ±12.288 V,
±6.144 V, ±10.24 V, ±5.12 V, and ±2.56 V, as well as
unipolar input ranges of 0 V to 12.288 V, 0 V to
10.24 V, 0 V to 6.144 V, and 0 V to 5.12 V. The gain
and offset errors are accurately trimmed within the
specified values for each input range to ensure high
dc precision. The input range selection is done by
software programming of the device internal registers.
The devices offer a high resistive input impedance
(≥ 1 MΩ) irrespective of the selected input range.
The multiSPI digital interface is backward-compatible
to the traditional SPI protocol. Additionally,
configurable features simplify interface to a wide
range of host controllers.
Device Information(1)
PART NUMBER
2 Applications
•
•
•
ADS868x
Channel-Isolated PLC Analog Input Modules
Test and Measurement
Battery Pack Monitoring
PACKAGE
BODY SIZE (NOM)
TSSOP (16)
5.00 mm × 4.40 mm
WQFN (16)
4.00 mm × 4.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Block Diagram
DVDD
AVDD
REFIO
ADS868x
4.096-V
Reference
REFCAP
CONVST/CS
1 M:
SCLK
OVP
AIN_P
PGA
AIN_GND
OVP
2nd-Order
LPF
ADC
Driver
16-Bit
SAR ADC
Digital Logic
and Interface
SDI
SDO
1 M:
VBIAS
AGND
Oscillator
DGND
REFGND
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
ADS8681, ADS8685, ADS8689
SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
www.ti.com
Table of Contents
1
2
3
4
5
6
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
4
6
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
Absolute Maximum Ratings ...................................... 6
ESD Ratings.............................................................. 6
Recommended Operating Conditions....................... 6
Thermal Information .................................................. 6
Electrical Characteristics........................................... 7
Timing Requirements: Conversion Cycle................ 11
Timing Requirements: Asynchronous Reset........... 11
Timing Requirements: SPI-Compatible Serial
Interface ................................................................... 11
6.9 Timing Requirements: Source-Synchronous Serial
Interface (External Clock) ........................................ 12
6.10 Timing Requirements: Source-Synchronous Serial
Interface (Internal Clock).......................................... 12
6.11 Typical Characteristics .......................................... 16
7
Detailed Description ............................................ 23
7.1 Overview ................................................................. 23
7.2 Functional Block Diagram ....................................... 23
7.3
7.4
7.5
7.6
8
Feature Description.................................................
Device Functional Modes........................................
Programming...........................................................
Register Maps .........................................................
24
36
41
49
Application and Implementation ........................ 57
8.1 Application Information............................................ 57
8.2 Typical Application ................................................. 57
9
Power Supply Recommendations...................... 61
9.1 Power Supply Decoupling ....................................... 61
9.2 Power Saving .......................................................... 61
10 Layout................................................................... 62
10.1 Layout Guidelines ................................................. 62
10.2 Layout Example .................................................... 63
11 Device and Documentation Support ................. 64
11.1
11.2
11.3
11.4
11.5
11.6
11.7
Documentation Support ........................................
Related Links ........................................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
64
64
64
64
64
64
65
12 Mechanical, Packaging, and Orderable
Information ........................................................... 65
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision B (December 2016) to Revision C
Page
•
Deleted per channel from ALARM → High, Low Threshold bullet in Features section.......................................................... 1
•
Added pin 1 indicator to RUM pinout drawing ........................................................................................................................ 4
•
Deleted offers a low impedance of 30 kΩ from footnotes 2 and 3 in Absolute Maximum Ratings table ............................... 6
•
Changed test conditions of Input Overvoltage Protection Circuit, VOVP parameter................................................................ 7
•
Deleted multichannel reference from Overview section ....................................................................................................... 23
•
Changed the input voltage range for each analog channel to the input voltage range in Analog Input Structure section .. 24
•
Changed Input Overvoltage Protection Limits When AVDD = 5 V table name from Input Overvoltage Protection
Limits When AVDD = 5 V or Offers a Low Impedance of 30 kΩ.......................................................................................... 25
•
Changed AVDD is floating with an impedance 30 kΩ to AVDD is floating in Input Protection Circuit section..................... 26
•
Changed Input Overvoltage Protection Limits When AVDD = Floating table title from Input Overvoltage Protection
Limits When AVDD = Floating with Impedance 30 kΩ ......................................................................................................... 26
•
Added footnotes to List of Input Commands table ............................................................................................................... 43
Changes from Revision A (March 2016) to Revision B
Page
•
Added ADS8685 and ADS8689 devices to document .......................................................................................................... 1
•
Changed Features bullets: added High Speed bullet, deleted Aggregate Throughput sub-bullet from Excellent
Performance bullet, added Extended to Industrial Temperature Range bullet, and deleted Packages bullet ....................... 1
•
Changed Description section ................................................................................................................................................. 1
•
Deleted Device Comparison Table ........................................................................................................................................ 4
•
Added maximum throughput to conditions statement of Electrical Characteristics table and all Timing Requirements
tables ..................................................................................................................................................................................... 7
2
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
•
Changed Power-Supply Requirements sub-section parameter names in Electrical Characteristics table for clarity
(specification values did not change) ..................................................................................................................................... 9
•
Changed conditions of Timing Requirements tables ........................................................................................................... 11
•
Changed Typical Characteristics section: added maximum throughput to conditions statement and deleted
redundant plots for the ADS868x ........................................................................................................................................ 16
•
Changed Input Alarm section: corrected alarm reset configuration in last two paragraphs and both images ..................... 35
•
Added fifth paragraph of Multiple Devices: Daisy-Chain Topology section.......................................................................... 38
•
Added RST (Falling Edge) to Device Functional States figure ............................................................................................ 39
•
Changed bulleted list in Data Transfer Frame section pertaining to the end of the data transfer frame F .......................... 42
•
Changed Output Data Word section..................................................................................................................................... 44
•
Changed several value after reset notations from h to b throughout bit registers of the Register Maps section ............... 49
•
Changed ALARM_H_TH_REG Register Field Descriptions table ...................................................................................... 56
•
Changed ALARM_L_TH_REG Register Field Descriptions table ....................................................................................... 56
•
Changed Application and Implementation section for clarity and improved description of device functionality ................. 57
•
Changed first sentence of Power Saving section................................................................................................................. 61
Changes from Original (February 2016) to Revision A
•
Page
Released to production .......................................................................................................................................................... 1
Copyright © 2016–2018, Texas Instruments Incorporated
Product Folder Links: ADS8681 ADS8685 ADS8689
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
www.ti.com
5 Pin Configuration and Functions
PW Package
16-Pin TSSOP
Top View (Not to Scale)
DGND
1
16
DVDD
AVDD
2
15
RVS
AGND
3
14
ALARM/SDO-1/GPO
REFIO
4
13
SDO-0
REFGND
5
12
SCLK
REFCAP
6
11
CONVST/CS
AIN_P
7
10
SDI
AIN_GND
8
9
RST
RVS
DVDD
DGND
AVDD
RUM Package
16-Pin WQFN
Top View (Not to Scale)
AGND
ALARM/SDO-1/GPO
REFIO
SDO-0
SDI
CONVST/CS
RST
REFCAP
AIN_GND
SCLK
AIN_P
REFGND
Pin Functions
NO.
NAME
TYPE (1)
DESCRIPTION
TSSOP
WQFN
AGND
3
1
P
Analog ground pin. Decouple with the AVDD pin.
AIN_GND
8
6
AI
Analog input: negative. Decouple with the AIN_P pin.
AIN_P
7
5
AI
Analog input: positive. Decouple with the AIN_GND pin.
ALARM/SDO-1/GPO
14
12
DO
Multi-function output pin. Active high alarm.
Data output 1 for serial communication. General-purpose output pin.
AVDD
2
16
P
Analog supply pin. Decouple with the AGND pin.
CONVST/CS
11
9
DI
Dual-functionality pin.
Active high logic: conversion start input pin; a CONVST rising edge brings the device from
acquisition phase to conversion phase.
Active low logic: chip-select input pin; the device takes control of the data bus when CS is low;
the SDO-x pins go to tri-state when CS is high.
DGND
1
15
P
Digital ground pin. Decouple with the DVDD pin.
(1)
4
AI = analog input, AIO = analog input/output, DI = digital input, DO = digital output, and P = power supply.
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
Pin Functions (continued)
NAME
NO.
TYPE (1)
DESCRIPTION
TSSOP
WQFN
DVDD
16
14
P
REFCAP
6
4
AO
REFGND
5
3
P
REFIO
4
2
AIO
RST
9
7
DI
Active low logic input to reset the device.
RVS
15
13
DO
Multi-function output pin for serial interface; see the RESET State section.
With CS held high, RVS reflects the status of the internal ADCST signal.
With CS low, the status of RVS depends on the output protocol selection.
SCLK
12
10
DI
Serial communication: clock input pin for the serial interface.
All system-synchronous data transfer protocols are timed with respect to the SCLK signal.
SDI
10
8
DI
Dual function: data input pin for serial communication.
Chain data input during serial communication in daisy-chain mode.
SDO-0
13
11
DO
Serial communication: data output 0
Digital supply pin. Decouple with the DGND pin.
ADC reference buffer decoupling capacitor pin. Decouple with the REFGND pin.
Reference ground pin; short to the analog ground plane. Decouple with the REFIO and
REFCAP pins.
Internal reference output and external reference input pin. Decouple with REFGND.
Copyright © 2016–2018, Texas Instruments Incorporated
Product Folder Links: ADS8681 ADS8685 ADS8689
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
AIN_P, AIN_GND to GND
MIN
MAX
AVDD = 5 V (2)
–20
20
AVDD = floating (3)
–11
11
UNIT
V
AVDD to GND or DVDD to GND
–0.3
7
V
REFCAP to REFGND or REFIO to REFGND
–0.3
5.7
V
GND to REFGND
–0.3
0.3
V
Digital input pins to GND
–0.3
DVDD + 0.3
V
Digital output pins to GND
–0.3
DVDD + 0.3
V
Operating, TA
–40
125
Storage, Tstg
–65
150
Temperature
(1)
(2)
(3)
°C
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, and do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
AVDD = 5 V.
AVDD = floating.
6.2 ESD Ratings
VALUE
V(ESD)
Electrostatic discharge
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
Analog input pins
(AIN_P, AIN_GND)
±4000
All other pins
±2000
Charged device model (CDM), per JEDEC specification JESD22-C101
(1)
(2)
(2)
UNIT
V
±500
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
MIN
NOM
MAX
UNIT
AVDD
Analog supply voltage
4.75
5
5.25
V
DVDD
Digital supply voltage
1.65
3.3
AVDD
V
6.4 Thermal Information
ADS8681, ADS8685, ADS8689
THERMAL METRIC (1)
PW (TSSOP)
RUM (WQFN)
16 PINS
16 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance
95.7
31.9
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
29.3
27.9
°C/W
RθJB
Junction-to-board thermal resistance
41.5
7.4
°C/W
ψJT
Junction-to-top characterization parameter
1.5
0.3
°C/W
ψJB
Junction-to-board characterization parameter
40.8
7.4
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
N/A
1.9
°C/W
(1)
6
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
6.5 Electrical Characteristics
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
ANALOG INPUTS
Input range = ±3 × VREF
Full-scale input span (1)
(AIN_P to AIN_GND)
VIN
–12.288
12.288
Input range = ±2.5 × VREF
–10.24
10.24
Input range = ±1.5 × VREF
–6.144
6.144
Input range = ±1.25 × VREF
–5.12
5.12
Input range = ±0.625 × VREF
–2.56
2.56
Input range = 3 × VREF
0
12.288
Input range = 2.5 × VREF
0
10.24
Input range = 1.5 × VREF
0
6.144
Input range = 1.25 × VREF
0
5.12
–12.288
12.288
Input range = ±2.5 × VREF
–10.24
10.24
Input range = ±1.5 × VREF
–6.144
6.144
Input range = ±1.25 × VREF
–5.12
5.12
Input range = ±0.625 × VREF
–2.56
2.56
Input range = 3 × VREF
0
12.288
Input range = 2.5 × VREF
0
10.24
Input range = 1.5 × VREF
0
6.144
Input range = 1.25 × VREF
0
Input range = ±3 × VREF
AIN_P
AIN_GND
RIN
Operating input range
Operating input range
All input ranges
Input impedance
At TA = 25°C
IIN
With voltage
at the AIN_P
pin = VIN
Input current
V
5.12
–0.1
0
0.1
Input range = ±3 × VREF
1.02
1.2
1.38
Input range = ±1.5 × VREF
1.02
1.2
1.38
Input range = 3 × VREF
1.02
1.2
1.38
Input range = 1.5 × VREF
1.02
1.2
1.38
Input range = ±2.5 × VREF
0.85
1
1.15
Input range = ±1.25 × VREF
0.85
1
1.15
Input range = ±0.625 × VREF
0.85
1
1.15
Input range = 2.5 × VREF
0.85
1
1.15
Input range = 1.25 × VREF
0.85
1
1.15
7
25
Input impedance drift
V
Input range = ±3 × VREF
(VIN – 2.5) / RIN
Input range = ±2.5 × VREF
(VIN – 2.2) / RIN
Input range = ±1.5 × VREF
(VIN – 2.0) / RIN
Input range = ±1.25 × VREF
(VIN – 2.0) / RIN
Input range = ±0.625 × VREF
(VIN – 1.6) / RIN
Input range = 3 × VREF
(VIN – 2.6) / RIN
Input range = 2.5 × VREF
(VIN – 2.5) / RIN
Input range = 1.5 × VREF
(VIN – 2.7) / RIN
Input range = 1.25 × VREF
(VIN – 2.5) / RIN
V
MΩ
ppm/°C
µA
INPUT OVERVOLTAGE PROTECTION CIRCUIT
VOVP
All input ranges
AVDD = 5 V, all input ranges
–20
20
AVDD = floating, all input ranges
–11
11
V
INPUT BANDWIDTH
f–3
dB
f–0.1 dB
(1)
Small-signal Input
bandwidth
–3 dB
All input ranges
15
–0.1 dB
All input ranges
2.5
kHz
Ideal input span, does not include gain or offset error.
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Electrical Characteristics (continued)
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
SYSTEM PERFORMANCE
Resolution
16
NMC
No missing codes
16
DNL
Differential nonlinearity (2)
All input ranges
All bipolar ranges (3)
INL
Integral nonlinearity
EO
EG
(2)
All unipolar
ranges (4)
Bits
Bits
–0.7
±0.4
0.7
–1.2
±0.5
1.2
ADS8681
–1.5
±0.5
1.5
ADS8685, ADS8689
–1.2
±0.5
1.2
All bipolar ranges (3)
–1
±0.2
1
All unipolar ranges (4)
–2
±0.2
2
Offset error (5)
At TA = 25°C
Offset error drift with temperature
All input ranges
Gain error (6)
At TA = 25°C, all input ranges
Gain error drift with temperature (7)
All input ranges
LSB
LSB
mV
–3
±0.75
3
–0.025
±0.01
0.025
ppm/°C
%FSR
–5
±1
5
ppm/°C
90.25
92
90.5
92
90.25
92
DYNAMIC CHARACTERISTICS
Input range =
±3 × VREF
ADS8681
Input range =
±2.5 × VREF
ADS8681
ADS8685, ADS8689
ADS8685, ADS8689
90.5
92
89.5
91.5
Input range = ±1.25 × VREF
89.5
91.5
Input range = ±0.625 × VREF
87.75
90
Input range = 3 × VREF
Input range = ±1.5 × VREF
Signal-to-noise ratio (8)
SNR
89.25
91
Input range = 2.5 × VREF
89
91
Input range = 1.5 × VREF
88
90
Input range = 1.25 × VREF
Total harmonic distortion (9) (8)
THD
87.5
All input
ranges
–107
ADS8685, ADS8689
–112
Input range =
±3 × VREF
ADS8681
Input range =
±2.5 × VREF
ADS8681
ADS8685, ADS8689
Signal-to-noise + distortion (8)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
8
Spurious-free dynamic range (8)
92
90.4
92
90.15
92
92
89.4
91.5
Input range = ±1.25 × VREF
89.4
91.5
Input range = ±0.625 × VREF
87.65
90
Input range = 3 × VREF
SFDR
90.15
90.4
Input range = ±1.5 × VREF
SINAD
90
ADS8681
ADS8685, ADS8689
89
91
Input range = 2.5 × VREF
88.75
91
Input range = 1.5 × VREF
87.75
90
Input range = 1.25 × VREF
87.25
All input
ranges
dB
dB
dB
90
ADS8681
109
ADS8685, ADS8689
114
dB
This specification indicates the endpoint INL, not best-fit INL.
Bipolar ranges are ±12.288 V, ±10.24 V, ±6.144 V, ±5.12 V, and ±2.56 V.
Unipolar ranges are 0 V–12.288 V, 0 V–10.24 V, 0 V–6.144 V, and 0 V–5.12 V.
Measured relative to actual measured reference.
Excludes internal reference accuracy error.
Excludes internal reference temperature drift.
All specifications expressed in decibels (dB) refer to the full-scale input (FSR) and are tested with a 1-kHz input signal 0.25 dB below
full-scale, unless otherwise specified.
Calculated on the first nine harmonics of the input frequency.
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SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
Electrical Characteristics (continued)
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
SAMPLING DYNAMICS
tCONV
tACQ
fcycle
Conversion time
Acquisition time
Maximum throughput rate
without latency
ADS8681
665
ADS8685
1000
ADS8689
5000
ADS8681
335
ADS8685
1000
ADS8689
5000
ns
ns
ADS8681
1000
ADS8685
500
ADS8689
100
kSPS
INTERNAL REFERENCE OUTPUT
VREFIO
On the REFIO pin
(configured as an output)
dVREFIO/dTA
Internal reference temperature drift
COUT_REFIO
Decoupling capacitor on REFIO pin
VREFCAP
Reference voltage to the ADC
(on the REFCAP pin)
TSSOP (PW)
4.095
4.096
4.097
WQFN (RUM)
4.094
4.096
4.098
TSSOP (PW)
4
7
WQFN (RUM)
5
At TA = 25°C
4.7
At TA = 25°C
4.095
REFCAP temperature drift
COUT_REFCAP
Decoupling capacitor on REFCAP pin
Turn-on time
ppm/°C
µF
4.096
4.097
0.5
2
10
COUT_REFCAP = 10 µF, COUT_REFIO = 10 µF
V
V
ppm/°C
μF
20
ms
EXTERNAL REFERENCE INPUT
VREFIO_EXT
External reference voltage on REFIO
REFIO pin configured as an input
4.046
4.096
4.146
V
AVDD COMPARATOR
VTH_HIGH
High threshold voltage
5.3
V
VTH_LOW
Low threshold voltage
4.7
V
POWER-SUPPLY REQUIREMENTS
AVDD
DVDD
IAVDD_DYN
IAVDD_STC
Analog power-supply voltage
4.75
5
5.25
1.65
3.3
AVDD
2.7
3.3
AVDD
ADS8681
8.2
10.5
ADS8685
5.6
7.25
ADS8689
4
5
ADS8681
7.0
8.75
ADS8685
4.4
5.5
ADS8689
2.7
3.25
Internal
reference
ADS8681
4.7
6.25
ADS8685, ADS8689
3.5
4.7
External
reference
ADS8681
3.5
4.5
ADS8685, ADS8689
2.3
3
Operating range
Digital power-supply voltage
Supply range for specified performance
Analog supply current,
device converting at maximum
throughput
Analog supply current,
device not converting
Internal
reference
External
reference
V
mA
mA
IAVDD_STDBY
Analog supply current,
device in STANDBY mode
Internal reference
2.8
External reference
1.6
IAVDD_PD
Analog supply current,
device in PD mode
Internal reference
10
External reference
10
IDVDD_DYN
Digital supply current,
maximum throughput
IDVDD_STDBY
Digital supply current,
device in STANDBY mode
1
μA
IDVDD_PD
Digital supply current,
device in PD mode
1
μA
0.2
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μA
0.25
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Electrical Characteristics (continued)
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
DIGITAL INPUTS (CMOS)
VIH
VIL
DVDD > 2.35 V
0.7 ×
DVDD
DVDD +
0.3
DVDD ≤ 2.35 V
0.8 ×
DVDD
DVDD +
0.3
DVDD > 2.35 V
–0.3
0.3 ×
DVDD
DVDD ≤ 2.35 V
–0.3
0.2 ×
DVDD
Digital high input voltage logic level
Digital low input voltage logic level
V
V
Input leakage current
100
nA
Input pin capacitance
5
pF
DIGITAL OUTPUTS (CMOS)
VOH
Digital high output voltage logic level
IO = 500-μA source
VOL
Digital low output voltage logic level
IO = 500-μA sink
Floating state leakage current
Only for digital output pins
0.8 ×
DVDD
DVDD
V
0
0.2 ×
DVDD
V
Internal pin capacitance
1
µA
5
pF
TEMPERATURE RANGE
TA
10
Operating free-air temperature
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–40
125
°C
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6.6 Timing Requirements: Conversion Cycle
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
MIN
TYP
MAX
UNIT
TIMING REQUIREMENTS
fcycle
Sampling frequency
tcycle
ADC cycle time period
tacq
Acquisition time
ADS8681
1000
ADS8685
500
ADS8689
100
kSPS
1/fcycle
ADS8681
335
ADS8685
1000
ADS8689
5000
ns
TIMING SPECIFICATIONS
tconv
Conversion time
ADS8681
665
ADS8685
1000
ADS8689
5000
ns
6.7 Timing Requirements: Asynchronous Reset
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
MIN
TYP
MAX
UNIT
TIMING REQUIREMENTS
twl_RST
Pulse duration: RST high
100
ns
TIMING SPECIFICATIONS
tD_RST_POR
Delay time for POR reset: RST rising to RVS rising
tD_RST_APP
Delay time for application reset: RST rising to CONVST/CS rising
tNAP_WKUP
Wake-up time: NAP mode
tPWRUP
Power-up time: PD mode
20
ms
1
µs
20
20
µs
ms
6.8 Timing Requirements: SPI-Compatible Serial Interface
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
MIN
TYP
MAX
UNIT
66.67
MHz
TIMING REQUIREMENTS
fCLK
Serial clock frequency
tCLK
Serial clock time period
tPH_CK
SCLK high time
0.45
0.55
tCLK
tPL_CK
SCLK low time
0.45
0.55
tCLK
tSU_CSCK
Setup time: CONVST/CS falling to first SCLK capture edge
7.5
ns
tSU_CKDI
Setup time: SDI data valid to SCLK capture edge
7.5
ns
tHT_CKDI
Hold time: SCLK capture edge to (previous) data valid on SDI
7.5
ns
tHT_CKCS
Delay time: last SCLK capture edge to CONVST/CS rising
7.5
ns
1/fCLK
TIMING SPECIFICATIONS
tDEN_CSDO
Delay time: CONVST/CS falling edge to data enable
9.5
ns
tDZ_CSDO
Delay time: CONVST/CS rising to SDO-x going to 3-state
10
ns
tD_CKDO
Delay time: SCLK launch edge to (next) data valid on SDO-x
12
ns
tD_CSRVS
Delay time: CONVST/CS rising edge to RVS falling
14
ns
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6.9 Timing Requirements: Source-Synchronous Serial Interface (External Clock)
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
MIN
TYP
MAX
UNIT
66.67
MHz
TIMING REQUIREMENTS
fCLK
Serial clock frequency
tCLK
Serial clock time period
tPH_CK
SCLK high time
0.45
0.55
tCLK
tPL_CK
SCLK low time
0.45
0.55
tCLK
1/fCLK
TIMING SPECIFICATIONS
tDEN_CSDO
Delay time: CONVST/CS falling edge to data enable
9.5
ns
tDZ_CSDO
Delay time: CONVST/CS rising to SDO-x going to 3-state
10
ns
tD_CKRVS_r
Delay time: SCLK rising edge to RVS rising
14
ns
tD_CKRVS_f
Delay time: SCLK falling edge to RVS falling
14
ns
tD_RVSDO
Delay time: RVS rising to (next) data valid on SDO-x
2.5
ns
tD_CSRVS
Delay time: CONVST/CS rising edge to RVS displaying internal device state
15
ns
6.10 Timing Requirements: Source-Synchronous Serial Interface (Internal Clock)
all minimum and maximum specifications are at TA = –40°C to +125°C; typical specifications are at TA = 25°C; AVDD = 5 V,
DVDD = 3.3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
MIN
TYP
MAX
UNIT
TIMING SPECIFICATIONS
tDEN_CSDO
Delay time: CONVST/CS falling edge to data enable
9.5
ns
tDZ_CSDO
Delay time: CONVST/CS rising to SDO-x going to 3-state
10
ns
tDEN_CSRVS
Delay time: CONVST/CS falling edge to first rising edge on RVS
50
ns
tD_RVSDO
Delay time: RVS rising to (next) data valid on SDO-x
2.5
ns
tINTCLK
Time period: internal clock
15
tCYC_RVS
Time period: RVS signal
15
tWH_RVS
RVS high time
0.4
0.6
tINTCLK
tWL_RVS
RVS low time
0.4
0.6
tINTCLK
tD_CSRVS
Delay time: CONVST/CS rising edge to RVS displaying internal
device state
15
ns
12
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ns
ns
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The CS falling edge can
be issued after tconv_max
or when RVS goes high.
tcycle
CONVST/CS
tconv
tacq
ADCST (Internal)
RVS
Figure 1. Conversion Cycle Timing Diagram
trst
twl_RST
RST
td_rst
RVS
Figure 2. Asynchronous Reset Timing Diagram
tcycle
tconv_max
Data Read Time
CONVST/CS
tD_CSRVS
tD_CSRVS
RVS
tHT_CKCS
tSU_CSCK
CPOL = 0
SCLK
CPOL = 1
tD_CSDO
tD_CKDO
tDEN_CSDO
M
SDO-0
M-1
M-2
L+1
L
tSU_CKDI
tHT_CKDI
SDI
Figure 3. Standard SPI Interface Timing Diagram for CPHA = 0
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tcycle
Data Read Time
tconv_max
CONVST/CS
tD_CSRVS
tD_CSRVS
RVS
tHT_CKCS
tSU_CSCK
CPOL = 0
SCLK
CPOL = 1
tD_CKDO
tDEN_CSDO
SDO-0
0
M
M-1
L+1
tDZ_CSDO
L
tSU_CKDI
tHT_CKDI
SDI
Figure 4. Standard SPI Interface Timing Diagram for CPHA = 1
tcycle
tconv_max
Data Read Time
CONVST/CS
tD_CSRVS
tD_CSRVS
RVS
tHT_CKCS
tSU_CSCK
CPOL = 0
SCLK
CPOL = 1
tD_CSDO
tD_CKDO
tDEN_CSDO
SDO-0
M
M-2
M-4
L+3
L+1
SDO-1
M-1
M-3
M-5
L+2
L
tSU_CKDI
tHT_CKDI
SDI
Figure 5. multiSPI Interface Timing Diagram for Dual SDO-x and CPHA = 0
14
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tcycle
tconv_max
Data Read Time
CONVST/CS
tD_CSRVS
tD_CSRVS
RVS
tHT_CKCS
tSU_CSCK
CPOL = 0
SCLK
CPOL = 1
tD_CKDO
tDEN_CSDO
SDO-0
tDZ_CSDO
0
M
M-2
L+3
L+1
0
M-1
M-1
L+2
L
SDO-1
tSU_CKDI
tHT_CKDI
SDI
Figure 6. multiSPI Interface Timing Diagram for Dual SDO-x and CPHA = 1
tconv
Data Read Time
CONVST/CS
tSU_CSCK
SCLK
tDEN_CSDO
SDO-0
M-1
M-k
M-k-1
L+1
M-k-2
tD_CKRVS_r
tD_CKRVS_f
tD_CSRD
tDZ_CSDO
tD_RVSDO
M
L
tD_CSRVS
RVS
Figure 7. multiSPI Source-Synchronous External Clock Serial Interface Timing Diagram
tconv_max
Data Read Time
CONVST/CS
SCLK
tDEN_CSDO
SDO-0
tDZ_CSDO
M
tDEN_CSRVS
M-1
M-k
tD_RVSDO
M-k-1
M-k-2
L+1
L
tD_CSRVS
tWH_RVS
RVS
tCYC_RVS
tWL_RVS
Figure 8. multiSPI Source-Synchronous Internal Clock Serial Interface Timing Diagram
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6.11 Typical Characteristics
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
15
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
9
3
Analog Input Current (uA)
Analog Input Current (uA)
15
-3
-9
-15
-12.288
-8.192
-4.096
0
4.096
Input Voltage (V)
8.192
-40C
25C
125C
9
3
-3
-9
-15
-12.288
12.288
-8.192
D001
-4.096
0
4.096
Input Voltage (V)
8.192
12.288
D002
Range = ±12.288 V
Figure 9. Input I-V Characteristic Across Input Ranges
Figure 10. Input I-V Characteristic Across Temperature
1600
400
1200
200
Number of Devices
Input Impedance Drift (ppm)
1400
0
±12.288 V
±10.24 V
±6.144 V
±5.12 V
±2.56 V
-200
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
1000
800
600
400
200
0
-400
-40
-7
26
59
Free- Air Temperature (qC)
92
1.02 1.06
125
1.1
D003
1.14 1.18 1.22 1.26
Input Impedance (M:)
1.3
1.34 1.38
D004
Number of samples = 3398
Figure 12. Typical Distribution of Input Impedance
45000
40000
40000
35000
35000
30000
30000
Number of Hits
Number of Hits
Figure 11. Input Impedance Drift vs Temperature
45000
25000
20000
15000
20000
15000
10000
10000
5000
5000
0
0
32766
32767
32768
32769
Output Codes
32770
32766
32771
Figure 13. DC Histogram for Mid-Scale Inputs (±12.288 V)
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32767
D007
Mean = 32768.24, sigma = 0.57, input = 0 V
16
25000
32768
32769
Output Codes
32770
32771
D008
D003
Mean = 32767.24, sigma = 0.56, input = 0 V
Figure 14. DC Histogram for Mid-Scale Inputs (±10.24 V)
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Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
40000
45000
Number of Hits
Number of Hits
30000
20000
30000
15000
10000
0
0
32766 32767 32768 32769 32770 32771 32772
Output Codes
32766
Mean = 32768.21, sigma = 0.62, input = 0 V
Figure 15. DC Histogram for Mid-Scale Inputs (±6.144 V)
32771
32772
D010
Figure 16. DC Histogram for Mid-Scale Inputs (±5.12 V)
50000
30000
40000
24000
Number of Hits
Number of Hits
32768 32769 32770
Output Codes
Mean = 32768.11, sigma = 0.62, input = 0 V
36000
18000
12000
30000
20000
10000
6000
0
0
32765
32765 32766 32767 32768 32769 32770 32771 32772
Output Codes
D011
Mean = 32768.22, sigma = 0.75, input = 0 V
32766
32767 32768 32769
Output Codes
32770
32771
D012
Mean = 32767.73, sigma = 0.60, input = 6.144 V
Figure 17. DC Histogram for Mid-Scale Inputs
(±2.56 V)
Figure 18. DC Histogram for Mid-Scale Inputs
(0 V–12.288 V)
40000
40000
30000
30000
Number of Hits
Number of Hits
32767
D009
20000
10000
20000
10000
0
0
32766
32767
32768 32769 32770
Output Codes
32771
32772
Mean = 32768.41, sigma = 0.62, input = 5.12 V
Figure 19. DC Histogram for Mid-Scale Inputs
(0 V–10.24 V)
32766
32767
D013
32768 32769 32770
Output Codes
32771
32772
D014
Mean = 32768.42, sigma = 0.76, input = 3.072 V
Figure 20. DC Histogram for Mid-Scale Inputs
(0 V–6.144 V)
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Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
40000
Differential Nonlinearity (LSB)
0.7
Number of Hits
30000
20000
10000
0
0.5
0.3
0.1
-0.1
-0.3
-0.5
-0.7
32766
32767
32768 32769 32770
Output Codes
32771
32772
0
16384
D015
Mean = 32768.32, sigma = 0.78, input = 2.56 V
49152
65535
D016
All input ranges
Figure 21. DC Histogram for Mid-Scale Inputs (0 V–5.12 V)
Figure 22. Typical DNL for All Codes
1.2
0.7
Maximum
Minimum
0.5
0.8
Integral Nonlinearity (LSB)
Differential Nonlinearity (LSB)
32768
Codes (LSB)
0.3
0.1
-0.1
-0.3
0.4
0
-0.4
-0.8
-0.5
-0.7
-40
-1.2
-7
26
59
Free-Air Temperature (0C)
92
125
0
16384
D017
32768
Codes (LSB)
49152
65535
D018
All input ranges
Figure 23. DNL vs Temperature
Figure 24. Typical INL for All Codes (All Bipolar Ranges)
1.5
1.2
Maximum
Minimum
Integral Nonlinearity (LSB)
Integral Nonlinearity (LSB)
1
0.5
0
-0.5
0.6
0
-0.6
-1
-1.5
0
16384
32768
Codes (LSB)
49152
65536
D023
Figure 25. Typical INL for All Codes (All Unipolar Ranges)
18
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-1.2
-40
-7
26
59
Free-Air Temperature (0C)
92
125
D027
Figure 26. INL vs Temperature (All Bipolar Ranges)
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Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
1.5
1
± 12.288 V
± 10.24 V
± 6.144 V
0.75
0.75
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
0.5
Offset Error (mV)
Integral Nonlinearity (LSB)
Maximum
Minimum
0
-0.75
0.25
0
-0.25
-0.5
-0.75
-1.5
-40
-7
26
59
Free-Air Temperature (0C)
92
-1
-40
125
Figure 27. INL vs Temperature (All Unipolar Ranges)
92
125
D036
0.025
± 12.288 V
± 10.24 V
± 6.144 V
0.015
Gain Error (%FSR)
10
Number of Devices
26
59
Free-Air Temperature (0C)
Figure 28. Offset Error vs
Temperature Across Input Ranges
12.5
7.5
5
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
0.005
-0.005
-0.015
2.5
-0.025
-40
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3
Offset Drift (ppm/ºC)
-7
D037
Figure 29. Typical Histogram for Offset Drift
26
59
Free-Air Temperature (0C)
92
125
D038
Figure 30. Gain Error vs Temperature Across Input Ranges
1.1
25
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
1
0.9
20
Gain Error (%FSR)
0.8
Number of Units
-7
D032
15
10
0.7
0.6
0.5
0.4
0.3
0.2
5
0.1
0
-0.1
0
0
0.5
1
1.5
2 2.5 3 3.5
Gain Drift (ppm/ºC)
4
4.5
0
5
2000
D039
Figure 31. Typical Histogram for Gain Error Drift
4000
6000
Source Resistance (:)
8000
10000
D040
Figure 32. Gain Error vs External Resistance (REXT)
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Typical Characteristics (continued)
0
0
-40
-40
Amplitude (dB)
Amplitude (dB)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
-80
-120
-160
-80
-120
-160
-200
-200
0
100000
200000
300000
Input Frequency (Hz)
400000
500000
0
Number of points = 64k, fIN = 1 kHz
100000
150000
Input Frequency (Hz)
200000
250000
D001
Number of points = 64k, fIN = 1 kHz
Figure 33. Typical FFT Plot (All Ranges) for the ADS8681
Figure 34. Typical FFT Plot (All Ranges) for the ADS8685
0
94.5
Signal-to-Noise Ratio (dB)
-40
Amplitude (dB)
50000
D041
-80
-120
-160
-200
0
20000
40000
60000
Input Frequency (Hz)
80000
± 12.288 V
± 10.24 V
± 6.144 V
0-10.24 V
0-6.144 V
0-5.12 V
93
91.5
90
88.5
100
100000
± 5.12 V
± 2.56 V
0-12.288 V
1000
Input Frequency (Hz)
D002
10000
D050
Number of points = 64k, fIN = 1 kHz
Figure 35. Typical FFT Plot (All Ranges) for the ADS8689
Figure 36. SNR vs Input Frequency
96
94
Signal-to-Noise Ratio (dB)
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
Signal-to-Noise + Distortion Ratio (dB)
± 12.288 V
± 10.24 V
± 6.144 V
95
94
93
92
91
90
89
88
-40
-7
26
59
Free-Air Temperature (0C)
92
125
93
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
92
91
90
89
88
100
1000
Input Frequency (Hz)
D051
10000
D052
fIN = 1 kHz
Figure 37. SNR vs Temperature
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Figure 38. SINAD vs Input Frequency
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Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
-80
± 12.288 V
± 10.24 V
± 6.144 V
95
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
Total Harmonic Distortion Ratio (dB)
Signal-to-Noise + Distortion Ratio (dB)
96
94
93
92
91
90
89
88
-40
-7
26
59
Free-AirTemperature (0C)
92
±12.288 V
±10.24 V
±6.144 V
0-10.24 V
0-6.144 V
0-5.12 V
-90
-100
-110
-120
100
125
±5.12 V
±2.56 V
0-12.288 V
1000
Input Frequency (Hz)
D053
10000
D054
fIN = 1 kHz
Figure 39. SINAD vs Temperature
Figure 40. THD vs Input Frequency
9
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
8
-90
IAVDD Dynamic (mA)
Total Harmonic Distortion (dB)
-80
-100
-110
7
6
5
4
3
ADS8681
ADS8685
ADS8689
2
-120
-40
-7
26
59
Free-Air Temperature (0C)
92
1
-40
125
-7
D055
26
59
Free-Air Temperature (qC)
92
125
D003
fIN = 1 kHz
Figure 41. THD vs Temperature
Figure 42. AVDD Current vs Temperature
9
5
8
4.5
IAVDD Static (mA)
IAVDD Current (mA)
7
6
5
4
4
3.5
3
3
2
ADS8681
ADS8685
ADS8689
ADS8681
ADS8685
1
0
200
400
600
Throughput (ksps)
800
Figure 43. AVDD Current vs Throughput
1000
D005
2.5
-40
-7
26
59
Free-Air Temperature (qC)
92
125
D004
Figure 44. AVDD Current vs Temperature
(During Sampling)
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Typical Characteristics (continued)
at TA = 25°C, AVDD = 5 V, DVDD = 3 V, VREF = 4.096 V (internal), and maximum throughput (unless otherwise noted)
3
2.8
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
± 12.288 V
± 10.24 V
± 6.144 V
2.5
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
2.7
IAVDD PD (uA)
IAVDD Standby (mA)
± 12.288 V
± 10.24 V
± 6.144 V
2.6
2
1.5
1
2.5
0.5
2.4
-40
-7
26
59
Free-Air Temperature (0C)
92
Figure 45. AVDD Current vs Temperature
(Standby Mode)
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125
D067
0
-40
-7
26
59
Free-Air Temperature (0C)
92
125
D068
Figure 46. AVDD Current vs Temperature
(Power-Down Mode)
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7 Detailed Description
7.1 Overview
The ADS868x devices belong to a family of high-speed, high-performance, easy-to-use integrated data
acquisition system. This single-channel device supports true bipolar input voltage swings up to ±12.288 V,
operating on a single 5-V analog supply. The device features an enhanced SPI interface (multiSPI) that allows
the sampling rate to be maximized even with lower speed host controllers.
The device consists of a high-precision successive approximation register (SAR) analog-to-digital converter
(ADC) and a power-optimized analog front-end (AFE) circuit for signal conditioning that includes:
• A high-resistive input impedance (≥ 1 MΩ) that is independent of the sampling rate
• A programmable gain amplifier (PGA) with a pseudo-differential input configuration supporting nine softwareprogrammable unipolar and bipolar input ranges
• A second-order, low-pass antialiasing filter
• An ADC driver amplifier that ensures quick settling of the SAR ADC input for high accuracy
• An input overvoltage protection circuit up to ±20 V
The device also features a low temperature drift, 4.096-V internal reference with a fast-settling buffer and a
multiSPI serial interface with daisy-chain (DAISY) and ALARM features.
The integration of the precision AFE circuit with high input impedance and a precision ADC operating from a
single 5-V supply offers a simplified end solution without requiring external high-voltage bipolar supplies and
complicated driver circuits.
7.2 Functional Block Diagram
DVDD
AVDD
REFIO
ADS868x
4.096-V
Reference
REFCAP
CONVST/CS
1 M:
SCLK
OVP
AIN_P
PGA
AIN_GND
OVP
2nd-Order
LPF
ADC
Driver
16-Bit
SAR ADC
Digital Logic
and Interface
SDI
SDO
1 M:
VBIAS
AGND
Oscillator
DGND
REFGND
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7.3 Feature Description
7.3.1 Analog Input Structure
The device features a pseudo-differential input structure, meaning that the single-ended analog input signal is
applied at the positive input AIN_P and the negative input AIN_GND is tied to GND. Figure 47 shows the
simplified circuit schematic for the AFE circuit, including the input overvoltage protection circuit, PGA, low-pass
filter (LPF), and high-speed ADC driver.
1 M:
OVP
AIN_P
PGA
AIN_GND
OVP
2nd-Order
LPF
ADC
Driver
ADC
1 M:
CONVST/CS
SCLK
SDI
SDO
VB
Figure 47. Simplified Analog Front-End Circuit Schematic
The device can support multiple unipolar or bipolar, single-ended input voltage ranges based on the configuration
of the program registers. As explained in the RANGE_SEL_REG register, the input voltage range can be
configured to bipolar ±3 × VREF, ±2.5 × VREF, ±1.5 × VREF, ±1.25 × VREF, and ±0.625 × VREF or unipolar 0 to 3 ×
VREF, 0 to 2.5 × VREF, 0 to 1.5 × VREF and 0 to 1.25 × VREF. With the internal or external reference voltage set to
4.096 V, the input ranges of the device can be configured to bipolar ranges of ±12.288 V, ±10.24 V, ±6.144 V,
±5.12 V, and ±2.56 V or unipolar ranges of 0 V to 12.288 V, 0 V to 10.24 V, 0 V to 6.144 V, and 0 V to 5.12 V.
The device samples the voltage difference (AIN_P – AIN_GND) between the analog input and the AIN_GND pin.
The device allows a ±0.1-V range on the AIN_GND pin. This feature is useful in modular systems where the
sensor or signal-conditioning block is further away from the ADC on the board and when a difference in the
ground potential of the sensor or signal conditioner from the ADC ground is possible. In such cases, running
separate wires from the AIN_GND pin of the device to the sensor or signal-conditioning ground is recommended.
In order to obtain optimum performance, the input currents and impedances along each input path are
recommended to be matched. The two single-ended signals to AIN_P and AIN_GND must be routed as
symmetrically as possible from the signal source to the ADC input pins.
If the analog input pin (AIN_P) to the device is left floating, the output of the ADC corresponds to an internal
biasing voltage. The output from the ADC must be considered as invalid if the device is operated with floating
input pins. This condition does not cause any damage to the device, which becomes fully functional when a valid
input voltage is applied to the pins.
7.3.2 Analog Input Impedance
The device presents a resistive input impedance ≥ 1 MΩ on each of the analog inputs. The input impedance is
independent of the ADC sampling frequency or the input signal frequency. The primary advantage of such highimpedance inputs is the ease of driving the ADC inputs without requiring driving amplifiers with low output
impedance. Bipolar, high-voltage power supplies are not required in the system because this ADC does not
require any high-voltage, front-end drivers. In most applications, the signal sources or sensor outputs can be
directly connected to the ADC input, thus significantly simplifying the design of the signal chain.
In order to maintain the dc accuracy of the system, matching the external source impedance on the AIN_P input
pin with an equivalent resistance on the AIN_GND pin is recommended. This matching helps cancel any
additional offset error contributed by the external resistance.
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Feature Description (continued)
7.3.3 Input Protection Circuit
The device features an internal overvoltage protection (OVP) circuit on each of the analog inputs. Use the
internal protection circuit only as a secondary protection scheme. The external protection devices in the end
application are highly recommended to be used to protect against surges, electrostatic discharge (ESD), and
electrical fast transient (EFT) conditions. A conceptual block diagram of the internal OVP circuit is shown in
Figure 48.
AVDD
VP+
RFB
0V
ESD
AVDD
VP-
RS
1 MŸ
AIN_P
D1p
D2p
RS
V±
AVDD
AIN_GND
VOUT
D1n
1 MŸ
V+
+
D2n
RDC
ESD
VB
GND
Figure 48. Input Overvoltage Protection Circuit Schematic
As shown in Figure 48, the combination of the 1-MΩ (or, 1.2 MΩ for appropriate input ranges) input resistors
along with the PGA gain-setting resistors RFB and RDC limit the current flowing into the input pin. A combination
of anti-parallel diodes, D1 and D2 are added to protect the internal circuitry and set the overvoltage protection
limits.
Table 1 explains the various operating conditions for the device when powered on. This table indicates that when
the device is properly powered up (AVDD = 5 V) or offers a low impedance of < 30 kΩ, the internal overvoltage
protection circuit can withstand up to ±20 V on the analog input pins.
Table 1. Input Overvoltage Protection Limits When AVDD = 5 V (1)
INPUT CONDITION
(VOVP = ±20 V)
CONDITION
RANGE
TEST
CONDITION
ADC
OUTPUT
COMMENTS
|VIN| < |VRANGE|
Within operating range
All input
ranges
Valid
|VRANGE| < |VIN| < |VOVP|
Beyond operating range but
within overvoltage range
All input
ranges
Saturated
ADC output is saturated, but device is internally protected
(not recommended for extended time).
|VIN| > |VOVP|
Beyond overvoltage range
All input
ranges
Saturated
This usage condition can cause irreversible damage to the
device.
(1)
Device functions as per data sheet specifications.
GND = 0 V, AIN_GND = 0 V, |VRANGE| is the maximum input voltage for any selected input range, and |VOVP| is the break-down voltage
for the internal OVP circuit. Assume that RS is approximately 0 Ω.
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The results indicated in Table 1 are based on an assumption that the analog input pin is driven by a very low
impedance source (RS is approximately 0 Ω). However, if the source driving the input has higher impedance, the
current flowing through the protection diodes reduces further, thereby increasing the OVP voltage range. Higher
source impedances result in gain errors and contribute to overall system noise performance.
Figure 49 shows the voltage versus current response of the internal overvoltage protection circuit when the
device is powered on. According to this current-to-voltage (I-V) response, the current flowing into the device input
pin is limited by the 1-MΩ (or 1.2 MΩ for appropriate input ranges) input impedance. However, for voltages
beyond ±20 V, the internal node voltages surpass the break-down voltage for internal transistors, thus setting the
limit for overvoltage protection on the input pin.
The same overvoltage protection circuit also provides protection to the device when the device is not powered on
and AVDD is floating. This condition can arise when the input signals are applied before the ADC is fully
powered on. The overvoltage protection limits for this condition are shown in Table 2.
Table 2. Input Overvoltage Protection Limits When AVDD = Floating (1)
INPUT CONDITION
(VOVP = ±11 V)
CONDITION
RANGE
TEST
CONDITION
ADC OUTPUT
COMMENTS
|VIN| < |VOVP|
Within overvoltage range
All input ranges
Invalid
Device is not functional but is protected internally by the
OVP circuit.
|VIN| > |VOVP|
Beyond overvoltage range
All input ranges
Invalid
This usage condition can cause irreversible damage to the
device.
(1)
AVDD = floating, GND = 0 V, AIN_GND = 0 V, |VRANGE| is the maximum input voltage for any selected input range, and |VOVP| is the
break-down voltage for the internal OVP circuit. Assume that RS is approximately 0 Ω.
Figure 50 shows the I-V response of the internal overvoltage protection circuit when the device is not powered
on. According to this I-V response, the current flowing into the device input pin is limited by the 1-MΩ input
impedance. However, for voltages beyond ±11 V, the internal node voltage surpasses the break-down voltage for
internal transistors, thus setting the limit for overvoltage protection on the input pin.
30
24
18
20
Input Current (uA)
Input Current (uA)
12
10
0
-10
6
0
-6
-12
-20
-30
-30
-18
-24
-18
-12
-6
0
6
Input voltage (V)
12
18
24
Figure 49. I-V Curve for the Input OVP Circuit
(AVDD = 5 V)
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30
-24
-20
-16
-12
-8
D005
-4
0
4
Input voltage (V)
8
12
16
20
D006
Figure 50. I-V Curve for the Input OVP Circuit
(AVDD = Floating)
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7.3.4 Programmable Gain Amplifier (PGA)
The device features a programmable gain amplifier (PGA) as part of the analog signal-conditioning circuit that
converts the original single-ended input signal into a fully-differential signal to drive the internal SAR ADC. The
PGA also adjusts the common-mode level of the input signal before feeding it into the SAR ADC to ensure
maximum usage of the ADC input dynamic range. Depending on the range of the input signal, the PGA gain can
be adjusted by setting the RANGE_SEL[3:0] bits in the configuration register (see the RANGE_SEL_REG
register). The default or power-on state for the RANGE_SEL[3:0] bits is 0000, corresponding to an input signal
range of ±3 × VREF. Table 3 lists the various configurations of the RANGE_SEL[3:0] bits for the different analog
input voltage ranges.
The PGA uses a precisely-matched network of resistors for multiple gain configurations. Matching between these
resistors is accurately trimmed to keep the overall gain error low across all input ranges.
Table 3. Input Range Selection Bits Configuration
RANGE_SEL[3:0]
ANALOG INPUT RANGE
BIT 3
BIT 2
BIT 1
BIT 0
±3 × VREF
0
0
0
0
±2.5 × VREF
0
0
0
1
±1.5 × VREF
0
0
1
0
±1.25 × VREF
0
0
1
1
±0.625 × VREF
0
1
0
0
0–3 × VREF
1
0
0
0
0–2.5 × VREF
1
0
0
1
0–1.5 × VREF
1
0
1
0
0–1.25 × VREF
1
0
1
1
7.3.5 Second-Order, Low-Pass Filter (LPF)
In order to mitigate the noise of the front-end amplifier and gain resistors of the PGA, the AFE circuit of the
device features a second-order, antialiasing LPF at the output of the PGA. The magnitude and phase response
of the analog antialiasing filter are shown in Figure 51 and Figure 52, respectively. For maximum performance,
the –3-dB cutoff frequency for the antialiasing filter is typically set to 15 kHz. The performance of the filter is
consistent across all input ranges supported by the ADC.
45
3
0
Phase (Degree)
Magnitude (dB)
0
-3
±12.288 V
±10.24 V
±6.144 V
±5.12 V
±2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
-6
-9
-12
-15
10
100
-45
-90
1k
Input Frequency (Hz)
10k
100k
-135
10
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
100
D058
Figure 51. Second-Order LPF Magnitude Response
1k
Input Frequency (Hz)
10k
100k
D059
Figure 52. Second-Order LPF Phase Response
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7.3.6 ADC Driver
In order to meet the performance of the device at the maximum sampling rate, the sample-and-hold capacitors at
the input of the ADC must be successfully charged and discharged during the acquisition time window. This drive
requirement at the input of the ADC necessitates the use of a high-bandwidth, low-noise, and stable amplifier
buffer. Such an input driver is integrated in the front-end signal path of the analog input channel of the device.
7.3.7 Reference
The device can operate with either an internal voltage reference or an external voltage reference using the
internal buffer. The internal or external reference selection is determined by programming the INTREF_DIS bit of
the RANGE_SEL_REG register. The internal reference source is enabled (INTREF_DIS = 0) by default after
reset or when the device powers up. The INTREF_DIS bit must be programmed to logic 1 to disable the internal
reference source whenever an external reference source is used.
7.3.7.1 Internal Reference
The device features an internal reference source with a nominal output value of 4.096 V. In order to select the
internal reference, the INTREF_DIS bit of the RANGE_SEL_REG register must be programmed to logic 0. When
the internal reference is used, the REFIO pin becomes an output with the internal reference value. A 4.7-µF
(minimum) decoupling capacitor is recommended to be placed between the REFIO pin and REFGND, as shown
in Figure 53. The capacitor must be placed as close to the REFIO pin as possible. The output impedance of the
internal band-gap circuit creates a low-pass filter with this capacitor to band-limit the noise of the reference. The
use of a smaller capacitor value allows higher reference noise in the system that can potentially degrade SNR
and SINAD performance. The REFIO pin must not be used to drive external ac or dc loads because of limited
current output capability. The REFIO pin can be used as a source if followed by a suitable op amp buffer (such
as the OPA320).
AVDD
4.096 VREF
RANGE_SEL_REG[6] = 0
(INTREF_DIS)
REFIO
4.7 PF
REFCAP
10 PF
1 PF
REFGND
ADC
AGND
Figure 53. Device Connections for Using an Internal 4.096-V Reference
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The device internal reference is factory-trimmed to ensure the initial accuracy specification. The histogram in
Figure 54 shows the distribution of the internal voltage reference output taken from more than 3420 production
devices.
1000
900
Number of Devices
800
700
600
500
400
300
200
100
0
4.094 4.0945 4.095 4.0955 4.096 4.0965 4.097 4.0975 4.098
REFIO Voltage (V)
D060
Figure 54. Internal Reference Accuracy Histogram at Room Temperature
The initial accuracy specification for the internal reference can be degraded if the die is exposed to any
mechanical or thermal stress. Heating the device when being soldered to a printed circuit board (PCB) and any
subsequent solder reflow is a primary cause for shifts in the VREF value. The main cause of thermal hysteresis is
a change in die stress and is therefore a function of the package, die-attach material, and molding compound, as
well as the layout of the device itself.
Number of Devices
In order to illustrate this effect, 30 devices were soldered using lead-free solder paste with the manufacturer
suggested reflow profile, as explained in the AN-2029 Handling & Process Recommendations application report.
The internal voltage reference output is measured before and after the reflow process and the typical shift in
value is shown in Figure 55. Although all tested units exhibit a positive shift in their output voltages, negative
shifts are also possible. The histogram in Figure 55 shows the typical shift for exposure to a single reflow profile.
Exposure to multiple reflows, which is common on PCBs with surface-mount components on both sides, causes
additional shifts in the output voltage. If the PCB is to be exposed to multiple reflows, solder the ADS868x in the
second pass to minimize device exposure to thermal stress.
7
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
-7
-6
-5
-4
-3
-2
-1
Error in REFIO Voltage (mV)
0
1
D070
Figure 55. Solder Heat Shift Distribution Histogram
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The internal reference is also temperature compensated to provide excellent temperature drift over an extended
industrial temperature range of –40°C to +125°C. Figure 56 and Figure 57 show the variation of the internal
reference voltage across temperature for different values of the AVDD supply voltage. The temperature drift of
the internal reference is also a function of the package type. Figure 58 and Figure 59 show histogram distribution
of the reference voltage drift for the TSSOP (PW) and WQFN (RUM) packages, respectively.
4.1
4.097
AVDD = 5.25 V
AVDD = 5 V
AVDD = 4.75 V
REFIO Voltage (V) - TSSOP
4.099
REFIO Voltage (V) - QFN
4.098
AVDD = 5.25 V
AVDD = 5 V
AVDD = 4.75 V
4.096
4.097
4.096
4.095
4.094
4.093
4.092
4.095
4.094
4.093
4.092
4.091
4.09
4.091
4.09
-40
-7
26
59
Free-Air Temperature (0C)
92
125
4.089
-40
-7
D061
Figure 56. REFIO Voltage Variation Across AVDD and
Temperature (PW Package)
26
59
Free-Air Temperature (0C)
92
125
D612
Figure 57. REFIO Voltage Variation Across AVDD and
Temperature (RUM Package)
8
14
6
10
Number of Devices
Number of Devices
12
8
6
4
4
2
2
0
0
3.1
3.5
3.9
4.3 4.7 5.1 5.5 5.9
REFIO Drift (ppm/ºC)
6.3
6.7
7.1
D062
AVDD = 5 V, number of devices = 30, ΔT = –40°C to +125°C
Figure 58. Internal Reference Temperature Drift Histogram
(PW Package)
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2.2
3
3.8 4.6 5.4 6.2 7 7.8 8.6 9.4 10.2 11
REFIO Drift (ppm/ºC)
D622
AVDD = 5 V, number of devices = 30, ΔT = –40°C to +125°C
Figure 59. Internal Reference Temperature Drift Histogram
(RUM Package)
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7.3.7.2 External Reference
For applications that require a better reference voltage or a common reference voltage for multiple devices, the
device provides a provision to use an external reference source along with an internal buffer to drive the ADC
reference pin. In order to select the external reference mode, the INTREF_DIS bit of the RANGE_SEL_REG
register must be programmed to logic 1. In this mode, an external 4.096-V reference must be applied at the
REFIO pin, which functions as an input. Any low-power, low-drift, or small-size external reference can be used in
this mode because the internal buffer is optimally designed to handle the dynamic loading on the REFCAP pin
that is internally connected to the ADC reference input. The output of the external reference must be
appropriately filtered to minimize the resulting effect of the reference noise on system performance. A typical
connection diagram for this mode is shown in Figure 60.
AVDD
4.096 VREF
AVDD
RANGE_SEL_REG[6] = 1
(INTREF_DIS)
OUT
REFIO
REF5040
(See the device datasheet for
a detailed pin configuration.)
CREF
REFCAP
1 PF
10 PF
REFGND
ADC
AGND
Figure 60. Device Connections for Using an External 4.096-V Reference
The output of the internal reference buffer appears at the REFCAP pin. A minimum capacitance of 10 µF must
be placed between the REFCAP and REFGND pins. Place another capacitor of 1 µF as close to the REFCAP
pin as possible for decoupling high-frequency signals. Do not use the internal buffer to drive external ac or dc
loads because of the limited current output capability of this buffer.
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The performance of the internal buffer output is very stable across the entire operating temperature range of
–40°C to +125°C. Figure 61 (for the PW package) and Figure 63 (for the RUM package) show the variation in
the REFCAP output across temperature for different values of the AVDD supply voltage. The typical specified
value of the reference buffer drift over temperature is 0.5 ppm/°C, as shown in Figure 62 (for the PW package)
and Figure 64 (for the RUM package), and the maximum specified temperature drift is equal to 2 ppm/°C.
24
4.1
AVDD = 5.25 V
AVDD = 5 V
AVDD = 4.75 V
4.098
20
Number of Devices
REFCAP Voltage (V) - TSSOP
4.099
4.097
4.096
4.095
4.094
4.093
4.092
16
12
8
4
4.091
4.09
-40
0
-7
26
59
Free-Air Temperature (0C)
92
0.2
125
0.4
0.6
D063
0.8 1.0 1.2 1.4 1.6
REFCAP Drift (ppm/ºC)
1.8
2.0
D064
AVDD = 5 V, number of devices = 30, ΔT = –40°C to +125°C
Figure 61. Reference Buffer Output (REFCAP) Variation vs
Supply and Temperature (PW Package)
Figure 62. Reference Buffer Temperature Drift Histogram
(PW Package)
4.098
18
AVDD = 5.25 V
AVDD = 5 V
AVDD = 4.75 V
16
14
4.096
Number of Devices
REFCAP Voltage (V) - QFN
4.097
4.095
4.094
4.093
4.092
10
8
6
4
4.091
4.09
-40
12
2
0
-7
26
59
Free-Air Temperature (0C)
92
125
0.2
0.4
0.6
D632
0.8 1.0 1.2 1.4 1.6
REFCAP Drift (ppm/ºC)
1.8
2.0
D642
WQFN (RUM) package, AVDD = 5 V, number of devices = 30,
ΔT = –40°C to +125°C
Figure 63. Reference Buffer Output (REFCAP) Variation vs
Supply and Temperature (RUM Package)
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Figure 64. Reference Buffer Temperature Drift Histogram
(RUM Package)
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7.3.8 ADC Transfer Function
The device supports a pseudo-differential input supporting both bipolar and unipolar input ranges. The output of
the device is in straight-binary format for both bipolar and unipolar input ranges.
The ideal transfer characteristic for all input ranges is shown in Figure 65. The full-scale range (FSR) for each
input signal is equal to the difference between the positive full-scale (PFS) input voltage and the negative fullscale (NFS) input voltage. The LSB size is equal to FSR / 216. For a reference voltage of VREF = 4.096 V, the
LSB values corresponding to the different input ranges are listed in Table 4.
ADC Output Code
FFFFh
8000h
0001h
1 LSB
NFS
FSR / 2
FSR ± 1 LSB
PFS
Analog Input
(AIN_P ± AIN_GND)
Figure 65. Device Transfer Function (Straight-Binary Format)
Table 4. ADC LSB Values for Different Input Ranges (VREF = 4.096 V)
INPUT RANGE
POSITIVE FULL-SCALE
(V)
NEGATIVE FULL-SCALE
(V)
FULL-SCALE RANGE
(V)
LSB
±3 × VREF
12.288
–12.288
24.576
375.0 µV
±2.5 × VREF
10.24
–10.24
20.48
312.5 µV
±1.5 × VREF
6.144
–6.144
12.288
185.5 µV
±1.25 × VREF
5.12
–5.12
10.24
156.25 µV
±0.625 × VREF
2.56
–2.56
5.12
78.125 µV
0 to 3 × VREF
12.288
0
12.288
185.5 µV
0 to 2.5 × VREF
10.24
0
10.24
156.25 µV
0 to 1.5 × VREF
6.144
0
6.144
93.75 µV
0 to 1.25 × VREF
5.12
0
5.12
78.125 µV
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7.3.9 Alarm Features
The device features an active-high alarm output on the ALARM/SDO-1/GPO pin, provided that the pin is
configured for alarm functionality. To enable the ALARM output on the multi-function pin, se the
SDO1_CONFIG[1:0] bits of the SDO_CTL_REG register to 01b (see the SDO_CTL_REG register).
The device features two types of alarm functions: an input alarm and an AVDD alarm.
• For the input alarm, the voltage at the input of the ADC is monitored and compared against userprogrammable high and low threshold values. The device sets an active high alarm output when the
corresponding digital value of the input signal goes beyond the high or low threshold set by the user; see the
Input Alarm section for a detailed explanation of the input alarm feature functionality.
• For the AVDD alarm, the analog supply voltage (AVDD) of the ADC is monitored and compared against the
specified typical low threshold (4.7 V) and high threshold (5.3 V) values of the AVDD supply. The device sets
an active high alarm output if the value of AVDD crosses the specified low (4.7 V) and high threshold (5.3 V)
values in either direction.
When the alarm functionality is turned on, both the input and AVDD alarm functions are enabled by default.
These alarm functions can be selectively disabled by programming the IN_AL_DIS and VDD_AL_DIS bits
(respectively) of the RST_PWRCTL_REG register.
Each alarm (input alarm or AVDD alarm) has two types of alarm flags associated with it: the active alarm flag
and the tripped alarm flag. All the alarm flags can be read in the ALARM_REG register. Both flags are set when
the associated alarm is triggered. However while the active alarm is cleared at the end of the current ADC
conversion (and set again if the alarm condition persists), the tripped flag is cleared only after ALARM_REG is
read.
The ALARM output flags are updated internally at the end of every conversion. These output flags can be read
during any data frame that the user initiates by bringing the CONVST/CS signal to a low level.
The ALARM output flags can be read in three different ways: either via the ALARM output pin, by reading the
internal ALARM registers, or by appending the ALARM flags to the data output.
• A high level on the ALARM pin indicates an over- or undervoltage condition on AVDD or on the analog input
channel of the device. This pin can be wired to interrupt the host input.
• The internal ALARM flag bits in the ALARM_REG register are updated at the end of conversion. After
receiving an ALARM interrupt on the output pin, the internal alarm flag registers can be read to obtain more
details on the conditions that generated the alarm.
• The alarm output flags can be selectively appended to the data output bit stream (see the
DATAOUT_CTL_REG register for configuration details).
Figure 66 shows a functional block diagram for the device alarm functionality.
AVDD High Alarm
AVDD Low Alarm
Input Alarm Threshold
+/Hysteresis
ALARM
Other Input Alarm
Input Active Alarm Flag
+
ADC Output
ADCST Rising
(End of Conversion)
S
Q
R
Q
Input Tripped Alarm Flag
Alarm Flag Read
ADC
SDO
Figure 66. Alarm Functionality Schematic
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7.3.9.1 Input Alarm
The device features a high and a low alarm on the analog input. The alarms corresponding to the input signal
have independently-programmable thresholds and a common hysteresis setting that can be controlled through
the ALARM_H_TH_REG and ALARM_L_TH_REG registers.
The device sets the input high alarm when the digital output exceeds the high alarm upper limit [high alarm
threshold (T)]. The alarm resets when the digital output is less than or equal to the high alarm lower limit [high
alarm (T) – H – 1). This function is shown in Figure 67.
Alarm Threshold
Alarm Threshold
Similarly, the input low alarm is triggered when the digital output falls below the low alarm lower limit [low alarm
threshold (T)]. The alarm resets when the digital output is greater than or equal to the low alarm higher limit [low
alarm (T) + H + 1]. This function is shown in Figure 68.
H_ALARM On
H_ALARM Off
(T ± H ± 1)
(T)
Figure 67. High ALARM Hysteresis
L_ALARM On
L_ALARM Off
(T)
ADC Output
(T + H + 1)
ADC Output
Figure 68. Low ALARM Hysteresis
7.3.9.2 AVDD Alarm
The device features a high and a low alarm on the analog voltage supply, AVDD. Unlike the input signal alarm,
the AVDD alarm has fixed trip points that are set by design. The device features an internal analog comparator
that constantly monitors the analog supply against the high and low threshold voltages. The high alarm is set if
AVDD exceeds a typical value of 5.3 V and the low alarm is asserted if AVDD drops below 4.7 V. This feature is
specially useful for debugging unusual device behavior caused by a glitch or brown-out condition on the analog
AVDD supply.
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7.4 Device Functional Modes
The device features the multiSPI digital interface for communication and data transfer between the device and
the host controller. The multiSPI interface supports many data transfer protocols that the host uses to exchange
data and commands with the device. The host can transfer data into the device using one of the standard SPI
modes. However, the device can be configured to output data in a number of ways to suit the application
demands of throughput and latency. The data output in these modes can be controlled either by the host or the
device, and the timing can either be system synchronous or source synchronous. For detailed explanation of the
supported data transfer protocols, see the Data Transfer Protocols section.
This section describes the main components of the digital interface module as well as supported configurations
and protocols. As shown in Figure 69, the interface module is comprised of shift registers (both input and output),
configuration registers, and a protocol unit. During any particular data frame, data are transferred both into and
out of the device. As a result, the host always perceives the device as a 32-bit input-output shift register, as
shown in Figure 69.
Interface Module
SDI
Output Register (Data and Flags from Device)
D31
D30
D1
D0
Unified Shift Register
LSB
LSB+1
MSB-1
MSB
B31
B30
B1
B0
SDO-0
ADC
RST
CONVST
Input Register
Digital
Control
Logic
Command Processor
CS
SCLK
ALARM/SDO-1/GPO
SCLK
Counter
Configuration Registers
RVS
Figure 69. Device Interface Module
The Pin Configuration and Functions section provides descriptions of the interface pins; the Data Transfer Frame
section details the functions of shift registers, the SCLK counter, and the command processor; the Data Transfer
Protocols section details supported protocols; and the Register Maps section explains the configuration registers
and bit settings.
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Device Functional Modes (continued)
7.4.1 Host-to-Device Connection Topologies
The multiSPI interface and device configuration registers offer great flexibility in the ways a host controller can
exchange data or commands with the device. This section describes how to select the hardware connection
topology to meet different system requirements.
7.4.1.1 Single Device: All multiSPI Options
Figure 70 shows the pin connection between a host controller and a stand-alone device to exercise all options
provided by the multiSPI interface.
DVDD
Isolation
(Optional)
RST
CONVST/CS
SCLK
Device
Host
Controller
SDI
SDO-0
ALARM/SDO-1/GPO
RVS
Figure 70. All multiSPI Protocols Pin Configuration
7.4.1.2 Single Device: Standard SPI Interface
Figure 71 shows the minimum pin interface for applications using a standard SPI protocol.
DVDD
Isolation
(Optional)
RST
(Optional)
CONVST/CS
SCLK
Device
Host
Controller
SDI
SDO-0
ALARM/SDO-1/GPO
RVS
(Optional)
Figure 71. Standard SPI Protocol Pin Configuration
The CONVST/CS, SCLK, SDI, and SDO-0 pins constitute a standard SPI port of the host controller. The RST pin
can be tied to DVDD. The RVS pin can be monitored for timing benefits. The ALARM/SDO-1/GPO pin may not
have any external connection.
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Device Functional Modes (continued)
7.4.1.3 Multiple Devices: Daisy-Chain Topology
Host Controller
Device 2
RVS
SDO-0
SDI
SCLK
RVS
SDO-0
SDI
SCLK
CONVST/CS
Device 1
CONVST/CS
RVS
SDO-0
SDI
SCLK
CONVST/CS
Isolation
(Optional)
SDI
SDO
SCLK
CONVST/CS
A typical connection diagram showing multiple devices in a daisy-chain topology is shown in Figure 72.
Device N
Figure 72. Daisy-Chain Connection Schematic
The CONVST/CS and SCLK inputs of all devices are connected together and controlled by a single CONVST/CS
and SCLK pin of the host controller, respectively. The SDI input pin of the first device in the chain (device 1) is
connected to the SDO-x pin of the host controller, the SDO-0 output pin of device 1 is connected to the SDI input
pin of device 2, and so forth. The SDO-0 output pin of the last device in the chain (device N) is connected to the
SDI pin of the host controller.
To operate multiple devices in a daisy-chain topology, the host controller must program the configuration
registers in each device with identical values. The devices must operate with a single SDO-0 output, using the
external clock with any of the legacy, SPI-compatible protocols for data read and data write operations. In the
SDO_CTL_REG register, bits 7-0 must be programmed to 00h.
All devices in the daisy-chain topology sample their analog input signals on the rising edge of the CONVST/CS
signal and the data transfer frame starts with a falling edge of the same signal. At the launch edge of the SCLK
signal, every device in the chain shifts out the MSB to the SDO-0 pin. On every SCLK capture edge, each device
in the chain shifts in data received on its SDI pin as the LSB bit of the unified shift register; see Figure 69.
Therefore, in a daisy-chain configuration, the host controller receives the data of device N, followed by the data
of device N-1, and so forth (in MSB-first fashion). On the rising edge of the CONVST/CS signal, each device
decodes the contents in its unified and takes appropriate action.
For N devices connected in a daisy-chain topology, an optimal data transfer frame must contain 32 × N SCLK
capture edges (see Figure 73). A shorter data transfer frame can result in an erroneous device configuration and
must be avoided. For a data transfer frame with > 32 × N SCLK capture edges, the host controller must
appropriately align the configuration data for each device before bringing CONVST/CS high.
The overall throughput of the system is proportionally reduced with the number of devices connected in a daisychain topology.
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Device Functional Modes (continued)
A typical timing diagram for three devices connected in a daisy-chain topology and using the SPI-00-S protocol is
shown in Figure 73.
CONVST/CS
SCLK
1
2
31
32
33
Configuration Data: Device 3
{SDO}HOST
{SDI}1
B95
B94
34
63
64
65
Configuration Data: Device 2
B65
B64
B63
B62
66
95
96
Configuration Data: Device 1
B33
B32
B31
B30
B1
B0
Configuration Data: Device 2
{SDO-0}1
{SDI}2
{D31}1
{D30}1
{D1}1
{D0}1
B95
B94
B65
B64
B63
B62
B33
B32
Configuration Data: Device 3
{SDO-0}2
{SDI}3
{D31}2
{D30}2
{D1}2
{D0}2
{D31}1
Output Data: Device 3
{SDO-0}3
{SDI}HOST
{D31}3
{D30}3
{D30}1
{D1}1
{D0}1
B95
Output Data: Device 2
{D1}3
{D0}3
{D31}2
{D30}2
B94
B65
B64
Output Data: Device 1
{D1}2
{D0}2
{D31}1
{D30}1
{D1}1
{D0}1
Figure 73. Three Devices in Daisy-Chain Mode Timing Diagram
7.4.2 Device Operational Modes
As shown in Figure 74, the device supports three functional states: RESET, ACQ, and CONV. The device state
is determined by the status of the CONVST/CS and RST control signals provided by the host controller.
Power-Up
ACQ
CS
T/
(R
isi
ng
)
ge
Ed
RS
n
sio
S
NV
CO
d
En
o
o
fC
RS
er
nv
T
(F
al
lin
g
T
Ed
(R
ge
isi
ng
Ed
ge
)
)
RST (Falling Edge)
CONV
RESET
Figure 74. Device Functional States
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Device Functional Modes (continued)
7.4.2.1 RESET State
The device features an active-low RST pin that is an asynchronous digital input. In order to enter a RESET state,
the RST pin must be pulled low and kept low for the twl_RST duration (as specified in the Timing Requirements:
Asynchronous Reset table).
The device features two different types of reset functions: an application reset or a power-on reset (POR). The
functionality of the RST pin is determined by the state of the RSTn_APP bit in the RST_PWRCTL_REG register.
• In order to configure the RST pin to issue an application reset, the RSTn_APP bit in the RST_PWRCTL_REG
register must be configured to 1b. In this RESET state, all configuration registers (see the Register Maps
section) are reset to their default values, the RVS pins remain low, and the SDO-x pins are tri-stated.
• The default configuration for the RST pin is to issue a power-on reset when pulled to a low level. The
RSTn_APP bit is set to 0b in this state. When a POR is issued, all internal circuitry of the device (including
the PGA, ADC driver, and voltage reference) are reset. When the device comes out of the POR state, the
tD_RST_POR time duration must be allowed for (see the Timing Requirements: Asynchronous Reset table) in
order for the internal circuitry to accurately settle.
In order to exit any of the RESET states, the RST pin must be pulled high with CONVST/CS and SCLK held low.
After a delay of tD_RST_POR or tD_RST_APP (see the Timing Requirements: Asynchronous Reset table), the device
enters ACQ state and the RVS pin goes high.
To operate the device in any of the other two states (ACQ or CONV), the RST pin must be held high. With the
RST pin held high, transitions on the CONVST/CS pin determine the functional state of the device. A typical
conversion cycle is illustrated in Figure 1.
7.4.2.2 ACQ State
In ACQ state, the device acquires the analog input signal. The device enters ACQ state on power-up, after any
asynchronous reset, or after the end of every conversion.
The falling edge of the RST falling edge takes the device from an ACQ state to a RESET state. A rising edge of
the CONVST/CS signal takes the device from ACQ state to a CONV state.
The device offers a low-power NAP mode to reduce power consumption in the ACQ state; see the NAP Mode
section for more details on NAP mode.
7.4.2.3 CONV State
The device moves from ACQ state to CONV state on the rising edge of the CONVST/CS signal. The conversion
process uses an internal clock and the device ignores any further transitions on the CONVST/CS signal until the
ongoing conversion is complete (that is, during the time interval of tconv).
At the end of conversion, the device enters ACQ state. The cycle time for the device is given by Equation 1:
t cycle-min tconv tacq-min
(1)
NOTE
The conversion time, tconv, can vary within the specified limits of tconv_min and tconv_max (as
specified in the Timing Requirements: Conversion Cycle table). After initiating a
conversion, the host controller must monitor for a low-to-high transition on the RVS pin or
wait for the tconv_max duration to elapse before initiating a new operation (data transfer or
conversion). If RVS is not monitored, substitute tconv in Equation 1 with tconv_max.
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7.5 Programming
The device features nine configuration registers (as described in the Register Maps section) and supports two
types of data transfer operations: data write (the host configures the device), and data read (the host reads data
from the device).
7.5.1 Data Transfer Frame
A data transfer frame between the device and the host controller begins at the falling edge of the CONVST/CS
pin and ends when the device starts conversion at the subsequent rising edge. The host controller can initiate a
data transfer frame by bringing the CONVST/CS signal low (as shown in Figure 75) after the end of the CONV
phase, as described in the CONV State section.
Frame F
CONVST/CS
RVS
As per output protocol selection.
td_RVS
SCLK
N SCLKs
SDI
Valid Command
SDO-x
Data Output or OSR Contents
SCLK Counter
SCLK Counter
0
N
Output Data Word
Input Shift Register (ISR)
D31
D0
B31
B0
D31
D0
B31
B0
Output Shift Register (OSR)
Command Processor
Figure 75. Data Transfer Frame
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Programming (continued)
For a typical data transfer frame F:
1. The host controller pulls CONVST/CS low to initiate a data transfer frame. On the falling edge of the
CONVST/CS signal:
– RVS goes low, indicating the beginning of the data transfer frame.
– The internal SCLK counter is reset to 0.
– The device takes control of the data bus. As illustrated in Figure 75, the contents of the output data word
are loaded into the 32-bit output shift register (OSR).
– The internal configuration register is reset to 0000h, corresponding to a NOP command.
2. During the frame, the host controller provides clocks on the SCLK pin:
– On each SCLK capture edge, the SCLK counter is incremented and the data bit received on the SDI pin
is shifted into the LSB of the input shift register.
– On each launch edge of the output clock (SCLK in this case), the MSB of the output shift register data is
shifted out on the selected SDO-x pins.
– The status of the RVS pin depends on the output protocol selection (see the Protocols for Reading From
the Device section).
3. The host controller pulls the CONVST/CS pin high to end the data transfer frame. On the rising edge of
CONVST/CS:
– The SDO-x pins go to tri-state.
– As illustrated in Figure 75, the contents of the input shift register are transferred to the command
processor for decoding and further action.
– RVS output goes low, indicating the beginning of conversion.
After pulling CONVST/CS high, the host controller must monitor for a low-to-high transition on the RVS pin or
wait for the tconv_max time (see the Timing Requirements: Conversion Cycle table) to elapse before initiating a new
data transfer frame.
At the end of the data transfer frame F:
• If the SCLK counter = 32, then the device treats the frame F as an optimal data transfer frame for any read or
write operation. At the end of an optimal data transfer frame, the command processor treats the 32-bit
contents of the input shift register as a valid command word.
• If the SCLK counter is < 32, then the device treats the frame F as a short data transfer frame.
– The data write operation to the device in invalid and the device treats this frame as an NOP command.
– The output data bits transferred during a short frame on the SDO-x pins are still valid data. The host
controller can use the short data transfer frame to read only the required number of MSB bits from the 32bit output shift register.
• If the SCLK counter is > 32, then the device treats the frame F as a long data transfer frame. At the end of a
long data transfer frame, the command processor treats the 32-bit contents of the input shift register as a
valid command word. There is no restriction on the maximum number of clocks that can be provided within
any data transfer frame F. However, when the host controller provides a long data transfer frame, the last 32
bits shifted into the device prior to the CONVST/CS rising edge must constitute the desired command.
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Programming (continued)
7.5.2 Input Command Word and Register Write Operation
Any data write operation to the device is always synchronous to the external clock provided on the SCLK pin.
The device allows either one byte or two bytes (equivalent to half a word) to be read or written during any device
programming operation. Table 5 lists the input commands supported by the device. The input commands
associated with reading or writing two bytes in a single operation are suffixed as HWORD.
For any HWORD command, the LSB of the 9-bit address is always ignored and considered as 0b. For example,
regardless whether address 04h or 05h is entered for any particular HWORD command, the device always
exercises the command on address 04h.
Table 5. List of Input Commands
OPCODE
B[31:0]
00000000_000000000_
00000000_00000000
11000_xx__
(1)
COMMAND
ACRONYM
NOP
COMMAND DESCRIPTION
No operation
•
•
•
•
Command used to clear any (or a group of) bits of a register.
Any bit marked 1 in the data field results in that particular bit of the specified register being
reset to 0, leaving the other bits unchanged.
Half-word command (that is, the command functions on 16 bits at a time).
LSB of the 9-bit address is always ignored and considered as 0b. (2)
Command used to perform a 16-bit read operation.
Half-word command (that is, the device outputs 16 bits of register data at a time).
LSB of the 9-bit address is always ignored and considered as 0b.
Upon receiving this command, the device sends out 16 bits of the register in the next frame.
CLEAR_HWORD
11001_xx__
00000000_00000000
READ_HWORD
•
•
•
•
01001_xx__
00000000_00000000
READ
•
Same as the READ_HWORD except that only eight bits of the register (byte read) are
returned in the next frame.
11010_00__
•
•
Half-word write command (two bytes of input data are written into the specified address).
LSB of the 9-bit address is always ignored and considered as 0b.
11010_01__
•
•
•
Half-word write command.
LSB of the 9-bit address is always ignored and considered as 0b.
With this command, only the MS byte of the 16-bit data word is written at the specified
register address. The LS byte is ignored.
•
•
•
Half-word write command.
LSB of the 9-bit address is always ignored and considered as 0b.
With this command, only the LS byte of the 16-bit data word is written at the specified register
address. The MS byte is ignored.
•
•
Command used to set any (or a group of) bits of a register.
Any bit marked 1 in the data field results in that particular bit of the specified register being set
to 1, leaving the other bits unchanged.
Half-word command (that is, the command functions on 16 bits at a time).
LSB of the 9-bit address is always ignored and considered as 0b.
WRITE
11010_10__
11011_xx__
All other input command
combinations
(1)
(2)
SET_HWORD
•
•
NOP
No operation
is realized by adding a 0 at the MSB location followed by an 8-bit register address as defined in Table 10. The for register 0x04h is 0x0-0000-0100b.
An HWORD command operates on a set of 16 bits in the register map that is usually identified as two registers of eight bits each. For
example, the command 11000_xx_ is treated the same as the command 11000_xx_ for bits 15:0 of the RST_PWRCTL_REG register.
All input commands (including the CLEAR_HWORD, WRITE, and SET_HWORD commands listed in Table 5)
used to configure the internal registers must be 32 bits long. If any of these commands are provided in a
particular data frame F, that command gets executed at the rising edge of the CONVST/CS signal.
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7.5.3 Output Data Word
The data read from the device can be synchronized to the external clock on the SCLK pin or to an internal clock
of the device by programming the configuration registers (see the Data Transfer Protocols section for details).
In any data transfer frame, the contents of the internal output shift register are shifted out on the SDO-x pins. The
output data for any frame (F+1) is determined by the command issued in frame F and the status of
DATA_VAL[2:0] bits:
• If DATA_VAL[2:0] bits in DATAOUT_CTL_REG register are set to 1xxb, then the output data word for frame
(F+1) contains fixed data pattern as described in the DATAOUT_CTL_REG register.
• If a valid READ command is issued in frame F, the output data word for frame (F+1) contains 8-bit register
data, followed by 0's.
• If a valid READ_HWORD command is issued in frame F, the output data word for frame (F+1) contains 16-bit
register data, followed by 0's.
• For all other combinations, the output data word for frame (F+1) contains the latest 16-bit conversion result.
Program the DATAOUT_CTL_REG register to append various data flags to the conversion result. The data
flags are appended as per following sequence:
1. DEVICE_ADDR[3:0] bits are appended if the DEVICE_ADDR_INCL bit is set to 1
2. AVDD ALARM FLAGS are appended if the VDD_ACTIVE_ALARM_INCL bit is set to 1
3. INPUT ALARM FLAGS are appended if the IN_ACTIVE_ALARM_INCL bit is set to 1
4. ADC INPUT RANGE FLAGS are appended if the RANGE_INCL bit is set to 1
5. PARITY bits are appended if the PAR_EN bit is set to 1
6. All the remaining bits in the 32-bit output data word are set to 0.
Table 6 shows the output data word with all data flags enabled.
Table 6. Output Data Word With All Data Flags Enabled
DEVICE_ADDR_INCL = 1b, VDD_ACTIVE_ALARM_INCL = 1b, IN_ACTIVE_ALARM_INCL = 1b, RANGE_INCL = 1b, and PAR_EN = 1b
D[31:16]
D[15:12]
D[11:10]
D[9:7]
D[7:4]
D[3:2]
D[1:0]
Conversion result
Device address
AVDD alarm flags
Input alarm flags
ADC input range
Parity bits
00b
Table 7 shows output data word with only some of the data flags enabled.
Table 7. Output Data Word With Only Some Data Flags Enabled
DEVICE_ADDR_INCL = 0b, VDD_ACTIVE_ALARM_INCL = 1b, IN_ACTIVE_ALARM_INCL = 0b, RANGE_INCL = 1b, and PAR_EN = 1b
44
D[31:16]
D[15:14]
D[13:10]
D[9:8]
D[7:0]
Conversion result
AVDD alarm flags
ADC input range
Parity bits
00000000b
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7.5.4 Data Transfer Protocols
The device features a multiSPI interface that allows the host controller to operate at slower SCLK speeds and
still achieve the required cycle time with a faster response time.
• For any data write operation, the host controller can use any of the four legacy, SPI-compatible protocols to
configure the device, as described in the Protocols for Configuring the Device section.
• For any data read operation from the device, the multiSPI interface module offers the following options:
– Legacy, SPI-compatible protocol with a single SDO-x (see the Legacy, SPI-Compatible (SYS-xy-S)
Protocols with a Single SDO-x section)
– Legacy, SPI-compatible protocol with dual SDO-x (see the Legacy, SPI-Compatible (SYS-xy-S) Protocols
with Dual SDO-x section)
– ADC master clock or source-synchronous (SRC) protocol for data transfer (see the Source-Synchronous
(SRC) Protocols section)
7.5.4.1 Protocols for Configuring the Device
As described in Table 8, the host controller can use any of the four legacy, SPI-compatible protocols (SPI-00-S,
SPI-01-S, SPI-10-S, or SPI-11-S) to write data into the device.
Table 8. SPI Protocols for Configuring the Device
PROTOCOL
SCLK POLARITY
(At CS Falling Edge)
SCLK PHASE
(Capture Edge)
SDI_CTL_REG
SDO_CTL_REG
DIAGRAM
SPI-00-S
SPI-01-S
Low
Rising
00h
00h
Figure 76
Low
Falling
01h
00h
Figure 76
SPI-10-S
High
Falling
02h
00h
Figure 77
SPI-11-S
High
Rising
03h
00h
Figure 77
On power-up or after coming out of any asynchronous reset, the device supports the SPI-00-S protocol for data
read and data write operations. To select a different SPI-compatible protocol, program the SDI_MODE[1:0] bits in
the SDI_CNTL_REG register. This first write operation must adhere to the SPI-00-S protocol. Any subsequent
data transfer frames must adhere to the newly-selected protocol. The SPI protocol selected by the configuration
of the SDI_MODE[1:0] is applicable to both read and write operations.
Figure 76 and Figure 77 detail the four protocols using an optimal data frame; see the Timing Requirements:
SPI-Compatible Serial Interface table for associated timing parameters.
NOTE
As explained in the Data Transfer Frame section, a valid write operation to the device
requires a minimum of 32 SCLKs to be provided within a data transfer frame.
CONVST/CS
CONVST/CS
RVS
RVS
CPOL = 0
CPOL = 0
SCLK
SCLK
CPOL = 1
SDI
CPOL = 1
B31
B30
B29
B1
B0
Figure 76. Standard SPI Timing Protocol
(CPHA = 0, 32 SCLK Cycles)
SDI
B31
B30
B2
B1
B0
Figure 77. Standard SPI Timing Protocol
(CPHA = 1, 32 SCLK Cycles)
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7.5.4.2 Protocols for Reading From the Device
The protocols for the data read operation can be broadly classified into three categories:
1. Legacy, SPI-compatible protocols with a single SDO-x
2. Legacy, SPI-compatible protocols with dual SDO-x
3. ADC master clock or source-synchronous (SRC) protocol for data transfer
7.5.4.2.1 Legacy, SPI-Compatible (SYS-xy-S) Protocols with a Single SDO-x
As shown in Table 9, the host controller can use any of the four legacy, SPI-compatible protocols (SPI-00-S, SPI01-S, SPI-10-S, or SPI-11-S) to read data from the device.
Table 9. SPI Protocols for Reading From the Device
PROTOCOL
SCLK POLARITY
(At CS Falling
Edge)
SPI-00-S
Low
SPI-01-S
Low
SPI-10-S
High
SPI-11-S
High
SCLK PHASE
(Capture Edge)
MSB BIT
LAUNCH EDGE
SDI_CTL_REG
SDO_CTL_REG
DIAGRAM
Rising
CS falling
00h
00h
Figure 78
Falling
1st SCLK rising
01h
00h
Figure 78
Falling
CS falling
02h
00h
Figure 79
Rising
1st SCLK falling
03h
00h
Figure 79
On power-up or after coming out of any asynchronous reset, the device supports the SPI-00-S protocol for data
read and data write operations. To select a different SPI-compatible protocol for both the data transfer
operations:
1. Program the SDI_MODE[1:0] bits in the SDI_CTL_REG register. This first write operation must adhere to the
SPI-00-S protocol. Any subsequent data transfer frames must adhere to the newly-selected protocol.
2. Set the SDO_MODE[1:0] bits = 00b in the SDO_CTL_REG register.
NOTE
The SPI transfer protocol selected by configuring the SDI_MODE[1:0] bits in the
SDI_CTL_REG register determines the data transfer protocol for both write and read
operations. Either data can be read from the device using the selected SPI protocol by
configuring the SDO_MODE[1:0] bits = 00b in the SDO_CTL_REG register, or one of the
SRC protocols can be selected for data read, as explained in the Source-Synchronous
(SRC) Protocols section.
When using any of the SPI-compatible protocols, the RVS output remains low throughout the data transfer frame;
see the Timing Requirements: SPI-Compatible Serial Interface table for associated timing parameters.
Figure 78 and Figure 79 explain the details of the four protocols. As explained in the Data Transfer Frame
section, the host controller can use a short data transfer frame to read only the required number of MSB bits
from the 32-bit output data word.
If the host controller uses a long data transfer frame with SDO_CNTL_REG[7:0] = 00h, then the device exhibits
daisy-chain operation (see the Multiple Devices: Daisy-Chain Topology section).
46
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CONVST/CS
CONVST/CS
RVS
RVS
CPOL = 0
CPOL = 0
SCLK
SCLK
CPOL = 1
CPOL = 1
M
SDO-0
M-1
L+1
M-2
L
SDO-0
Figure 78. Standard SPI Timing Protocol
(CPHA = 0, Single SDO-x)
0
M
M-1
L+1
L
Figure 79. Standard SPI Timing Protocol
(CPHA = 1, Single SDO-x)
7.5.4.2.2 Legacy, SPI-Compatible (SYS-xy-S) Protocols with Dual SDO-x
The device provides an option to increase the SDO-x bus width from one bit (default, single SDO-x) to two bits
(dual SDO-x) when operating with any of the data transfer protocols. In order to operate the device in dual SDO
mode, the SDO1_CONFIG[1:0] bits in the SDO_CTL_REG register must be set to 11b. In this mode, the
ALARM/SDO-1/GPO pin functions as SDO-1.
In dual SDO mode, two bits of data are launched on the two SDO-x pins (SDO-0 and SDO-1) on every SCLK
launch edge, as shown in Figure 80 and Figure 81.
CONVST/CS
CONVST/CS
RVS
RVS
CPOL = 0
CPOL = 0
SCLK
SCLK
CPOL = 1
SDO-0
SDO-1
CPOL = 1
M
M-1
M-2
M-3
L+3
M-4
L+2
M-5
L+1
SDO-0
0
M
M-2
L+3
L+1
0
M-1
M-1
L+2
L
L
SDO-1
Figure 80. Standard SPI Timing Protocol
(CPHA = 0, Dual SDO-x)
Figure 81. Standard SPI Timing Protocol
(CPHA = 1, Dual SDO-x)
NOTE
For any particular SPI protocol, the device follows the same timing specifications for single
and dual SDO modes. The only difference is that the device requires half as many SCLK
cycles to output the same number of bits when in single SDO mode, thus reducing the
minimum required SCLK frequency for a certain sampling rate of the ADC.
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7.5.4.2.3 Source-Synchronous (SRC) Protocols
The multiSPI interface supports an ADC master clock or source-synchronous mode of data transfer between the
device and host controller. In this mode, the device provides an output clock that is synchronous with the output
data. Furthermore, the host controller can also select the output clock source and data bus width options in this
mode of operation. In all SRC modes of operation, the RVS pin provides the output clock, synchronous to the
device data output.
The SRC protocol allows the clock source (internal or external) and the width of the output bus to be configured,
similar to the SPI protocols.
7.5.4.2.3.1 Output Clock Source Options
The device allows the output clock on the RVS pin to be synchronous to either the external clock provided on the
SCLK pin or to the internal clock of the device. This selection is done by configuring the SSYNC_CLK bit, as
explained in the SDO_CTL_REG register. The timing diagram and specifications for operating the device with an
SRC protocol in external CLK mode are provided in Figure 7 and the Timing Requirements: Source-Synchronous
Serial Interface (External Clock) table. The timing diagram and specifications for operating the device with an
SRC protocol in internal CLK mode are provided in Figure 8 and the Timing Requirements: Source-Synchronous
Serial Interface (Internal Clock) table.
7.5.4.2.3.2 Output Bus Width Options
The device provides an option to increase the SDO-x bus width from one bit (default, single SDO-x) to two bits
(dual SDO-x) when operating with any of the SRC protocols. In order to operate the device in dual SDO mode,
the SDO1_CONFIG[1:0] bits in the SDO_CTL_REG register must be set to 11b. In this mode, the ALARM/SDO1/GPO pin functions as SDO-1.
NOTE
For any particular SRC protocol, the device follows the same timing specifications for
single and dual SDO modes. The only difference is that the device requires half as many
clock cycles to output the same number of bits when in single SDO mode, thus reducing
the minimum required clock frequency for a certain sampling rate of the ADC.
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7.6 Register Maps
7.6.1 Device Configuration and Register Maps
The device features nine configuration registers, mapped as described in Table 10. Each configuration registers
is comprised of four registers, each containing a data byte.
Table 10. Configuration Registers Mapping
ADDRESS
REGISTER NAME
00h
DEVICE_ID_REG
REGISTER FUNCTION
04h
RST_PWRCTL_REG
08h
SDI_CTL_REG
SDI data input control register
0Ch
SDO_CTL_REG
SDO-x data input control register
10h
DATAOUT_CTL_REG
14h
RANGE_SEL_REG
Device ID register
Reset and power control register
Output data control register
Input range selection control register
20h
ALARM_REG
24h
ALARM_H_TH_REG
ALARM output register
ALARM high threshold and hysteresis register
28h
ALARM_L_TH_REG
ALARM low threshold register
7.6.1.1 DEVICE_ID_REG Register (address = 00h)
This register contains the unique identification numbers associated to a device that is used in a daisy-chain
configuration involving multiple devices.
Figure 82. DEVICE_ID_REG Register
31
30
29
15
14
13
28
27
Reserved
R-00h
12
11
26
25
24
23
10
9
8
7
Reserved
R-0000h
22
21
Reserved
R-0000b
6
5
20
19
4
3
18
17
16
DEVICE_ADDR[3:0]
R/W-0000b
2
1
0
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 00h
Address for bits 15-8 = 01h
Address for bits 23-16 = 02h
Address for bits 31-24 = 03h
Table 11. DEVICE_ID_REG Register Field Descriptions
Bit
(1)
Field
Type
Reset
Description
31-24
Reserved
R
00h
Reserved. Reads return 00h.
23-20
Reserved
R
0000b
Reserved. Reads return 0000b.
19-16
DEVICE_ADDR[3:0] (1)
R
0000b
These bits can be used to identify up to 16 different devices in a
system.
15-0
Reserved
R
0000h
Reserved. Reads return 0000h.
These bits are useful in daisy-chain mode.
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7.6.1.2 RST_PWRCTL_REG Register (address = 04h)
This register controls the reset and power-down features offered by the converter.
Any write operation to the RST_PWRCTL_REG register must be preceded by a write operation with the register
address set to 05h and the register data set to 69h.
Figure 83. RST_PWRCTL_REG Register
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
4
3
2
1
0
IN_AL_DIS
Reserved
RSTn_APP
NAP_EN
PWRDN
R/W-0b
R-0b
R/W-b
R/W-b
R/W-0b
Reserved
R-0000h
15
14
13
12
11
10
9
8
7
6
WKEY[7:0]
Reserved
R/W-00h
R-00b
5
VDD_AL_
DIS
R/W-0b
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 04h
Address for bits 15-8 = 05h
Address for bits 23-16 = 06h
Address for bits 31-24 = 07h
Table 12. RST_PWRCTL_REG Register Field Descriptions
Field
Type
Reset
Description
31-16
Bit
Reserved
R
0000h
Reserved. Reads return 0000h.
15-8
WKEY[7:0]
R/W
00h
This value functions as a protection key to enable writes to bits 5-0.
Bits are written only if WKEY is set to 69h first.
7-6
(1)
(2)
50
Reserved
R
00b
Reserved. Reads return 00b
5
VDD_AL_DIS
R/W
0b
0b = VDD alarm is enabled
1b = VDD alarm is disabled
4
IN_AL_DIS
R/W
0b
0b = Input alarm is enabled
1b = Input alarm is disabled
3
Reserved
R
0b
Reserved. Reads return 0h.
R/W
0b
0b = RST pin functions as a POR class reset (causes full device
initialization)
1b = RST pin functions as an application reset (only user-programmed
modes are cleared)
(1)
2
RSTn_APP
1
NAP_EN (2)
R/W
0b
0b = Disables the NAP mode of the converter
1b = Enables the converter to enter NAP mode if CONVST/CS is held
high after the current conversion completes
0
PWRDN (2)
R/W
0b
0b = Puts the converter into active mode
1b = Puts the converter into power-down mode
Setting this bit forces the RST pin to function as an application reset until the next power cycle.
See the Electrical Characteristics table for details on the latency encountered when entering and exiting the associated low-power
mode.
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7.6.1.3 SDI_CTL_REG Register (address = 08h)
This register configures the protocol used for writing data to the device.
Figure 84. SDI_CTL_REG Register
31
30
29
28
27
26
25
15
14
13
12
11
10
9
24
23
Reserved
R-0000h
8
7
22
21
20
19
18
17
6
5
4
3
2
1
0
SDI_MODE
[1:0]
R/W-b
Reserved
Reserved
R-00h
R-000000b
16
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 08h
Address for bits 15-8 = 09h
Address for bits 23-16 = 0Ah
Address for bits 31-24 = 0Bh
Table 13. SDI_CTL_REG Register Field Descriptions
Field
Type
Reset
Description
31-16
Bit
Reserved
R
0000h
Reserved. Reads return 0000h.
15-8
Reserved
R
00h
Reserved. Reads return 00h.
7-2
Reserved
R
000000b
Reserved. Reads return 000000b.
1-0
SDI_MODE[1:0]
R/W
00b
These bits select the protocol for reading from or writing to the device.
00b = Standard SPI with CPOL = 0 and CPHASE = 0
01b = Standard SPI with CPOL = 0 and CPHASE = 1
10b = Standard SPI with CPOL = 1 and CPHASE = 0
11b = Standard SPI with CPOL = 1 and CPHASE = 1
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7.6.1.4 SDO_CTL_REG Register (address = 0Ch)
This register controls the data protocol used to transmit data out from the SDO-x pins of the device.
Figure 85. SDO_CTL_REG Register
31
30
29
28
27
26
25
15
14
13
12
11
10
9
8
SDO1_
CONFIG
[1:0]
R/W-00b
Reserved
GPO_VAL
Reserved
R-000b
R/W-0b
R-00b
24
23
Reserved
R-0000h
7
22
21
20
19
18
17
16
6
5
4
3
2
1
0
Reserved
SSYNC_CLK
Reserved
SDO_
MODE[1:0]
R-0b
R/W-b
R-0h
R/W-b
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 0Ch
Address for bits 15-8 = 0Dh
Address for bits 23-16 = 0Eh
Address for bits 31-24 = 0Fh
Table 14. SDO_CTL_REG Register Field Descriptions
Bit
Field
Type
Reset
Description
31-16
Reserved
R
0000h
Reserved. Reads return 0h.
15-13
Reserved
R
000b
Reserved. Reads return 000b.
12
GPO_VAL
R/W
0b
1-bit value for the output on the GPO pin.
11-10
Reserved
R
00b
Reserved. Reads return 00b.
SDO1_CONFIG[1:0]
R/W
00b
Two bits are used to configure ALARM/SDO-1/GPO:
00b = SDO-1 is always tri-stated; 1-bit SDO mode
01b = SDO-1 functions as ALARM; 1-bit SDO mode
10b = SDO-1 functions as GPO; 1-bit SDO mode
11b = SDO-1 combined with SDO-0 offers a 2-bit SDO mode
Reserved
R
0b
Reserved. Reads return 0b.
R/W
0b
This bit controls the source of the clock selected for source-synchronous
transmission.
0b = External SCLK (no division)
1b = Internal clock (no division)
R
0000b
Reserved. Reads return 0000b.
00b
These bits control the data output modes of the device.
0xb = SDO mode follows the same SPI protocol as that used for SDI; see
the SDI_CTL_REG register
10b = Invalid configuration
11b = SDO mode follows the ADC master clock or source-synchronous
protocol
9-8
7
6
5-2
1-0
(1)
52
SSYNC_CLK
(1)
Reserved
SDO_MODE[1:0]
R/W
This bit takes effect only in the ADC master clock or source-synchronous mode of operation.
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7.6.1.5 DATAOUT_CTL_REG Register (address = 10h)
This register controls the data output by the device.
Figure 86. DATAOUT_CTL_REG Register
31
30
15
14
DEVICE_
Reserved ADDR_
INCL
R-0b
R/W-0b
29
28
27
26
13
12
11
10
25
Reserved
R-0000h
9
24
23
22
21
20
19
18
17
16
8
7
6
5
4
3
2
1
0
VDD_ACTIVE_
ALARM_INCL[1:0]
IN_ACTIVE_
ALARM_INCL[1:0]
Reserved
RANGE_
INCL
Reserved
PAR_EN
R/W-0b
R/W-0b
R-0b
R/W-0b
R-0000b
R/Wb
DATA_VAL
[2:0]
R/W-000b
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 10h
Address for bits 15-8 = 11h
Address for bits 23-16 = 12h
Address for bits 31-24 = 13h
Table 15. DATAOUT_CTL_REG Register Field Descriptions
Bit
Field
Type
Reset
Description
31-16
Reserved
R
0000h
Reserved. Reads return 0000h.
15
Reserved
R
0b
Reserved. Reads return 0b.
14
DEVICE_ADDR_INCL
R/W
0b
Control to include the 4-bit DEVICE_ADDR register value in the
SDO-x output bit stream.
0b = Do not include the register value
1b = Include the register value
13-12
VDD_ACTIVE_ALARM_INCL[1:0]
R/W
00b
Control to include the active VDD ALARM flags in the SDO-x output
bit stream.
00b = Do not include
01b = Include ACTIVE_VDD_H_FLAG
10b = Include ACTIVE_VDD_L_FLAG
11b = Include both flags
11-10
IN_ACTIVE_ALARM_INCL[1:0]
R/W
00b
Control to include the active input ALARM flags in the SDO-x output
bit stream.
00b = Do not include
01b = Include ACTIVE_IN_H_FLAG
10b = Include ACTIVE_IN_L_FLAG
11b = Include both flags
9
Reserved
R
0b
Reserved. Reads return 0h.
8
RANGE_INCL
R/W
0b
Control to include the 4-bit input range setting in the SDO-x output
bit stream.
0b = Do not include the range configuration register value
1b = Include the range configuration register value
Reserved
R
0000b
Reserved. Reads return 0000b.
0b = Output data does not contain parity information
1b = Two parity bits (ADC output and output data frame) are
appended to the LSBs of the output data
The ADC output parity bit reflects an even parity for the ADC output
bits only.
The output data frame parity bit reflects an even parity signature for
the entire output data frame, including the ADC output bits and any
internal flags or register settings.
7-4
3
2-0
(1)
PAR_EN (1)
R/W
0b
DATA_VAL[2:0]
R/W
000b
These bits control the data value output by the converter.
0xxb = Value output is the conversion data
100b = Value output is all 0's
101b = Value output is all 1's
110b = Value output is alternating 0's and 1's
111b = Value output is alternating 00's and 11's
Setting this bit increases the length of the output data by two bits.
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7.6.1.6 RANGE_SEL_REG Register (address = 14h)
This register controls the configuration of the internal reference and input voltage ranges for the converter.
Figure 87. RANGE_SEL_REG Register
31
30
29
28
27
26
25
24
15
14
13
12
11
10
9
8
Reserved
R-00h
23
22
Reserved
R-0000h
7
6
Reser
INTREF_ DIS
ved
R-0b
R/W-0b
21
20
19
18
17
16
5
4
3
2
1
0
Reserved
RANGE_SEL[3:0]
R-00b
R/W-b
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 14h
Address for bits 15-8 = 15h
Address for bits 23-16 = 16h
Address for bits 31-24 = 17h
Table 16. RANGE_SEL_REG Register Field Descriptions
Bit
54
Field
Type
Reset
Description
31-16
Reserved
R
0000h
Reserved. Reads return 0000h.
15-8
Reserved
R
00h
Reserved. Reads return 00h.
7
Reserved
R
0b
Reserved. Reads return 0b.
6
INTREF_DIS
R/W
0b
Control to disable the ADC internal reference.
0b = Internal reference is enabled
1b = Internal reference is disabled
5-4
Reserved
R
00b
Reserved. Reads return 00b.
3-0
RANGE_SEL[3:0]
R/W
0000b
These bits comprise the 4-bit register that selects the nine input ranges of
the ADC.
0000b = ±3 × VREF
0001b = ±2.5 × VREF
0010b = ±1.5 × VREF
0011b = ±1.25 × VREF
0100b = ±0.625 × VREF
1000b = 3 × VREF
1001b = 2.5 × VREF
1010b = 1.5 × VREF
1011b = 1.25 × VREF
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7.6.1.7 ALARM_REG Register (address = 20h)
This register contains the output alarm flags (active and tripped) for the input and AVDD alarm.
Figure 88. ALARM_REG Register
31
30
29
28
27
26
25
24
23
Reserved
R-0000h
15
14
13 12
11
10
9
8
7
ACTIVE_ ACTIVE_
ACTIVE_ ACTIVE_
TRP_
VDD_L_ VDD_H_ Reserved
IN_L_
IN_H_ Reserved VDD_L_
FLAG
FLAG
FLAG
FLAG
FLAG
R-0b
R-0b
R-00b
R-0b
R-0b
R-00b
R-0b
22
21
20
19
18
17
16
6
TRP_
VDD_H_
FLAG
R-0b
5
4
3
2
1
0
TRP_IN_ TRP_IN_
L_FLAG H_FLAG
R-0b
Reserved
OVW_
ALARM
R-000b
R-0b
R-0b
LEGEND: R = Read only; -n = value after reset; -0, -1 = Condition after application reset; -, - = Condition after power-on reset
Address for bits 7-0 = 20h
Address for bits 15-8 = 21h
Address for bits 23-16 = 22h
Address for bits 31-24 = 23h
Table 17. ALARM_REG Register Field Descriptions
Bit
31-16
Field
Type
Reset
Description
Reserved
R
0000h
Reserved. Reads return 0000h.
15
ACTIVE_VDD_L_FLAG
R
0b
Active ALARM output flag for low AVDD voltage.
0b = No ALARM condition
1b = ALARM condition exists
14
ACTIVE_VDD_H_FLAG
R
0b
Active ALARM output flag for high AVDD voltage.
0b = No ALARM condition
1b = ALARM condition exists
Reserved
R
00b
Reserved. Reads return 00b.
11
ACTIVE_IN_L_FLAG
R
0b
Active ALARM output flag for high input voltage.
0b = No ALARM condition
1b = ALARM condition exists
10
ACTIVE_IN_H_FLAG
R
0b
Active ALARM output flag for low input voltage.
0b = No ALARM condition
1b = ALARM condition exists
9-8
Reserved
R
00b
Reserved. Reads return 00b.
7
TRP_VDD_L_FLAG
R
0b
Tripped ALARM output flag for low AVDD voltage.
0b = No ALARM condition
1b = ALARM condition exists
6
TRP_VDD_H_FLAG
R
0b
Tripped ALARM output flag for high AVDD voltage.
0b = No ALARM condition
1b = ALARM condition exists
5
TRP_IN_L_FLAG
R
0b
Tripped ALARM output flag for high input voltage.
0b = No ALARM condition
1b = ALARM condition exists
4
TRP_IN_H_FLAG
R
0b
Tripped ALARM output flag for low input voltage.
0b = No ALARM condition
1b = ALARM condition exists
Reserved
R
000b
Reserved. Reads return 000b.
OVW_ALARM
R
0b
Logical OR outputs all tripped ALARM flags.
0b = No ALARM condition
1b = ALARM condition exists
13-12
3-1
0
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7.6.1.8 ALARM_H_TH_REG Register (address = 24h)
This register controls the hysteresis and high threshold for the input alarm.
Figure 89. ALARM_H_TH_REG Register
31
30
15
14
29
28
27
26
INP_ALRM_HYST[7:0]
R/W-00h
13
12
11
10
25
9
24
23
22
8
7
6
INP_ALRM_HIGH_TH[15:0]
R/W-FFFFh
21
20
19
Reserved
R-00h
4
3
5
18
17
16
2
1
0
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 24h
Address for bits 15-8 = 25h
Address for bits 23-16 = 26h
Address for bits 31-24 = 27h
Table 18. ALARM_H_TH_REG Register Field Descriptions
Field
Type
Reset
Description
31-24
Bit
INP_ALRM_HYST[7:0]
R/W
00h
INP_ALRM_HYST[7:2]: 6-bit hysteresis value for the input ALARM.
INP_ALRM_HYST[1:0] must be set to 00b.
23-16
Reserved
R
00h
Reserved. Reads return 00h.
15-0
INP_ALRM_HIGH_TH[15:0]
R/W
FFFFh
Threshold for comparison is INP_ALRM_HIGH_TH[15:0].
7.6.1.9 ALARM_L_TH_REG Register (address = 28h)
This register controls the low threshold for the input alarm.
Figure 90. ALARM_L_TH_REG Register
31
30
29
28
27
26
15
14
13
12
11
10
25
24
23
22
Reserved
R-0000h
9
8
7
6
INP_ALRM_LOW_TH[15:0]
R/W-0000h
21
20
19
18
17
16
5
4
3
2
1
0
LEGEND: R/W = Read/Write; R = Read only; -n = value after reset; -0, -1 = Condition after application reset;
LEGEND: -, - = Condition after power-on reset
Address for bits 7-0 = 28h
Address for bits 15-8 = 29h
Address for bits 23-16 = 2Ah
Address for bits 31-24 = 2Bh
Table 19. ALARM_L_TH_REG Register Field Descriptions
Bit
56
Field
Type
Reset
Description
32:16
Reserved
R
0000h
Reserved. Reads return 0000h.
15-0
INP_ALRM_LOW_TH[15:0]
R/W
0000h
Threshold for comparison is INP_ALRM_LOW_TH[15:0].
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The ADS868x is a fully-integrated data acquisition (DAQ) system based on a 16-bit successive approximation
(SAR) analog-to-digital converter (ADC). The device includes an integrated analog front-end (AFE) circuit to drive
the inputs of the ADC and an integrated precision reference with a buffer. As such, this device does not require
any additional external circuits for driving the reference or analog input pins of the ADC.
8.2 Typical Application
Isolation
Barrier
Local Power Supply
System Power Supply
x
x
Isolated
DC-DC
Converter
IGND
x
GND
x
VINP
Input Signal
SAR ADC
Digital Isolator
Digital Host
x
VINM
IGND
x
GND
GND
IGND
IGND
x
GND
NOTE: The potential difference between IGND and GND can be as high as the barrier breakdown voltage (often thousands
of volts).
x
Figure 91. 16-Bit Isolated DAQ System for High Common-Mode Rejection
8.2.1 Design Requirements
Design a 16-bit DAQ system for processing input signals up to ±12 V superimposed on large dc or ac commonmode offsets relative to the ground potential of the system main power supply. The specific performance
requirements are as follows:
• Input signal: ±12-V amplitude signal of a 1-kHz frequency superimposed on a ±75-V common-mode with
frequency between dc and 15 kHz
• CMRR > 100 dB over stipulated common-mode frequency range
• SNR > 90 dB
• THD < –104 dB
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Typical Application (continued)
8.2.2 Detailed Design Procedure
The design uses galvanic isolation between the DAQ system inputs and main power supply to achieve extremely
high CMRR, as indicated by Figure 91. The system not only tolerates large common-mode voltages beyond its
absolute maximum ratings but also delivers excellent performance largely independent of common-mode
amplitude and frequency (within the specified operating limits). The relevant performance characteristics are
illustrated in Figure 99, Figure 93, and Figure 94.
The system performance requirements by itself can be easily satisfied by using the ADS868x. This device
simplifies system design because the ADS868x eliminates the need for designing a discrete high-performance
signal chain needed with most other SAR ADCs. In addition, the use of galvanic isolation has the following
system design implications:
• A local floating supply is needed to power the ADS868x because the device cannot load the system main
power supply
• A digital isolator is required to facilitate data transfer between the isolated ADS868x serial interface and the
digital host controller
The floating power supply can be realized as an isolated transformer-based, push-pull converter followed by a
rectifier and low-dropout (LDO) regulator to largely eliminate the ADC power-supply ripple by taking advantage of
the high PSRR provided by most LDOs. A schematic of this design is shown in Figure 92.
xx
xx
xx
xx
Isolation
Barrier
S2
Main Power Supply
(> 4.3 V)
GND
Transformer
Driver
GND
VIN
IGND
S1
GND
Full-Wave Rectifier and
Smoothing Capacitor
LDO
VIN
D1
CS
M
D2
VOUT
Isolated 5 V
GND
IGND
IGND
LDO
IGND
VIN
VOUT
Isolated 3.3 V
1:NS (>1)
GND
IGND
Isolated Switching Power Supply
Figure 92. Isolated Power-Supply Design
Recommended components for the circuit shown in Figure 92 are given below:
• The SN6501 transformer driver is selected for its low input voltage requirement, small form-factor, and the
flexibility offered for easily adjusting the system isolation voltage rating by substituting the transformer
• A miniature printed circuit board (PCB)-mount, center-tapped transformer with a gain > 1 maintains line
regulation at the LDO outputs
• Schottky rectifiers for minimal forward voltage drop
• Smoothing capacitor for sufficiently low ripple at the LDO input
• The TPS7A4901 LDOs for an ultra-low noise contribution relative to the ADS868x and high PSRR over a
wide frequency range to attenuate output ripple to levels below the LDO output noise level
With regard to the digital isolator, the ISO7640FM is recommended for the following reasons:
• Supports > a 50-MHz SCLK and the required logic levels for operating the ADS868x at the full throughput
• Quad-channel device that facilitates excellent delay-matching between critical interface signals for reliable
operation at high speed
58
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Typical Application (continued)
0
0
-40
-40
Amplitude (dB)
Amplitude (dB)
8.2.3 Application Curves
-80
-120
-160
-200
0
100
200
300
Frequency (kHz)
400
500
0
100
D708
fSAMPLE = 1 MSPS, VIN = ±12 V, fIN = 1 kHz, VCM = 50 VDC,
SINAD = 91.6 dB, THD = –109 dB
Figure 93. FFT Plot With a DC Common-Mode at 1 MSPS
0
-40
-40
-120
400
500
D709
Figure 94. FFT Plot With an AC Common-Mode at 1 MSPS
0
-80
200
300
Frequency (kHz)
fSAMPLE = 1 Msps, VIN = ±12 V, fIN = 1 kHz,
VCM = 155 VPP, SINAD = 91.6 dB, THD = –109 dB
Amplitude (dB)
Amplitude (dB)
-120
-160
-200
-80
-120
-160
-160
-200
-200
0
50
100
150
Frequency (kHz)
200
0
250
50
D710
fSAMPLE = 500 kSPS, VIN = ±12 V, fIN = 1 kHz, VCM = 50 VDC,
SINAD = 92 dB, THD = –109 dB
-40
-40
Amplitude (dB)
0
-120
-160
200
250
D711
Figure 96. FFT Plot With an AC Common-Mode at
500 kSPS
0
-80
100
150
Frequency (kHz)
fSAMPLE = 500 kSPS, VIN = ±12 V, fIN = 1 kHz,
VCM = 155 VPP, SINAD = 91.5 dB, THD = –109 dB
Figure 95. FFT Plot With a DC Common-Mode at 500 kSPS
Amplitude (dB)
-80
-80
-120
-160
-200
-200
0
10
20
30
Frequency (kHz)
40
50
0
10
D712
fSAMPLE = 100 kSPS, VIN = ±12 V, fIN = 1 kHz, VCM = 50 VDC,
SINAD = 91.8 dB, THD = –110 dB
Figure 97. FFT Plot With a DC Common-Mode at 100 kSPS
20
30
Frequency (kHz)
40
50
D713
fSAMPLE = 100 kSPS, VIN = ±12 V, fIN = 1 kHz,
VCM = 155 VPP, SINAD = 91.2 dB, THD = –110 dB
Figure 98. FFT Plot With an AC Common-Mode at
100 kSPS
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Typical Application (continued)
160
150
CMRR (dB)
140
130
120
110
100
90
80
20
100
1000
1000020000
Common-Mode Signal Frequency (Hz)
D200
Figure 99. Common-Mode Rejection Ratio vs Frequency
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9 Power Supply Recommendations
The device uses two separate power supplies: AVDD and DVDD. The internal circuits of the device operate on
AVDD and DVDD is used for the digital interface. AVDD and DVDD can be independently set to any value within
the permissible range.
9.1 Power Supply Decoupling
The AVDD supply pins must be decoupled with AGND by using a minimum 10-µF and 1-µF capacitor on each
supply. Place the 1-µF capacitor as close to the supply pins as possible. Place a minimum 10-µF decoupling
capacitor very close to the DVDD supply to provide the high-frequency digital switching current. The effect of
using the decoupling capacitor is illustrated in the difference between the power-supply rejection ratio (PSRR)
performance of the device. Figure 100 shows the PSRR of the device without using a decoupling capacitor. The
PSRR improves when the decoupling capacitors are used, as shown in Figure 101.
-50
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
-40
-50
Power Supply Rejection Ratio
Power Supply Rejection Ratio
-30
-60
-70
-80
100
1k
10k
Input Frequency (Hz)
100k
± 12.288 V
± 10.24 V
± 6.144 V
± 5.12 V
± 2.56 V
0-12.288 V
0-10.24 V
0-6.144 V
0-5.12 V
-60
-70
-80
-90
-100
100
D057
Figure 100. PSRR Without a Decoupling Capacitor
1k
10k
Input Frequency (Hz)
100k
D056
Figure 101. PSRR With a Decoupling Capacitor
9.2 Power Saving
In normal mode of operation, the device does not power down between conversions, and therefore achieves high
throughput.However, the device offers two programmable low-power modes: NAP and power-down (PD) to
reduce power consumption when the device is operated at lower throughput rates.
9.2.1 NAP Mode
In NAP mode, the internal blocks of the device are placed into a low-power mode to reduce the overall power
consumption of the device in the ACQ state.
To enable NAP mode:
• Write 69h to register address 05h to unlock the RST_PWRCTL_REG register.
• The NAP_EN bit in the RST_PWRCTL_REG register must be set to 1b. The CONVST/CS pin must be kept
high at the end of the conversion process. The device then enters NAP mode at the end of conversion and
remains in NAP mode as long as the CONVST/CS pin is held high.
A falling edge on the CONVST/CS brings the device out of NAP mode; however, the host controller can initiate a
new conversion (CONVST/CS rising edge) only after the tNAP_WKUP time has elapsed (see the Timing
Requirements: Asynchronous Reset table).
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Power Saving (continued)
9.2.2 Power-Down (PD) Mode
The device also features a deep power-down mode (PD) to reduce the power consumption at very low
throughput rates.
The following steps must be taken to enter PD mode:
1. Write 69h to register address 05h to unlock the RST_PWRCTL_REG register.
2. Set the PWRDN bit in the RST_PWRCTL_REG register to 1b. The device enters PD mode on the rising
edge of the CONVST/CS signal.
In PD mode, all analog blocks within the device are powered down; however, the interface remains active and
the register contents are also retained. The RVS pin is high, indicating that the device is ready to receive the
next command.
In order to exit PD mode:
1. Clear the PWRDN bit in the RST_PWRCTL_REG register to 0b.
2. The RVS pin goes high, indicating that the device has started coming out of PD mode. However, the host
controller must wait for the tPWRUP time (see the Timing Requirements: Asynchronous Reset table) to elapse
before initiating a new conversion.
10 Layout
10.1 Layout Guidelines
Figure 102 illustrates a PCB layout example for the ADS868x.
• Partition the PCB into analog and digital sections. Care must be taken to ensure that the analog signals are
kept away from the digital lines. This layout helps keep the analog input and reference input signals away
from the digital noise. In this layout example, the analog input and reference signals are routed on the lower
side of the board and the digital connections are routed on the top side of the board.
• Using a single dedicated ground plane is strongly encouraged.
• Power sources to the ADS868x must be clean and well-bypassed. Using a 1-μF, X7R-grade, 0603-size
ceramic capacitor with at least a 10-V rating in close proximity to the analog (AVDD) supply pins is
recommended. For decoupling the digital supply pin (DVDD), a 1-μF, X7R-grade, 0603-size ceramic capacitor
with at least a 10-V rating is recommended. Placing vias between the AVDD, DVDD pins and the bypass
capacitors must be avoided. All ground pins must be connected to the ground plane using short, lowimpedance paths.
• There are two decoupling capacitors used for the REFCAP pin. The first is a small, 1-μF, 0603-size ceramic
capacitor placed close to the device pins for decoupling the high-frequency signals and the second is a
10-μF, 0805-size ceramic capacitor to provide the charge required by the reference circuit of the device. A
capacitor with an ESR less than 0.2 Ω is recommended for the 10-μF capacitor. Both of these capacitors
must be directly connected to the device pins without any vias between the pins and capacitors.
• The REFIO pin also must be decoupled with a minimum of 4.7-μF ceramic capacitor if the internal reference
of the device is used. The capacitor must be placed close to the device pins.
62
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Product Folder Links: ADS8681 ADS8685 ADS8689
ADS8681, ADS8685, ADS8689
www.ti.com
SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
10.2 Layout Example
GND
External
Reference
DVDD
AVDD
1µF
GND
1µF
DVDD
4.7 µF
16
AVDD
2
GND
RVS
GND
REFIO
1µF
ALARM/SDO-1/GPO
4
SDO-0
REFGND
5
SCLK
REFCAP
6
CONVST/CS
SDI
10 µF
GND
RST
Analog Input
GND
Optional
Figure 102. Board Layout for the ADS868x
Copyright © 2016–2018, Texas Instruments Incorporated
Product Folder Links: ADS8681 ADS8685 ADS8689
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ADS8681, ADS8685, ADS8689
SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
www.ti.com
11 Device and Documentation Support
11.1 Documentation Support
11.1.1 Related Documentation
For related documentation see the following:
• Texas Instruments, OPA320 Precision, 20MHz, 0.9pA, Low-Noise, RRIO, CMOS Operational Amplifier with
Shutdown data sheet
• Texas Instruments, SN6501 Transformer Driver for Isolated Power Supplies data sheet
• Texas Instruments, TPS7A49 36-V, 150-mA, Ultralow-Noise, Positive Linear Regulator data sheet
• Texas Instruments, ISO764xFM Low-Power Quad-Channel Digital Isolators data sheet
• Texas Instruments, AN-2029 Handling and Process Recommendations application report
11.2 Related Links
The table below lists quick access links. Categories include technical documents, support and community
resources, tools and software, and quick access to order now.
Table 20. Related Links
PARTS
PRODUCT FOLDER
ORDER NOW
TECHNICAL
DOCUMENTS
TOOLS &
SOFTWARE
SUPPORT &
COMMUNITY
ADS8681
Click here
Click here
Click here
Click here
Click here
ADS8685
Click here
Click here
Click here
Click here
Click here
ADS8689
Click here
Click here
Click here
Click here
Click here
11.3 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
11.4 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.5 Trademarks
E2E is a trademark of Texas Instruments.
multiSPI is a trademark of Motorola Mobility LLC.
All other trademarks are the property of their respective owners.
11.6 Electrostatic Discharge Caution
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could cause the device not to meet its published specifications.
64
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Product Folder Links: ADS8681 ADS8685 ADS8689
ADS8681, ADS8685, ADS8689
www.ti.com
SBAS633C – FEBRUARY 2016 – REVISED OCTOBER 2018
11.7 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
Copyright © 2016–2018, Texas Instruments Incorporated
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65
PACKAGE OPTION ADDENDUM
www.ti.com
6-Feb-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
Lead/Ball Finish
MSL Peak Temp
(2)
(6)
(3)
Op Temp (°C)
Device Marking
(4/5)
ADS8681IPW
ACTIVE
TSSOP
PW
16
90
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8681
ADS8681IPWR
ACTIVE
TSSOP
PW
16
2000
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8681
ADS8681IRUMR
ACTIVE
WQFN
RUM
16
3000
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8681
ADS8681IRUMT
ACTIVE
WQFN
RUM
16
250
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8681
ADS8685IPW
ACTIVE
TSSOP
PW
16
90
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8685
ADS8685IPWR
ACTIVE
TSSOP
PW
16
2000
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8685
ADS8689IPW
ACTIVE
TSSOP
PW
16
90
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8689
ADS8689IPWR
ACTIVE
TSSOP
PW
16
2000
Green (RoHS
& no Sb/Br)
NIPDAU
Level-3-260C-168 HR
-40 to 125
ADS8689
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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