16-Bit, 250 kSPS PulSAR
ADC in MSOP/QFN
AD7685
FEATURES
APPLICATION DIAGRAM
APPLICATIONS
0.5
INL (LSB)
IN–
REF VDD VIO
SDI
AD7685
1.8V TO VDD
SCK
3- OR 4-WIRE INTERFACE
(SPI, DAISY CHAIN, CS)
SDO
GND
CNV
Figure 2.
Table 1. MSOP, QFN (LFCSP)/SOT-23
14-/16-/18-Bit PulSAR ADC
Type
18-Bit True
Differential
16-Bit True
Differential
16-Bit Pseudo
100
kSPS
250
kSPS
AD7691
AD7684
AD7687
AD7680
AD7685
400 kSPS
to
500 kSPS
AD7690
AD7982
AD7688
AD7693
AD7686
AD7683
AD7940
AD7694
AD7942
AD7946
1000
kSPS
AD7982
AD7980
ADC
Driver
ADA4941
ADA4841
ADA4941
ADA4841
ADA4841
ADA4841
The AD7685 is a 16-bit, charge redistribution successive
approximation, analog-to-digital converter (ADC) that operates
from a single power supply, VDD, between 2.3 V to 5.5 V. It
contains a low power, high speed, 16-bit sampling ADC with no
missing codes, an internal conversion clock, and a versatile serial
interface port. The part also contains a low noise, wide bandwidth,
short aperture delay, track-and-hold circuit. On the CNV rising
edge, it samples an analog input IN+ between 0 V to REF with
respect to a ground sense IN−. The reference voltage, REF, is
applied externally and can be set up to the supply voltage.
1.0
0
Power dissipation scales linearly with throughput.
–0.5
–1.0
0
16384
32768
CODE
49152
65536
02968-005
–1.5
–2.0
IN+
2.5V TO 5V
GENERAL DESCRIPTION
POSITIVE INL = +0.33LSB
NEGATIVE INL = –0.50LSB
1.5
0 TO VREF
Differential
14-Bit Pseudo
Differential
Battery-powered equipment
Medical instruments
Mobile communications
Personal digital assistants (PDAs)
Data acquisition
Instrumentation
Process controls
2.0
0.5V TO VDD
02968-001
16-bit resolution with no missing codes
Throughput: 250 kSPS
INL: ±0.6 LSB typical, ±2 LSB maximum (±0.003% of FSR)
SINAD: 93.5 dB @ 20 kHz
THD: −110 dB @ 20 kHz
Pseudo differential analog input range
0 V to VREF with VREF up to VDD
No pipeline delay
Single-supply operation 2.3 V to 5.5 V with
1.8 V to 5 V logic interface
Serial interface SPI®-/QSPI™-/MICROWIRE™-/DSP-compatible
Daisy-chain multiple ADCs, BUSY indicator
Power dissipation
1.4 μW @ 2.5 V/100 SPS
1.35 mW @ 2.5 V/100 kSPS, 4 mW @ 5 V/100 kSPS
Standby current: 1 nA
10-lead package: MSOP (MSOP-8 size) and
3 mm × 3 mm QFN (LFCSP) (SOT-23 size)
Pin-for-pin-compatible with 10-lead MSOP/QFN PulSAR® ADCs
Figure 1. Integral Nonlinearity vs. Code.
The SPI-compatible serial interface also features the ability,
using the SDI input, to daisy chain several ADCs on a single
3-wire bus or provides an optional BUSY indicator. It is
compatible with 1.8 V, 2.5 V, 3 V, or 5 V logic using the
separate supply VIO.
The AD7685 is housed in a 10-lead MSOP or a 10-lead QFN
(LFCSP) with operation specified from −40°C to +85°C.
Rev. B
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2004–2007 Analog Devices, Inc. All rights reserved.
AD7685
TABLE OF CONTENTS
Features .............................................................................................. 1
Driver Amplifier Choice ........................................................... 16
Applications....................................................................................... 1
Voltage Reference Input ............................................................ 16
Application Diagram........................................................................ 1
Power Supply............................................................................... 16
General Description ......................................................................... 1
Supplying the ADC from the Reference.................................. 17
Revision History ............................................................................... 2
Digital Interface.......................................................................... 17
Specifications..................................................................................... 3
CS Mode 3-Wire, No BUSY Indicator..................................... 18
Timing Specifications....................................................................... 5
CS Mode 3-Wire with BUSY Indicator ................................... 19
Absolute Maximum Ratings............................................................ 7
CS Mode 4-Wire, No BUSY Indicator..................................... 20
ESD Caution.................................................................................. 7
CS Mode 4-Wire with BUSY Indicator ................................... 21
Pin Configuration and Function Descriptions............................. 8
Terminology ...................................................................................... 9
Typical Performance Characteristics ........................................... 10
Theory of Operation ...................................................................... 13
Circuit Information.................................................................... 13
Converter Operation.................................................................. 13
Typical Connection Diagram ................................................... 14
Analog Inputs.............................................................................. 15
Chain Mode, No BUSY Indicator ............................................ 22
Chain Mode with BUSY Indicator........................................... 23
Application Hints ........................................................................... 24
Layout .......................................................................................... 24
Evaluating the Performance of the AD7685............................... 24
True 16-Bit Isolated Application Example .............................. 25
Outline Dimensions ....................................................................... 26
Ordering Guide .......................................................................... 27
REVISION HISTORY
3/07—Rev. A to Rev. B
Changes to Features and Table 1 .................................................... 1
Changes to Table 3............................................................................ 4
Moved Figure 3 and Figure 4 to Page............................................. 6
Inserted Figure 6; Renumbered Sequentially................................ 8
Changes to Figure 13 and Figure 14............................................. 11
Changes to Figure 27...................................................................... 14
Changes to Table 9.......................................................................... 16
Changes to Figure 32...................................................................... 17
Changes to Figure 43...................................................................... 22
Changes to Figure 45...................................................................... 23
Updated Outline Dimensions ....................................................... 26
Changes to Ordering Guide .......................................................... 27
12/04—Rev. 0 to Rev. A
Changes to Specifications ................................................................ 3
Changes to Figure 17 Captions ..................................................... 11
Changes to Power Supply Section ................................................ 17
Changes to Digital Interface Section............................................ 18
Changes to CS Mode 4-Wire No Busy Indicator Section ......... 21
Changes to CS Mode 4-Wire with Busy Indicator Section ....... 22
Changes to Chain Mode, No Busy Indicator Section ................ 23
Changes to Chain Mode with Busy Indicator Section............... 24
Added True 16-Bit Isolated Application Example Section ....... 26
Added Figure 47.............................................................................. 26
Changes to Ordering Guide .......................................................... 28
4/04—Revision 0: Initial Revision
Rev. B | Page 2 of 28
AD7685
SPECIFICATIONS
VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = –40°C to +85°C, unless otherwise noted.
Table 2.
Parameter
RESOLUTION
ANALOG INPUT
Voltage Range
Absolute Input Voltage
Analog Input CMRR
Leakage Current at 25°C
Input Impedance
ACCURACY
No Missing Codes
Differential Linearity Error
Integral Linearity Error
Transition Noise
Gain Error 2 , TMIN to TMAX
Gain Error Temperature Drift
Offset Error2, TMIN to TMAX
Offset Temperature Drift
Power Supply Sensitivity
THROUGHPUT
Conversion Rate
Transient Response
AC ACCURACY
Signal-to-Noise Ratio
Spurious-Free Dynamic
Range
Total Harmonic Distortion
Signal-to-(Noise + Distortion)
Conditions
Min
16
IN+ − IN−
IN+
0
−0.1
IN−
fIN = 250 kHz
Acquisition phase
−0.1
A Grade
Typ
Max
VREF
VDD +
0.1
+0.1
−6
REF = VDD = 5 V
VDD = 4.5 V to 5.5 V
VDD = 2.3 V to 4.5 V
VDD = 5 V ± 5%
fIN = 20 kHz,
VREF = 5 V
fIN = 20 kHz,
VREF = 2.5 V
fIN = 20 kHz
fIN = 20 kHz
fIN = 20 kHz,
VREF = 5 V
fIN = 20 kHz,
VREF = 5 V,
−60 dB input
fIN = 20 kHz,
VREF = 2.5 V
+6
0.5
±2
±0.3
±0.1
±0.7
±0.3
±0.05
0
0
VREF
VDD +
0.1
+0.1
−0.1
Min
16
16
−1
−3
±1.6
±3.5
±0.7
±1
0.5
±2
±0.3
±0.1
±0.7
±0.3
±0.05
0
0
+3
VREF
VDD +
0.1
+0.1
−0.1
65
1
See the
Analog Inputs section
16
−1
−2
±30
±1.6
±3.5
250
200
1.8
C Grade
Typ
Max
0
−0.1
65
1
See the
Analog Inputs section
±30
250
200
1.8
B Grade
Typ
Max
0
−0.1
65
1
See the
Analog Inputs section
15
VDD = 4.5 V to 5.5 V
VDD = 2.3 V to 4.5 V
Full-scale step
Min
16
±0.5
±0.6
0.45
±2
±0.3
±0.1
±0.7
±0.3
±0.05
0
0
+1.5
+2
±15
±1.6
±3.5
250
200
1.8
Unit
Bits
V
V
V
dB
nA
Bits
LSB 1
LSB
LSB
LSB
ppm/°C
mV
mV
ppm/°C
LSB
kSPS
kSPS
μs
90
90
92
91.5
93.5
dB 3
86
86
88
87.5
88.5
dB
−110
dB
−110
93.5
dB
dB
33.5
dB
88.5
dB
−115
dB
−100
−100
89
−106
90
−106
92
91.5
32
86
Intermodulation Distortion 4
85.5
87.5
−110
1
87
LSB means least significant bit. With the 5 V input range, 1 LSB is 76.3 μV.
See Terminology section. These specifications do include full temperature range variation but do not include the error contribution from the external reference.
All specifications in dB are referred to a full-scale input FS. Tested with an input signal at 0.5 dB below full-scale, unless otherwise specified.
4
fIN1 = 21.4 kHz, fIN2 = 18.9 kHz, each tone at −7 dB below full scale.
2
3
Rev. B | Page 3 of 28
AD7685
VDD = 2.3 V to 5.5 V, VIO = 2.3 V to VDD, VREF = VDD, TA = –40°C to +85°C, unless otherwise noted.
Table 3.
Parameter
REFERENCE
Voltage Range
Load Current
SAMPLING DYNAMICS
−3 dB Input Bandwidth
Aperture Delay
DIGITAL INPUTS
Logic Levels
VIL
VIH
IIL
IIH
DIGITAL OUTPUTS
Data Format
Pipeline Delay
Conditions
VOL
VOH
POWER SUPPLIES
VDD
VIO
VIO Range
Standby Current 1, 2
Power Dissipation
ISINK = +500 μA
ISOURCE = −500 μA
TEMPERATURE RANGE 3
Specified Performance
Min
Typ
0.5
Max
Unit
VDD + 0.3
250 kSPS, REF = 5 V
50
V
μA
VDD = 5 V
2
2.5
MHz
ns
–0.3
0.7 × VIO
−1
−1
0.3 × VIO
VIO + 0.3
+1
+1
Serial 16 bits straight binary
Conversion results available immediately
after completed conversion
0.4
VIO − 0.3
Specified performance
Specified performance
2.3
2.3
1.8
VDD and VIO = 5 V, 25°C
VDD = 2.5 V, 100 SPS throughput
VDD = 2.5 V, 100 kSPS throughput
VDD = 2.5 V, 200 kSPS throughput
VDD = 5 V, 100 kSPS throughput
VDD = 5 V, 250 kSPS throughput
TMIN to TMAX
1
1.4
1.35
2.7
4
10
−40
1
With all digital inputs forced to VIO or GND as required.
During acquisition phase.
3
Contact sales for extended temperature range.
2
Rev. B | Page 4 of 28
5.5
VDD + 0.3
VDD + 0.3
50
V
V
μA
μA
V
V
2.4
4.8
6
15
V
V
V
nA
μW
mW
mW
mW
mW
+85
°C
AD7685
TIMING SPECIFICATIONS
−40°C to +85°C, VIO = 2.3 V to 5.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.
Table 4. VDD = 4.5 V to 5.5 V 1
Parameter
Conversion Time: CNV Rising Edge To Data Available
Acquisition Time
Time Between Conversions
CNV Pulse Width (CS Mode)
SCK Period (CS Mode)
SCK Period (Chain Mode)
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
SCK Low Time
SCK High Time
SCK Falling Edge to Data Remains Valid
SCK Falling Edge to Data Valid Delay
VIO Above 4.5 V
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV or SDI Low to SDO D15 MSB Valid (CS Mode)
VIO Above 4.5 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
SDI Valid Setup Time from CNV Rising Edge (CS Mode)
SDI Valid Hold Time from CNV Rising Edge (CS Mode)
SCK Valid Setup Time from CNV Rising Edge (Chain Mode)
SCK Valid Hold Time from CNV Rising Edge (Chain Mode)
SDI Valid Setup Time from SCK Falling Edge (Chain Mode)
SDI Valid Hold Time from SCK Falling Edge (Chain Mode)
SDI High to SDO High (Chain Mode with Busy Indicator)
VIO Above 4.5 V
VIO Above 2.3 V
1
See Figure 3 and Figure 4 for load conditions.
Rev. B | Page 5 of 28
Symbol
tCONV
tACQ
tCYC
tCNVH
tSCK
tSCK
tSCKL
tSCKH
tHSDO
tDSDO
Min
0.5
1.8
4
10
15
Typ
Max
2.2
17
18
19
20
7
7
5
Unit
μs
μs
μs
ns
ns
ns
ns
ns
ns
ns
ns
ns
14
15
16
17
ns
ns
ns
ns
15
18
22
25
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
15
26
ns
ns
tEN
tDIS
tSSDICNV
tHSDICNV
tSSCKCNV
tHSCKCNV
tSSDISCK
tHSDISCK
tDSDOSDI
15
0
5
5
3
4
AD7685
−40°C to +85°C, VIO = 2.3 V to 4.5 V or VDD + 0.3 V, whichever is the lowest, unless otherwise stated.
Table 5. VDD = 2.3V to 4.5 V 1
Parameter
Conversion Time: CNV Rising Edge to Data Available
Acquisition Time
Time Between Conversions
CNV Pulse Width (CS Mode)
SCK Period (CS Mode)
SCK Period (Chain Mode)
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
SCK Low Time
SCK High Time
SCK Falling Edge to Data Remains Valid
SCK Falling Edge to Data Valid Delay
VIO Above 3 V
VIO Above 2.7 V
VIO Above 2.3 V
CNV or SDI Low to SDO D15 MSB Valid (CS Mode)
VIO Above 2.7 V
VIO Above 2.3 V
CNV or SDI High or Last SCK Falling Edge to SDO High Impedance (CS Mode)
SDI Valid Setup Time from CNV Rising Edge (CS Mode)
SDI Valid Hold Time from CNV Rising Edge (CS Mode)
SCK Valid Setup Time from CNV Rising Edge (Chain Mode)
SCK Valid Hold Time from CNV Rising Edge (Chain Mode)
SDI Valid Setup Time from SCK Falling Edge (Chain Mode)
SDI Valid Hold Time from SCK Falling Edge (Chain Mode)
SDI High to SDO High (Chain Mode with Busy Indicator)
tSCKL
tSCKH
tHSDO
tDSDO
Min
0.7
1.8
5
10
25
Typ
Max
3.2
29
35
40
12
12
5
Unit
μs
μs
μs
ns
ns
ns
ns
ns
ns
ns
ns
24
30
35
ns
ns
ns
18
22
25
ns
ns
ns
ns
ns
ns
ns
ns
ns
ns
tEN
tDIS
tSSDICNV
tHSDICNV
tSSCKCNV
tHSCKCNV
tSSDISCK
tHSDISCK
tDSDOSDI
30
0
5
8
5
4
36
See Figure 3 and Figure 4 for load conditions.
70% VIO
IOL
30% VIO
tDELAY
1.4V
TO SDO
CL
50pF
500µA
IOH
tDELAY
2V OR VIO – 0.5V1
0.8V OR 0.5V2
2V OR VIO – 0.5V1
0.8V OR 0.5V2
NOTES
1. 2V IF VIO ABOVE 2.5V, VIO – 0.5V IF VIO BELOW 2.5V.
2. 0.8V IF VIO ABOVE 2.5V, 0.5V IF VIO BELOW 2.5V.
Figure 4. Voltage Levels for Timing
Figure 3. Load Circuit for Digital Interface Timing
Rev. B | Page 6 of 28
02968-003
500µA
02968-002
1
Symbol
tCONV
tACQ
tCYC
tCNVH
tSCK
tSCK
AD7685
ABSOLUTE MAXIMUM RATINGS
Table 6.
Parameter
Analog Inputs
IN+ 1 , IN−1, REF
Supply Voltages
VDD, VIO to GND
VDD to VIO
Digital Inputs to GND
Digital Outputs to GND
Storage Temperature Range
Junction Temperature
θJA Thermal Impedance
θJC Thermal Impedance
Lead Temperature
Vapor Phase (60 sec)
Infrared (15 sec)
1
Rating
GND − 0.3 V to VDD + 0.3 V
or ±130 mA
−0.3 V to +7 V
±7 V
−0.3 V to VIO + 0.3 V
−0.3 V to VIO + 0.3 V
−65°C to +150°C
150°C
200°C/W (MSOP-10)
44°C/W (MSOP-10)
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
215°C
220°C
See the Analog Inputs section.
Rev. B | Page 7 of 28
AD7685
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
IN+ 3
IN– 4
9
SDI
TOP VIEW
(Not to Scale)
8
SCK
GND 5
7
SDO
6
CNV
REF 1
10 VIO
VDD 2
AD7685
9
SDI
IN+ 3
TOP VIEW
(Not to Scale)
8
SCK
IN– 4
GND 5
7
SDO
6
CNV
02968-005
10 VIO
AD7685
02968-004
REF 1
VDD 2
Figure 6. 10-Lead QFN (LFCSP) Pin Configuration
Figure 5. 10-Lead MSOP Pin Configuration
Table 7. Pin Function Descriptions
Pin No
1
Mnemonic
REF
Type 1
AI
2
3
4
5
6
VDD
IN+
IN−
GND
CNV
P
AI
AI
P
DI
7
8
9
SDO
SCK
SDI
DO
DI
DI
10
VIO
P
Description
Reference Input Voltage. The REF range is from 0.5 V to VDD. It is referred to the GND pin. This pin should
be decoupled closely to the pin with a 10 μF capacitor.
Power Supply.
Analog Input. It is referred to IN−. The voltage range, that is, the difference between IN+ and IN−, is 0 V to VREF.
Analog Input Ground Sense. Connect to the analog ground plane or to a remote sense ground.
Power Supply Ground.
Convert Input. This input has multiple functions. On its leading edge, it initiates the conversions and
selects the interface mode of the part, chain, or CS mode. In CS mode, it enables the SDO pin when low.
In chain mode, the data should be read when CNV is high.
Serial Data Output. The conversion result is output on this pin. It is synchronized to SCK.
Serial Data Clock Input. When the part is selected, the conversion result is shifted out by this clock.
Serial Data Input. This input provides multiple features. It selects the interface mode of the ADC as follows:
Chain mode is selected if SDI is low during the CNV rising edge. In this mode, SDI is used as a data input
to daisy chain the conversion results of two or more ADCs onto a single SDO line. The digital data level
on SDI is output on SDO with a delay of 16 SCK cycles.
CS mode is selected if SDI is high during the CNV rising edge. In this mode, either SDI or CNV can enable
the serial output signals when low, and if SDI or CNV is low when the conversion is complete, the BUSY
indicator feature is enabled.
Input/Output Interface Digital Power. Nominally at the same supply as the host interface (1.8 V, 2.5 V, 3 V, or 5 V).
1
AI = analog input, DI = digital input, DO = digital output, and P = power.
Rev. B | Page 8 of 28
AD7685
TERMINOLOGY
Integral Nonlinearity Error (INL)
INL refers to the deviation of each individual code from a line
drawn from negative full scale through positive full scale. The
point used as negative full scale occurs ½ LSB before the first
code transition. Positive full scale is defined as a level 1½ LSB
beyond the last code transition. The deviation is measured from
the middle of each code to the true straight line (see Figure 26).
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. DNL is the
maximum deviation from this ideal value. It is often specified in
terms of resolution for which no missing codes are guaranteed.
Offset Error
The first transition should occur at a level ½ LSB above analog
ground (38.1 μV for the 0 V to 5 V range). The offset error is
the deviation of the actual transition from that point.
Gain Error
The last transition (from 111 . . . 10 to 111 . . . 11) should
occur for an analog voltage 1½ LSB below the nominal full
scale (4.999886 V for the 0 V to 5 V range). The gain error is the
deviation of the actual level of the last transition from the ideal
level after the offset is adjusted out.
Spurious-Free Dynamic Range (SFDR)
The difference, in decibels (dB), between the rms amplitude of
the input signal and the peak spurious signal.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to SINAD by
ENOB = (SINADdB − 1.76)/6.02
and is expressed in bits.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in dB.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in dB.
Signal-to-(Noise + Distortion), SINAD
SINAD is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components below the Nyquist
frequency, including harmonics but excluding dc. The value for
SINAD is expressed in dB.
Aperture Delay
Aperture delay is a measure of the acquisition performance and
is the time between the rising edge of the CNV input and when
the input signal is held for a conversion.
Transient Response
The time required for the ADC to accurately acquire its input
after a full-scale step function is applied.
Rev. B | Page 9 of 28
AD7685
TYPICAL PERFORMANCE CHARACTERISTICS
1.5
1.0
0.5
0.5
DNL (LSB)
1.0
0
–0.5
0
–0.5
–1.0
–1.0
–1.5
–1.5
–2.0
0
16384
32768
CODE
49152
POSITIVE DNL = +0.21LSB
NEGATIVE DNL = –0.30LSB
1.5
65536
–2.0
02968-047
INL (LSB)
2.0
POSITIVE INL = +0.33LSB
NEGATIVE INL = –0.50LSB
0
16384
Figure 7. Integral Nonlinearity vs. Code
250000
32768
CODE
49152
65536
02968-008
2.0
Figure 10. Differential Nonlinearity vs. Code
140000
VDD = REF = 5V
125055
204292
VDD = REF = 2.5V
120000
200000
100000
COUNTS
COUNTS
150000
100000
80000
60966
59082
60000
40000
0
80E5
0
80E6
12
80E7
20000
20
0
0
0
80E8 80E9 80EA 80EB 80EC 80ED
CODE IN HEX
Figure 8. Histogram of a DC Input at the Code Center
0
–80
–100
–120
–140
–160
–180
0
20
40
60
80
FREQUENCY (kHz)
100
120
179
0
0
804E 804F 8050 8051 8052 8053 8054 8055 8056 8057 8058
CODE IN HEX
16384 POINT FFT
VDD = REF = 2.5V
fS = 250kSPS
fIN = 20.45kHz
SNR = 88.8dB
THD = –103.5dB
SFDR = –104.5dB
SECOND HARMONIC = –112.4dB
THIRD HARMONIC = –105.4dB
–20
AMPLITUDE (dB OF FULL SCALE)
–60
6956
213
0
02968-007
AMPLITUDE (dB OF FULL SCALE)
–40
2
Figure 11. Histogram of a DC Input at the Code Center
8192 POINT FFT
VDD = REF = 5V
fS = 250kSPS
fIN = 20.45kHz
SNR = 93.3dB
THD = –111.6dB
SFDR = –113.7dB
SECOND HARMONIC = –113.7dB
THIRD HARMONIC = –117.6dB
–20
8667
0
02968-009
0
27755
Figure 9. FFT Plot
–40
–60
–80
–100
–120
–140
–160
–180
0
20
40
60
80
FREQUENCY (kHz)
Figure 12. FFT Plot
Rev. B | Page 10 of 28
100
120
02968-010
29041
02968-006
50000
AD7685
100
–90
17
–95
SNR
–100
16
90
ENOB
THD, SFDR (dB)
SINAD
ENOB (Bits)
SINAD (dB)
95
15
14
85
–105
–110
THD
–115
SFDR
–120
2.7
3.1
3.5
3.9
4.3
4.7
REFERENCE VOLTAGE (V)
5.1
13
5.5
–130
2.3
02968-011
80
2.3
3.1
3.5
3.9
4.3
4.7
REFERENCE VOLTAGE (V)
5.1
5.5
Figure 16. THD, SFDR vs. Reference Voltage
Figure 13. SNR, SINAD, and ENOB vs. Reference Voltage
100
–60
95
–70
VREF = 5V, –10dB
VREF = 5V, –1dB
90
–80
VREF = 5V, –1dB
THD (dB)
SINAD (dB)
2.7
02968-014
–125
85
VREF = 2.5V, –1dB
VREF = 2.5V, –1dB
–90
80
–100
75
–110
0
50
100
FREQUENCY (kHz)
150
200
–120
02968-012
70
0
50
Figure 14. SINAD vs. Frequency
100
FREQUENCY (kHz)
150
200
02968-015
VREF = 5V, –10dB
Figure 17. THD vs. Frequency
100
–90
95
VREF = 5V
–100
VREF = 2.5V
THD (dB)
VREF = 2.5V
85
–110
VREF = 5V
80
–120
70
–55
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
125
Figure 15. SNR vs. Temperature
–130
–55
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
Figure 18. THD vs. Temperature
Rev. B | Page 11 of 28
105
125
02968-016
75
02968-013
SNR (dB)
90
AD7685
1000
–105
fS = 100kSPS
VDD = 5V
OPERATING CURRENTS (µA)
94
SNR
–110
92
THD (dB)
93
THD
–115
91
750
VDD = 2.5V
500
250
–8
–6
–4
INPUT LEVEL (dB)
–2
0
–120
0
–55
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
125
02968-020
VIO
90
–10
02968-017
SNR REFERENCE TO FULL SCALE (dB)
95
Figure 22. Operating Currents vs. Temperature
Figure 19. SNR and THD vs. Input Level
6
1000
5
fS = 100kSPS
OFFSET, GAIN ERROR (LSB)
750
VDD
500
250
3.5
1
OFFSET ERROR
0
–1
–2
GAIN ERROR
–3
–4
3.9
4.3
SUPPLY (V)
4.7
5.1
5.5
–6
–55
–35
–15
85
105
125
Figure 23. Offset and Gain Error vs. Temperature
Figure 20. Operating Currents vs. Supply
25
1000
VDD = 2.5V, 85°C
20
tDSDO DELAY (ns)
750
500
250
15
VDD = 2.5V, 25°C
10
VDD = 5V, 85°C
VDD = 5V, 25°C
5
VDD = 3.3V, 85°C
VDD = 3.3V, 25°C
VDD + VIO
0
–55
–35
–15
5
25
45
65
TEMPERATURE (°C)
85
105
125
02968-019
POWER-DOWN CURRENTS (nA)
5
25
45
65
TEMPERATURE (°C)
02968-021
3.1
02968-018
2.7
2
–5
VIO
0
2.3
3
0
0
20
40
60
80
SDO CAPACITIVE LOAD (pF)
100
120
Figure 24. tDSDO Delay vs. Capacitance Load and Supply
Figure 21. Power-Down Currents vs. Temperature
Rev. B | Page 12 of 28
02968-022
OPERATING CURRENTS (µA)
4
AD7685
THEORY OF OPERATION
IN+
SWITCHES CONTROL
MSB
REF
32,768C 16,384C
LSB
4C
2C
C
SW+
C
BUSY
COMP
GND
32,768C 16,384C
4C
2C
C
MSB
CONTROL
LOGIC
OUTPUT CODE
C
LSB
SW–
02968-023
CNV
IN–
Figure 25. ADC Simplified Schematic
CIRCUIT INFORMATION
CONVERTER OPERATION
The AD7685 is a fast, low power, single-supply, precise 16-bit
ADC using a successive approximation architecture.
The AD7685 is a successive approximation ADC based on a
charge redistribution DAC. Figure 25 shows the simplified
schematic of the ADC. The capacitive DAC consists of two
identical arrays of 16 binary weighted capacitors, which are
connected to the two comparator inputs.
The AD7685 is capable of converting 250,000 samples per
second (250 kSPS) and powers down between conversions.
When operating at 100 SPS, for example, it consumes typically
1.35 μW with a 2.5 V supply, ideal for battery-powered
applications.
The AD7685 provides the user with on-chip, track-and-hold
and does not exhibit any pipeline delay or latency, making it
ideal for multiple multiplexed channel applications.
The AD7685 is specified from 2.3 V to 5.5 V and can be
interfaced to any 1.8 V to 5 V digital logic family. It is housed in
a 10-lead MSOP or a tiny 10-lead QFN (LFCSP) that combines
space savings and allows flexible configurations.
It is pin-for-pin-compatible with the AD7686, AD7687, and
AD7688.
During the acquisition phase, terminals of the array tied to the
comparator’s input are connected to GND via SW+ and SW−.
All independent switches are connected to the analog inputs.
Therefore, the capacitor arrays are used as sampling capacitors
and acquire the analog signal on the IN+ and IN− inputs. When
the acquisition phase is complete and the CNV input goes high,
a conversion phase is initiated. When the conversion phase
begins, SW+ and SW− are opened first. The two capacitor
arrays are then disconnected from the inputs and connected to
the GND input. Therefore, the differential voltage between the
inputs IN+ and IN− captured at the end of the acquisition phase
is applied to the comparator inputs, causing the comparator to
become unbalanced. By switching each element of the capacitor
array between GND and REF, the comparator input varies by
binary weighted voltage steps (VREF/2, VREF/4 . . . VREF/65536).
The control logic toggles these switches, starting with the MSB,
to bring the comparator back into a balanced condition. After
the completion of this process, the part powers down and
returns to the acquisition phase, and the control logic generates
the ADC output code and a BUSY signal indicator.
Because the AD7685 has an on-board conversion clock, the
serial clock, SCK, is not required for the conversion process.
Rev. B | Page 13 of 28
AD7685
TYPICAL CONNECTION DIAGRAM
The ideal transfer characteristic for the AD7685 is shown in
Figure 26 and Table 8.
Figure 27 shows an example of the recommended connection
diagram for the AD7685 when multiple supplies are available.
ADC CODE (STRAIGHT BINARY)
Transfer Functions
111...111
111...110
111...101
000...010
000...001
–FS + 1 LSB
+FS – 1 LSB
+FS – 1.5 LSB
–FS + 0.5 LSB
02968-024
000...000
–FS
ANALOG INPUT
Figure 26. ADC Ideal Transfer Function
≥7V
REF1
5V
10µF2
100nF
1.8V TO VDD
≥7V
100nF
REF
VDD
IN+
0 TO VREF
3
≤–2V
AD7685
2.7nF
4
IN–
GND
VIO
SDI
SCK
SDO
3- OR 4-WIRE INTERFACE5
CNV
NOTES
1. SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.
2. CREF IS USUALLY A 10µF CERAMIC CAPACITOR (X5R).
3. SEE DRIVER AMPLIFIER CHOICE SECTION.
4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.
5. SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.
Figure 27. Typical Application Diagram with Multiple Supplies
Table 8. Output Codes and Ideal Input Voltages
Description
FSR – 1 LSB
Midscale + 1 LSB
Midscale
Midscale – 1 LSB
–FSR + 1 LSB
–FSR
1
2
Analog Input VREF = 5 V
4.999924 V
2.500076 V
2.5 V
2.499924 V
76.3 μV
0V
Digital Output Code Hexa
FFFF 1
8001
8000
7FFF
0001
0000 2
This is also the code for an overranged analog input (VIN+ − VIN− above VREF − VGND).
This is also the code for an underranged analog input (VIN+ − VIN− below VGND).
Rev. B | Page 14 of 28
02968-025
33Ω
AD7685
ANALOG INPUTS
Figure 28 shows an equivalent circuit of the input structure of
the AD7685.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN+ and IN−. Care must be taken to ensure that
the analog input signal never exceeds the supply rails by more
than 0.3 V because this will cause these diodes to begin to
forward-bias and start conducting current. These diodes can
handle a forward-biased current of 130 mA maximum. For
instance, these conditions could eventually occur when the
input buffer’s (U1) supplies are different from VDD. In such a
case, an input buffer with a short-circuit current limitation can
be used to protect the part.
VDD
D1
IN+
OR IN–
CIN
D2
02968-026
CPIN
RIN
GND
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN−) can be modeled as a parallel combination
of capacitor CPIN and the network formed by the series connection
of RIN and CIN. CPIN is primarily the pin capacitance. RIN is
typically 3 kΩ and is a lumped component made up of some
serial resistors and the on resistance of the switches. CIN is
typically 30 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to CPIN. RIN and CIN make a
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits the noise.
When the source impedance of the driving circuit is low, the
AD7685 can be driven directly. Large source impedances
significantly affect the ac performance, especially THD. The dc
performances are less sensitive to the input impedance. The
maximum source impedance depends on the amount of THD
that can be tolerated. The THD degrades as a function of the
source impedance and the maximum input frequency, as shown
in Figure 30.
–60
Figure 28. Equivalent Analog Input Circuit
–70
–80
–90
RS = 250Ω
–100
RS = 100Ω
80
RS = 50Ω
RS = 33Ω
–110
–120
VDD = 5V
50
40
1
10
100
FREQUENCY (kHz)
0
25
50
FREQUENCY (kHz)
75
100
Figure 30. THD vs. Analog Input Frequency and Source Resistance
VDD = 2.5V
60
1000
10000
02968-027
CMRR (dB)
70
02968-028
THD (dB)
This analog input structure allows the sampling of the
differential signal between IN+ and IN−. By using this
differential input, small signals common to both inputs are
rejected, as shown in Figure 29, which represents the typical
CMRR over frequency. For instance, by using IN− to sense a
remote signal ground, ground potential differences between
the sensor and the local ADC ground are eliminated.
Figure 29. Analog Input CMRR vs. Frequency
Rev. B | Page 15 of 28
AD7685
DRIVER AMPLIFIER CHOICE
VOLTAGE REFERENCE INPUT
Although the AD7685 is easy to drive, the driver amplifier
needs to meet the following requirements:
The AD7685 voltage reference input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
• The noise generated by the driver amplifier needs to be kept
as low as possible to preserve the SNR and transition noise
performance of the AD7685. Note that the AD7685 has a
noise much lower than most of the other 16-bit ADCs and,
therefore, can be driven by a noisier amplifier to meet a given
system noise specification. The noise coming from the
amplifier is filtered by the AD7685 analog input circuit lowpass filter made by RIN and CIN or by an external filter, if one
is used. Because the typical noise of the AD7685 is 35 μV
rms, the SNR degradation due to the amplifier is
⎞
⎟
⎟
⎟
⎟
⎠
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
POWER SUPPLY
• For ac applications, the driver should have a THD
performance commensurate with the AD7685. Figure 17
shows the AD7685’s THD vs. frequency.
• For multichannel, multiplexed applications, the driver
amplifier and the AD7685 analog input circuit must settle a
full-scale step onto the capacitor array at a 16-bit level
(0.0015%). In the amplifier’s data sheet, settling at 0.1% to
0.01% is more commonly specified. This could differ
significantly from the settling time at a 16-bit level and
should be verified prior to driver selection.
Table 9. Recommended Driver Amplifiers
The AD7685 is specified over a wide operating range from
2.3 V to 5.5 V. It has, unlike other low voltage converters, a
noise low enough to design a 16-bit resolution system with low
supply and respectable performance. It uses two power supply
pins: a core supply VDD and a digital input/output interface
supply VIO. VIO allows direct interface with any logic between
1.8 V and VDD. To reduce the number of supplies needed, the
VIO and VDD can be tied together. The AD7685 is independent of
power supply sequencing between VIO and VDD. Additionally,
it is very insensitive to power supply variations over a wide
frequency range, as shown in Figure 31, which represents PSRR
over frequency.
110
100
90
VDD = 5V
80
Typical Application
Very low noise and low power
5 V single-supply, low power
5 V single-supply, low power
Low power, low noise, and low frequency
Very low noise and high frequency
Very low noise and high frequency
Small, low power and low frequency
High frequency and low power
Rev. B | Page 16 of 28
70
VDD = 2.5V
60
50
40
30
1
10
100
FREQUENCY (kHz)
1000
Figure 31. PSRR vs. Frequency
10000
02968-029
where:
f–3dB is the input bandwidth in MHz of the AD7685
(2 MHz) or the cutoff frequency of the input filter, if one is
used.
N is the noise gain of the amplifier (for example, 1 in buffer
configuration).
eN is the equivalent input noise voltage of the op amp, in
nV/√Hz.
Amplifier
ADA4841-x
AD8605, AD8615
AD8655
OP184
AD8021
AD8022
AD8519
AD8031
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR43x reference.
PSRR (dB)
SNRLOSS
⎛
⎜
35
= 20log ⎜
⎜
π
⎜ 35 2 + f −3dB (Ne N )2
2
⎝
When REF is driven by a very low impedance source, for
example, a reference buffer using the AD8031 or the AD8605, a
10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for
optimum performance.
AD7685
The AD7685 powers down automatically at the end of each
conversion phase and, therefore, the power scales linearly with
the sampling rate, as shown in Figure 32. This makes the part
ideal for low sampling rate (even a few Hz) and low batterypowered applications.
10000
OPERATING CURRENTS (µA)
1000
VDD = 5V
VDD = 2.5V
100
10
1
VIO
0.1
1000
10000
SAMPLING RATE (SPS)
100000
02968-030
100
1000000
Figure 32. Operating Currents vs. Sampling Rate
SUPPLYING THE ADC FROM THE REFERENCE
For simplified applications, the AD7685, with its low operating
current, can be supplied directly using the reference circuit, as
shown in Figure 33. The reference line can be driven by either:
• The system power supply directly.
• A reference voltage with enough current output capability,
such as the ADR43x.
• A reference buffer, such as the AD8031, that can also filter
the system power supply, as shown in Figure 33.
5V
Though the AD7685 has a reduced number of pins, it offers
substantial flexibility in its serial interface modes.
The AD7685, when in CS mode, is compatible with SPI, QSPI,
digital hosts, and DSPs, for example, Blackfin® ADSP-BF53x or
ADSP-219x. This interface can use either 3-wire or 4-wire. A
3-wire interface using the CNV, SCK, and SDO signals minimizes
wiring connections, useful, for instance, in isolated applications.
A 4-wire interface using the SDI, CNV, SCK, and SDO signals
allows CNV, which initiates the conversions, to be independent
of the readback timing (SDI). This is useful in low jitter
sampling or simultaneous sampling applications.
The AD7685, when in chain mode, provides a daisy-chain
feature using the SDI input for cascading multiple ADCs on a
single data line similar to a shift register.
0.01
0.001
10
DIGITAL INTERFACE
5V
The mode in which the part operates depends on the SDI level
when the CNV rising edge occurs. The CS mode is selected if
SDI is high and the chain mode is selected if SDI is low. The
SDI hold time is such that when SDI and CNV are connected
together, the chain mode is always selected.
In either the CS mode or the chain mode, the AD7685 offers the
flexibility to optionally force a start bit in front of the data bits.
This start bit can be used as a BUSY signal indicator to
interrupt the digital host and trigger the data reading.
Otherwise, without a BUSY indicator, the user must time out
the maximum conversion time prior to readback.
The BUSY indicator feature is enabled as follows:
• In the CS mode, if CNV or SDI is low when the ADC
conversion ends (see Figure 37 and Figure 41).
• In the chain mode, if SCK is high during the CNV rising edge
(see Figure 45).
10Ω
5V
10kΩ
1µF
AD8031
10µF
1µF
1
REF
VDD
VIO
1OPTIONAL
REFERENCE BUFFER AND FILTER.
02968-031
AD7685
Figure 33. Example of Application Circuit
Rev. B | Page 17 of 28
AD7685
valid on both SCK edges. Although the rising edge can be used
to capture the data, a digital host using the SCK falling edge will
allow a faster reading rate provided it has an acceptable hold
time. After the 16th SCK falling edge or when CNV goes high,
whichever is earlier, SDO returns to high impedance.
CS MODE 3-WIRE, NO BUSY INDICATOR
This mode is usually used when a single AD7685 is connected
to an SPI-compatible digital host.
The connection diagram is shown in Figure 34, and the
corresponding timing is given in Figure 35.
CONVERT
With SDI tied to VIO, a rising edge on CNV initiates a
conversion, selects the CS mode, and forces SDO to high
impedance. Once a conversion is initiated, it will continue to
completion irrespective of the state of CNV. For instance, it
could be useful to bring CNV low to select other SPI devices,
such as analog multiplexers, but CNV must be returned high
before the minimum conversion time and held high until the
maximum conversion time to avoid the generation of the BUSY
signal indicator. When conversion is completed, the AD7685
enters the acquisition phase and powers down. When CNV
goes low, the MSB is output onto SDO. The remaining data bits
are then clocked by subsequent SCK falling edges. The data is
DIGITAL HOST
CNV
VIO
SDI
AD7685
DATA IN
SDO
02968-032
SCK
CLK
Figure 34. CS Mode 3-Wire, No BUSY Indicator
Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
1
2
3
14
tHSDO
16
tSCKH
tDSDO
tEN
SDO
15
D15
D14
D13
tDIS
D1
D0
Figure 35. CS Mode 3-Wire, No BUSY Indicator Serial Interface Timing (SDI High)
Rev. B | Page 18 of 28
02968-033
SCK
AD7685
powers down. The data bits are then clocked out, MSB first, by
subsequent SCK falling edges. The data is valid on both SCK
edges. Although the rising edge can be used to capture the data,
a digital host using the SCK falling edge will allow a faster reading
rate provided it has an acceptable hold time. After the optional
17th SCK falling edge, or when CNV goes high, whichever is
earlier, SDO returns to high impedance.
CS MODE 3-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7685 is connected
to an SPI-compatible digital host having an interrupt input.
The connection diagram is shown in Figure 36, and the
corresponding timing is given in Figure 37.
With SDI tied to VIO, a rising edge on CNV initiates a
conversion, selects the CS mode, and forces SDO to high
impedance. SDO is maintained in high impedance until the
completion of the conversion irrespective of the state of CNV.
Prior to the minimum conversion time, CNV could be used to
select other SPI devices, such as analog multiplexers, but CNV
must be returned low before the minimum conversion time and
held low until the maximum conversion time to guarantee the
generation of the BUSY signal indicator. When the conversion
is complete, SDO goes from high impedance to low. With a
pull-up on the SDO line, this transition can be used as an
interrupt signal to initiate the data reading controlled by the
digital host. The AD7685 then enters the acquisition phase and
CONVERT
VIO
DIGITAL HOST
CNV
VIO
AD7685
DATA IN
SDO
SCK
IRQ
02968-034
SDI
47kΩ
CLK
Figure 36. CS Mode 3-Wire with BUSY Indicator
Connection Diagram (SDI High)
SDI = 1
tCYC
tCNVH
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
1
2
3
tHSDO
15
16
17
tSCKH
tDSDO
SDO
tDIS
D15
D14
D1
D0
Figure 37. CS Mode 3-Wire with BUSY Indicator Serial Interface Timing (SDI High)
Rev. B | Page 19 of 28
02968-035
SCK
AD7685
conversion is complete, the AD7685 enters the acquisition
phase and powers down. Each ADC result can be read by
bringing low its SDI input, which consequently outputs the
MSB onto SDO. The remaining data bits are then clocked by
subsequent SCK falling edges. The data is valid on both SCK
edges. Although the rising edge can be used to capture the data,
a digital host using the SCK falling edge will allow a faster
reading rate, provided it has an acceptable hold time. After the
16th SCK falling edge, or when SDI goes high, whichever is
earlier, SDO returns to high impedance and another AD7685
can be read.
CS MODE 4-WIRE, NO BUSY INDICATOR
This mode is usually used when multiple AD7685s are
connected to an SPI-compatible digital host.
A connection diagram example using two AD7685s is shown in
Figure 38, and the corresponding timing is given in Figure 39.
With SDI high, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback (if SDI and CNV are low, SDO is
driven low). Prior to the minimum conversion time, SDI could
be used to select other SPI devices, such as analog multiplexers,
but SDI must be returned high before the minimum conversion
time and held high until the maximum conversion time to
avoid the generation of the BUSY signal indicator. When the
If multiple AD7685s are selected at the same time, the SDO
output pin handles this contention without damage or induced
latch-up. Meanwhile, it is recommended to keep this contention
as short as possible to limit extra power dissipation.
CS2
CS1
CONVERT
CNV
SDI
AD7685
DIGITAL HOST
CNV
SDO
SDI
AD7685
SCK
SDO
SCK
02968-036
DATA IN
CLK
Figure 38. CS Mode 4-Wire, No BUSY Indicator Connection Diagram
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSSDICNV
SDI(CS1)
tHSDICNV
SDI(CS2)
tSCK
tSCKL
SCK
1
2
14
3
tHSDO
16
17
18
D1
D0
D15
D14
30
31
32
D1
D0
tSCKH
tDSDO
tEN
D15
D14
D13
tDIS
02968-037
SDO
15
Figure 39. CS Mode 4-Wire, No BUSY Indicator Serial Interface Timing
Rev. B | Page 20 of 28
AD7685
as an interrupt signal to initiate the data readback controlled by
the digital host. The AD7685 then enters the acquisition phase
and powers down. The data bits are then clocked out, MSB first,
by subsequent SCK falling edges. The data is valid on both SCK
edges. Although the rising edge can be used to capture the data,
a digital host using the SCK falling edge will allow a faster
reading rate provided it has an acceptable hold time. After the
optional 17th SCK falling edge, or SDI going high, whichever is
earlier, the SDO returns to high impedance.
CS MODE 4-WIRE WITH BUSY INDICATOR
This mode is usually used when a single AD7685 is connected
to an SPI-compatible digital host, which has an interrupt input,
and it is desired to keep CNV, which is used to sample the
analog input, independent of the signal used to select the data
reading. This requirement is particularly important in
applications where low jitter on CNV is desired.
The connection diagram is shown in Figure 40, and the
corresponding timing is given in Figure 41.
CS1
CONVERT
VIO
DIGITAL HOST
CNV
SDI
AD7685
47kΩ
DATA IN
SDO
SCK
IRQ
02968-038
With SDI high, a rising edge on CNV initiates a conversion,
selects the CS mode, and forces SDO to high impedance. In this
mode, CNV must be held high during the conversion phase and
the subsequent data readback (if SDI and CNV are low, SDO is
driven low). Prior to the minimum conversion time, SDI could
be used to select other SPI devices, such as analog multiplexers,
but SDI must be returned low before the minimum conversion
time and held low until the maximum conversion time to
guarantee the generation of the BUSY signal indicator. When
the conversion is complete, SDO goes from high impedance to
low. With a pull-up on the SDO line, this transition can be used
CLK
Figure 40. CS Mode 4-Wire with BUSY Indicator Connection Diagram
tCYC
CNV
ACQUISITION
tCONV
tACQ
CONVERSION
ACQUISITION
tSSDICNV
SDI
tSCK
tHSDICNV
tSCKL
1
2
3
tHSDO
15
16
17
tSCKH
tDSDO
tDIS
tEN
SDO
D15
D14
D1
Figure 41. CS Mode 4-Wire with BUSY Indicator Serial Interface Timing
Rev. B | Page 21 of 28
D0
02968-039
SCK
AD7685
AD7685 enters the acquisition phase and powers down. The
remaining data bits stored in the internal shift register are then
clocked by subsequent SCK falling edges. For each ADC, SDI
feeds the input of the internal shift register and is clocked by the
SCK falling edge. Each ADC in the chain outputs its data MSB
first, and 16 × N clocks are required to readback the N ADCs.
The data is valid on both SCK edges. Although the rising edge
can be used to capture the data, a digital host using the SCK
falling edge will allow a faster reading rate and, consequently,
more AD7685s in the chain, provided the digital host has an
acceptable hold time. The maximum conversion rate may be
reduced due to the total readback time. For instance, with a 5 ns
digital host setup time and 3 V interface, up to eight AD7685s
running at a conversion rate of 220 kSPS can be daisy-chained
on a 3-wire port.
CHAIN MODE, NO BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7685s on a
3-wire serial interface. This feature is useful for reducing
component count and wiring connections, for example, in
isolated multiconverter applications or for systems with a
limited interfacing capacity. Data readback is analogous to
clocking a shift register.
A connection diagram example using two AD7685s is shown in
Figure 42, and the corresponding timing is given in Figure 43.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion and selects the
chain mode. In this mode, CNV is held high during the
conversion phase and the subsequent data readback. When the
conversion is complete, the MSB is output onto SDO and the
CONVERT
SDI
CNV
AD7685
SDO
DIGITAL HOST
AD7685
SDI
A
B
SCK
SCK
SDO
DATA IN
02968-040
CNV
CLK
Figure 42. Chain Mode Connection Diagram
SDIA = 0
tCYC
CNV
tACQ
CONVERSION
ACQUISITION
tSCK
tSCKL
tSSCKCNV
SCK
1
tHSCKCNV
2
3
14
15
tSSDISCK
16
17
18
DA15
DA14
30
31
32
DA1
DA0
tSCKH
tHSDISCK
tEN
SDOA = SDIB
DA15
DA14
DA13
DA1
DA0
DB15
DB14
DB13
DB1
DB0
tHSDO
tDSDO
SDOB
Figure 43. Chain Mode Serial Interface Timing
Rev. B | Page 22 of 28
02968-041
ACQUISITION
tCONV
AD7685
Figure 44) SDO is driven high. This transition on SDO can be
used as a BUSY indicator to trigger the data readback controlled
by the digital host. The AD7685 then enters the acquisition
phase and powers down. The data bits stored in the internal
shift register are then clocked out, MSB first, by subsequent
SCK falling edges. For each ADC, SDI feeds the input of the
internal shift register and is clocked by the SCK falling edge.
Each ADC in the chain outputs its data MSB first, and 16 × N + 1
clocks are required to readback the N ADCs. Although the
rising edge can be used to capture the data, a digital host also
using the SCK falling edge allows a faster reading rate and,
consequently, more AD7685s in the chain, provided the digital
host has an acceptable hold time. For instance, with a 5 ns
digital host setup time and 3 V interface, up to eight AD7685s
running at a conversion rate of 220 kSPS can be daisy-chained
to a single 3-wire port.
CHAIN MODE WITH BUSY INDICATOR
This mode can also be used to daisy chain multiple AD7685s on
a 3-wire serial interface while providing a BUSY indicator. This
feature is useful for reducing component count and wiring
connections, for example, in isolated multiconverter applications or
for systems with a limited interfacing capacity. Data readback is
analogous to clocking a shift register.
A connection diagram example using three AD7685s is shown
in Figure 44, and the corresponding timing is given in Figure 45.
When SDI and CNV are low, SDO is driven low. With SCK
high, a rising edge on CNV initiates a conversion, selects the
chain mode, and enables the BUSY indicator feature. In this
mode, CNV is held high during the conversion phase and the
subsequent data readback. When all ADCs in the chain have
completed their conversions, the nearend ADC (ADC C in
CONVERT
SDI
AD7685
CNV
SDO
SDI
AD7685
DIGITAL HOST
CNV
SDO
SDI
AD7685
A
B
C
SCK
SCK
SCK
DATA IN
SDO
IRQ
02968-042
CNV
CLK
Figure 44. Chain Mode with BUSY Indicator Connection Diagram
tCYC
ACQUISITION
tCONV
tACQ
ACQUISITION
CONVERSION
tSSCKCNV
SCK
tHSCKCNV
tSCKH
1
tEN
SDOA = SDIB
SDOB = SDIC
2
tSSDISCK
3
4
15
16
17
18
19
31
32
33
34
35
tSCKL
tHSDISCK
DA15 DA14 DA13
tDSDOSDI
tSCK
DA1
48
49
tDSDOSDI
DA0
tHSDO
tDSDO
tDSDOSDI
DB15 DB14 DB13
DB1
DB0 DA15 DA14
DA1
DA0
DC15 DC14 DC13
DC1
DC0 DB15 DB14
DB1
DB0 DA15 DA14
tDSDOSDI
SDOC
47
tDSDOSDI
Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing
Rev. B | Page 23 of 28
DA1
DA0
02968-043
CNV = SDIA
AD7685
APPLICATION HINTS
LAYOUT
The printed circuit board (PCB) that houses the AD7685
should be designed so that the analog and digital sections are
separated and confined to certain areas of the board. The
pinout of the AD7685 with all its analog signals on the left side
and all its digital signals on the right side eases this task.
Avoid running digital lines under the device because these
couple noise onto the die, unless a ground plane under the
AD7685 is used as a shield. Fast switching signals, such as CNV
or clocks, should never run near analog signal paths. Crossover
of digital and analog signals should be avoided
The AD7685 voltage reference input REF has a dynamic input
impedance and should be decoupled with minimal parasitic
inductances. This is done by placing the reference decoupling
ceramic capacitor close to, and ideally right up against, the REF
and GND pins and connected with wide, low impedance traces.
02968-044
At least one ground plane should be used. It could be common
or split between the digital and analog section. In the latter case,
the planes should be joined underneath the AD7685.
Figure 46. Example of Layout of the AD7685 (Top Layer)
Finally, the power supplies VDD and VIO should be decoupled
with ceramic capacitors, typically 100 nF, placed close to the
AD7685 and connected using short and wide traces to provide
low impedance paths and to reduce the effect of glitches on the
power supply lines.
An example layout following these rules is shown in Figure 46
and Figure 47.
Other recommended layouts for the AD7690 are outlined in
the documentation of the evaluation board (EVAL-AD7685CB).
The evaluation board package includes a fully assembled and
tested evaluation board, documentation, and software for
controlling the board from a PC via the universal evaluation
control board (EVAL-CONTROL BRD3).
Rev. B | Page 24 of 28
02968-045
EVALUATING THE PERFORMANCE OF THE AD7685
Figure 47. Example of Layout of the AD7685 (Bottom Layer)
AD7685
TRUE 16-BIT ISOLATED APPLICATION EXAMPLE
In applications where high accuracy and isolation are required,
for example, power monitoring, motor control, and some
medical equipment, the circuit given in Figure 48, using the
AD7685 and the ADuM1402C digital isolator, provides a
compact and high performance solution.
Multiple AD7685s are daisy-chained to reduce the number of
signals to isolate. Note that the SCKOUT, which is a readback of
the AD7685’s clock, has a very short skew with the DATA
signal. This skew is the channel-to-channel matching
propagation delay of the digital isolator (tPSKCD). This allows
running the serial interface at the maximum speed of the digital
isolator (45 Mbits/s for the ADuM1402C), which would have
been otherwise limited by the cascade of the propagation delays
of the digital isolator.
The complete analog chain runs on a 5 V single supply using
the ADR391 low dropout reference voltage and the rail-to-rail
CMOS AD8618 amplifier while offering true bipolar input range.
5V REF
5V
10µF
±10V INPUT
4kΩ
100nF
1kΩ
5V
REF VDD VIO
IN+
AD7685
2V REF
IN– GND
5V
100nF
SDO
SCK
CNV
SDI
1/4 AD8618
5V REF
±10V INPUT
VDD2 , VE2
GND1
GND2
VIA
VOA
VIB
VOB
VOC
VIC
VOD
VID
2.7V TO 5V
100nF
DATA
SCKOUT
SCKIN
5V
10µF
4kΩ
VDD1 , VE1
100nF
1kΩ
5V
REF VDD VIO
IN+
AD7685
2V REF
IN– GND
SDO
SCK
CNV
SDI
CONVERT
ADuM1402C
1/4 AD8618
5V REF
5V
10µF
±10V INPUT
4kΩ
100nF
1kΩ
5V
REF VDD VIO
IN+
AD7685
2V REF
IN– GND
SDO
SCK
CNV
SDI
1kΩ
1kΩ
5V
1/4 AD8618
5V REF
5V REF
5V
10µF
1kΩ
5V
REF VDD VIO
IN+
AD7685
2V REF
IN– GND
SDO
SCK
CNV
SDI
ADR391
5V
IN OUT
GND
1kΩ
2V REF
4kΩ
10µF
100nF
02968-046
±10V INPUT
4kΩ
100nF
1/4 AD8618
Figure 48. A True 16-Bit Isolated Simultaneous Sampling Acquisition System
Rev. B | Page 25 of 28
AD7685
OUTLINE DIMENSIONS
3.10
3.00
2.90
10
3.10
3.00
2.90
6
1
5
5.15
4.90
4.65
PIN 1
0.50 BSC
0.95
0.85
0.75
1.10 MAX
0.15
0.05
0.33
0.17
SEATING
PLANE
0.23
0.08
0.80
0.60
0.40
8°
0°
COPLANARITY
0.10
COMPLIANT TO JEDEC STANDARDS MO-187-BA
Figure 49.10-Lead Micro Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
3.00
BSC SQ
PIN 1
INDICATOR
1
10
1.50
BSC SQ
0.50
BSC
(BOT TOM VIEW)
6
0.80
0.75
0.70
SEATING
PLANE
0.80 MAX
0.55 TYP
SIDE VIEW
0.30
0.23
0.18
2.48
2.38
2.23
EXPOSED
PAD
TOP VIEW
0.50
0.40
0.30
5
1.74
1.64
1.49
0.05 MAX
0.02 NOM
PADDLE CONNECTED TO GND.
THIS CONNECTION IS NOT
REQUIRED TO MEET THE
ELECTRICAL PERFORMANCES
0.20 REF
Figure 50. 10-Terminal Quad Flat No Lead Package [QFN (LFCSP_WD)]
3 mm × 3 mm Body
(CP-10-9)
Dimensions shown in millimeters
Rev. B | Page 26 of 28
022207-A
INDEX
ARE A
AD7685
ORDERING GUIDE
Model
AD7685ACPZRL 1
AD7685ACPZRL71
AD7685ARM
AD7685ARMRL7
AD7685ARMZ1
AD7685ARMZRL71
AD7685BCPZRL1
AD7685BCPZRL71
AD7685BRM
AD7685BRMRL7
AD7685BRMZ1
AD7685BRMZRL71
AD7685CCPZRL1
AD7685CCPZRL71
AD7685CRM
AD7685CRMRL7
AD7685CRMZ1
AD7685CRMZRL71
EVAL-AD7685CB 2
EVAL-AD7685CBZ1, 2
EVAL-CONTROL BRD2 3
EVAL-CONTROL BRD33
1
2
3
Integral
Nonlinearity
±6 LSB max
±6 LSB max
±6 LSB max
±6 LSB max
±6 LSB max
±6 LSB max
±3 LSB max
±3 LSB max
±3 LSB max
±3 LSB max
±3 LSB max
±3 LSB max
±2 LSB max
±2 LSB max
±2 LSB max
±2 LSB max
±2 LSB max
±2 LSB max
No Missing
Code
15 Bits
15 Bits
15 Bits
15 Bits
15 Bits
15 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
16 Bits
Temperature
Range
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
Package Description
10-Lead QFN (LFCSP_WD)
10-Lead QFN (LFCSP_WD)
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
10-Lead QFN (LFCSP_WD)
10-Lead QFN (LFCSP_WD)
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
10-Lead QFN (LFCSP_WD)
10-Lead QFN (LFCSP_WD)
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
10-Lead MSOP
Evaluation Board
Evaluation Board
Controller Board
Controller Board
Package
Option
CP-10-9
CP-10-9
RM-10
RM-10
RM-10
RM-10
CP-10-9
CP-10-9
RM-10
RM-10
RM-10
RM-10
CP-10-9
CP-10-9
RM-10
RM-10
RM-10
RM-10
Branding
C4H
C4H
C37
C37
C4H
C4H
C3D
C3D
C01
C01
C3D
C3D
C4J
C4J
C00
C00
C4J
C4J
Z = RoHS Compliant Part.
This board can be used as a standalone evaluation board or in conjunction with the EVAL-CONTROL BRDx for evaluation/demonstration purposes.
These boards allow a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators.
Rev. B | Page 27 of 28
Ordering
Quantity
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
Tube, 50
Reel, 1,000
AD7685
NOTES
©2004–2007 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D02968-0-3/07(B)
Rev. B | Page 28 of 28
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