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DAC161S997
SNAS621A – JUNE 2013 – REVISED DECEMBER 2014
DAC161S997 16-Bit SPI-Programmable DAC for 4-20 mA Loops
1 Features
3 Description
•
•
•
•
•
•
•
•
The DAC161S997 is a very low power 16-bit ΣΔ
digital-to-analog converter (DAC) for transmitting an
analog output current over an industry standard 4-20
mA current loop. The DAC161S997 has a simple 4wire SPI for data transfer and configuration of the
DAC functions. To reduce power and component
count in compact loop-powered applications, the
DAC161S997 contains an internal ultra-low power
voltage reference and an internal oscillator. The low
power consumption of the DAC161S997 results in
additional current being available for the remaining
portion of the system. The loop drive of the
DAC161S997 interfaces to a Highway Addressable
Remote Transducer (HART) modulator, allowing
injection of FSK modulated digital data into the 4-20
mA current loop. This combination of specifications
and features makes the DAC161S997 ideal for 2- and
4-wire industrial transmitters. The DAC161S997 is
available in a 16-pin 4-mm × 4-mm WQFN package
and is specified over the extended industrial
temperature range of –40°C to +105°C.
1
16-Bit Resolution
Very-Low Supply Current of 100 µA
5 ppmFS/°C Gain Error
Pin-Programmable Power-Up Condition
Loop-Error Detection and Reporting
Programmable Output-Current Error Levels
Simple HART Modulator Interfacing
Highly Integrated Feature Set in Small Footprint
WQFN-16 (4- × 4-mm, 0.5-mm Pitch)
2 Applications
•
•
•
•
Two-Wire 4- to 20-mA Current-Loop Transmitter
Loop-Power Transmitters
Industrial Process Control
Actuator Control
Device Information(1)
PART NUMBER
DAC161S997
PACKAGE
WQFN (16)
BODY SIZE (NOM)
4.00 mm x 4.00 mm
(1) For all available packages, see the orderable addendum at
the end of the datasheet.
4 Simplified Schematic
LOOP+
LDO
VA
VD
Internal
Reference
SCLK
SPI
CSB
SDI
ÐÂ DAC
16
4-20 mA
Loop
+
IDAC
BASE
COMD
SDO
VD
COMA
80k
ERRB
DAC161S997
40
OUT
LOOP-
NC
C1
C2
C3
ERRLVL
HART
Modulator
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.
DAC161S997
SNAS621A – JUNE 2013 – REVISED DECEMBER 2014
www.ti.com
Table of Contents
1
2
3
4
5
6
7
8
Features ..................................................................
Applications ...........................................................
Description .............................................................
Simplified Schematic.............................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
1
2
3
4
7.1
7.2
7.3
7.4
7.5
7.6
7.7
4
4
4
4
5
6
7
Absolute Maximum Ratings ......................................
ESD Ratings ............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Timing Requirements ...............................................
Typical Characteristics ..............................................
Detailed Description .............................................. 8
8.1 Overview ................................................................... 8
8.2 Functional Block Diagram ......................................... 8
8.3
8.4
8.5
8.6
9
Feature Description................................................... 8
Device Functional Modes.......................................... 9
Programming .......................................................... 10
Register Maps ......................................................... 13
Application and Implementation ........................ 17
9.1 Application Information............................................ 17
9.2 Typical Application ................................................. 23
10 Power Supply Recommendations ..................... 29
11 Layout................................................................... 29
11.1 Layout Guidelines ................................................. 29
11.2 Layout Example .................................................... 29
12 Device and Documentation Support ................. 31
12.1 Trademarks ........................................................... 31
12.2 Electrostatic Discharge Caution ............................ 31
12.3 Glossary ................................................................ 31
13 Mechanical, Packaging, and Orderable
Information ........................................................... 31
5 Revision History
Changes from Original ( June 2013) to Revision A
•
2
Page
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section ................................................................................................. 1
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6 Pin Configuration and Functions
COMA
1
COMD
2
BASE
VA
C1
C2
16
15
14
13
RGH Package
16-Pin WQFN
(Top View)
12
C3
11
NC
DAP=COMA
SDO
OUT
8
9
7
4
ERRB
SCLK
6
ERRLVL
CSB
10
5
3
SDI
VD
Pin Functions
PIN
TYPE (1)
DESCRIPTION
NAME
NO.
BASE
16
A
External NPN base drive
COMA
1
P
Analog-block negative supply rail (local COMMON)
COMD
2
P
Digital-block negative supply rail (local COMMON)
CSB
6
I
SPI chip select
C1
14
A
External capacitor
C2
13
A
External capacitor, HART input
C3
12
A
External capacitor
EERB
7
O
Error flag output, open drain, active LOW
ERRLVL
10
I
Sets output-current level at power up and under-error conditions.
NC
11
–
Do not connect to this pin.
OUT
9
A
Loop output current source output
SCLK
4
I
SPI clock input
SDI
5
I
SPI data input
SDO
8
O
SPI data output
VA
15
P
Analog-block positive supply rail
3
P
Digital-block positive supply rail.
DAP
P
Die attach pad. Connect directly to local COMMON (COMA, COMD).
VD
DAP
(1)
G = Ground, I = Digital Input, O = Digital Output, P = Power, A = Analog
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SNAS621A – JUNE 2013 – REVISED DECEMBER 2014
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7 Specifications
7.1 Absolute Maximum Ratings (1)
Supply voltage (VA, VD to COMA, COMD)
MIN
MAX
UNIT
–0.3
6
V
Voltage between any two pins (2)
6
V
Current IN or OUT of any pin — except OUT pin (3)
5
mA
Output current at OUT
50
mA
Junction Temperature
150
°C
Operating Temperature
–40
105
°C
Storage temperature, Tstg
–65
150
°C
(1)
(2)
(3)
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating
Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
All voltages are measured with respect to COMA = COMD = 0 V, unless otherwise specified.
When the input voltage (VIN) at any pin exceeds power supplies (VIN < COMA or VIN > VA), the current at that pin must not exceed 5
mA, and the voltage (VIN) at that pin relative to any other pin must not exceed 6 V. See for Pin Descriptions for additional details of
input structures.
7.2 ESD Ratings
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001
(1) (2)
VALUE
UNIT
2000
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
The Human Body Model (HBM) is a 100 pF capacitor charged to the specified voltage then discharged through a 1.5 kΩ resistor into
each pin.
7.3 Recommended Operating Conditions
MIN
MAX
0
15
pF
0
V
BASE load to COMA
(COMA - COMD)
OUT load to COMA
UNIT
none
(VA - VD)
0
V
VA, VDD
Supply voltage
2.7
3.6
V
TA
Temperature
–40
105
°C
7.4 Thermal Information
DAC161S997
WQFN
UNIT
16 PINS
RθJA
(1)
4
Package thermal impedance (1)
35
°C/W
The package thermal impedance is calculated in accordance with JESD 51-7.
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7.5 Electrical Characteristics
Unless otherwise noted, these specifications apply for VA = VD = 3.3 V, COMA = COMD = 0 V, TA= 25°C, external bipolar
transistor: 2N3904, RE = 22 Ω, C1 = C2 = C3 = 2.2 nF.
PARAMETER
TEST CONDITIONS
MIN (1)
TYP
MAX (1)
UNIT
POWER SUPPLY
VA, VD
Supply voltage
VA = VD
-40°C ≤ TA ≤ 105°C
VA supply current
DACCODE = 0x0200 (2)
2.7
VD supply current
ICC
Total supply current
3.6
V
43
µA
57
µA
100
125
µA
DC ACCURACY
N
Resolution
16
(3)
0x2AAA < DACCODE < 0xD555
(4 mA < ILOOP < 20 mA)
INL
Integral non-linearity
DNL
Differential non-linearity
see
TUE
Total unadjusted error
0x2AAA < DACCODE < 0xD555
OE
Offset error
see
ΔOE
Offset error temperature coefficient
-40°C ≤ TA ≤ 105°C
GE
Gain error
see
ΔGE
Gain error temperature coefficient
-40°C ≤ TA ≤ 105°C
IERRL
LOW ERROR current
ERR_LOW = default
3.36
3.375
3.39
mA
IERRH
HIGH ERROR current
ERR_HIGH = default
21.70
21.75
21.82
mA
LTD
Long term drift — mean shift of 12
mA output current after 1000 hours
at 150°C
(4)
(5)
–1.5
bits
2.6
–0.2
0.2
0.01
–7.86
(6)
µA
0.84
µA
%FS
7.86
µA
0.48
ppmFS/°C
0.007
%FS
5
ppmFS/°C
90
ppmFS
LOOP CURRENT OUTPUT (OUT)
IOUTMIN
Minimum output current
Tested at DACCODE = 0x01C2 (7)
IOUTMAX
Maximum output current
Tested at DACCODE = 0xFFFF
ROUT
Output impedance
COMA to OUT voltage drop
0.19
mA
23.95
IOUT = 24 mA
mA
200
MΩ
960
mV
10
mA
BASE OUTPUT
IOUTSC
BASE short circuit output current
BASE forced to COMA potential
DYNAMIC CHARACTERISTICS
Output noise density
1 kHz
20
nA/rtHz
Integrated output noise
1 Hz to 1 kHz band
300
nARMS
Default setting of TIMEOUT in
CONFIG register
100
ms
INTERNAL TIMER
TM
Timeout period
DIGITAL INPUT CHARACTERISTICS
IIN
Digital input leakage current
VIL
Input low voltage
VIH
Input high voltage
CIN
Input capacitance
(1)
(2)
(3)
(4)
(5)
(6)
(7)
–10
10
µA
0.2 × VD
V
0.7 × VD
V
5
pF
Limits are ensured by testing, design, or statistical analysis at 25°C. Limits over the operating temperature range are ensured through
correlations using statistical quality control (SQC) method.
At code 0x0200 the BASE current is minimal, for example, device current contribution to power consumption is minimized. SPI is
inactive, for example, after transmitting code 0x200 to the DAC161S997, there are no more transitions in the channel during the supply
current measurement.
INL is measured using the best-fit method in the output current range of 4 mA to 20 mA.
Specified by design.
Offset is the y-intercept of the straight line defined by 4 mA and 20 mA points of the measured transfer characteristic.
Gain Error is the difference in slope of the straight line defined by measured 4 mA and 20 mA points of transfer characteristic, and that
of the ideal characteristic.
This must be treated as the minimum LOOP current ensured in self-powered mode.
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Electrical Characteristics (continued)
Unless otherwise noted, these specifications apply for VA = VD = 3.3 V, COMA = COMD = 0 V, TA= 25°C, external bipolar
transistor: 2N3904, RE = 22 Ω, C1 = C2 = C3 = 2.2 nF.
PARAMETER
TEST CONDITIONS
MIN (1)
MAX (1)
TYP
UNIT
DIGITAL OUTPUT CHARACTERISTICS
VOL
Output Low voltage
Isink = 200 μA
VOH
Output HIGH voltage
Isink = 200 μA
IOZH, IOZL
TRI-STATE leakage current
COUT
TRI-STATE output capacitance
0.4
V
10
µA
2.6
V
–10
5
pF
7.6 Timing Requirements
MIN
NOM
0
MAX
UNIT
10
MHz
fCLK
SCLK frequency
tH
SCLK high time
0.4 / FCLK
50
ns
tL
SCLK low time
0.4 / FCLK
50
ns
tCSB
CSB pulse width
5
40
ns
tCSS
CSB set-up time prior to SCLK rising edge
5
ns
tSCH
24th rising edge of SCLK to CSB rising edge
15
ns
tCSH
CSB hold time after the 24th falling edge of SCLK
tZSDO
CSB falling edge to SDO valid
10
tSDOZ
CSB rising edge to SDO HiZ
10
tDS
SDI data set-up time prior to SCLK rising edge
tDH
SDI data hold time after SCLK rising edge
tDO
SDO output data valid
6
10
ns
35
ns
10
6
ns
ns
10
ns
30
ns
Figure 1. SPI Timing Diagrams
6
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7.7 Typical Characteristics
Unless otherwise noted, data presented here was collected under these conditions VA = VD = 3.3 V, TA = 25°C, external
bipolar transistor: 2N3904, RE = 22 Ω, C1 = C2 = C3 = 2.2 nF.
0
Integration BW=1kHz
Integration BW=10kHz
MAGNITUDE RESPONSE (dB)
OUTPUT CURRENT RIPPLE A(rms)
6
5
4
3
2
1
0
-10
-20
-30
-40
-50
C1=C2=C3=2.2nF
HART Adaptation
C1=C2=C3=1nF
-60
-70
-80
0
4
8
12
16
20
OUTPUT CURRENT (mA)
24
Figure 2. Intergrated Noise vs ILOOP
1
10
100
1k
10k
FREQUENCY (Hz)
100k
Figure 3. ΣΔ Modulator Filter Response
1M
2.5
2.0
1.5
1.0
10k
INL ( A)
SETTLING TIME ( s)
100k
1k
100
0.0
-0.5
-1.0
10
-1.5
C1=C2=C3=2.2nF
HART Adaptation
C1=C2=C3=1nF
1
1
10
100
1k
10k
INPUT CODE STEP (lsb)
-2.0
-2.5
100k
Figure 4. Settling Time vs Input Step Size
120
-40 -20
0 20 40 60 80 100 120
TEMPERATURE (°C)
Figure 5. Output Linearity vs Temperature
120
C1=C2=C3=1nF
C1=C2=C3=2.2nF
C1=C2=C3=10nF
C1=C2=C3=100nF
100
C1=C2=C3=1nF
C1=C2=C3=2.2nF
C1=C2=C3=10nF
C1=C2=C3=100nF
100
80
PSRR (dB)
PSRR (dB)
Min INL
Max INL
0.5
60
80
60
40
40
20
20
0
0
1
10
100
1k
10k 100k
FREQUENCY (Hz)
1M
Figure 6. PSRR: ILOOP = 4 mA
1
10
100
1k
10k 100k
FREQUENCY (Hz)
1M
Figure 7. PSRR: ILOOP = 20 mA
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8 Detailed Description
8.1 Overview
The DAC161S997 is a 16-bit DAC realized as a ∑Δ modulator. The DAC’s output is a current pulse train that is
filtered by the on-board low pass RC filter. The final output current is a multiplied copy of the filtered modulator
output. This architecture ensures an excellent linearity performance, while minimizing power consumption of the
device.
The DAC161S997 eases the design of robust, precise, long-term stable industrial systems by integrating all
precision elements on-chip. Only a few external components are needed to realize a low-power, high-precision
industrial 4 - 20 mA transmitter.
In case of a fault, or during initial power-up the DAC161S997 will output current in either upper or lower error
current band. The choice of band is user selectable via a device pin. The error current value is user
programmable via SPI.
8.2 Functional Block Diagram
In d u s tri a l 4 -2 0 mA Tra n smi tte r
LDO
VD
VA
Internal
Reference
SCLK
SDI
IN
SPI
ÐÂ DAC
SPI
Sensor
LOOP+
SDO
16
+
IDAC
LOOP
SUPPLY
0-24 mA Loop
+
-
BASE
-
µC
COMD
CSB
VD
80k
INT
LOOP
RECEIVER
COMA
ERRB
40
DAC161S997
NC
C1
C2
OUT
C3
LOOP-
ERRLVL
HART Modulator
8.3 Feature Description
8.3.1 Error Detection And Reporting
By default, the DAC161S997 detects and reports several types of errors.
8.3.1.1 Loop Error
A loop error occurs when the device is unable to sustain the required output current at OUT pin, typically caused
by a drop in loop supply, or an increase in load impedance.
When a loop error occurs, the DAC161S997 changes the OUT-pin current to the value in the ERROR_LOW
register, unless the MASK_LOOP_ERR is set to 1. If the MASK_LOOP_ERR is not set, then the device also
periodically attempts to reassert the OUT current set in DACCODE by default. If the DACCODE-current output is
set, the DAC161S997 then stops reporting a loop error. The interval between reasserts is controlled by the
L_RETRY_TIME field in the ERROR_CONFIG register. If the DIS_RETRY_LOOP field in the ERROR_CONFIG
register is changed to 1, the device does not periodically check the loop and, instead, only checks the loop after
a read of the ERR_STATUS (0x09) register. If the loop error is not resolved, then the loop-error current persists.
8
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Feature Description (continued)
When a loop error occurs, the DAC161S997 sets the CURR_LOOP_STATUS and LOOP_STATUS fields in the
STATUS register to 1. The LOOP_STATUS field remains set to 1 until the STATUS register is read or the device
is reset. If the loop error is cleared, either by the device reasserting the loop current or by changing the OUT
current , then the CURR_LOOP_STATUS field clears.
8.3.1.2 SPI Timeout Error (Channel Error)
The DAC161S997 expects to receive periodic SPI write commands to ensure that the SPI connection is
functioning normally. If no SPI write command occurs within the time indicated by the SPI_TIMEOUT field in the
ERROR_CONFIG register, the device reports a SPI timeout error. Note that the SPI write command must be
properly formatted to avoid SPI Timeout errors (such as a write command that generates a frame error does not
prevent an imminent SPI Timeout error).
SPI Timeout error reporting is inhibited by MASK_SPI_TOUT. SPI Timeout errors are not reported on the loop if
MASK_SPI_ERR is set to 1.
Note that a write command to address 0 is not considered a valid write command and will not prevent a SPI
Timeout error.
8.3.1.3 Frame Error
If a SPI write command has an incorrect number of SCLK pulses, the device reports a frame error. The number
of SCLK pulses must be an integer and a multiple of 24. A frame error is always reported by ERRB being pulled
low. A frame error does not affect the loop current.
8.3.1.4 Alarm Current
By default, the DAC161S997 reports faults to the plant controller by forcing the OUT current into one of two error
bands. The error current bands are defined as either greater than 20 mA, or less than 4 mA. Loop errors are
reported by setting current of ERR_LOW.
If SPI Timeout Errors are reported on the loop (this is the default; it can be changed by setting the register
ERR_CONFIG:MASK_LOOP_ERR), the error band is controlled by the ERRLVL pin. When ERRLVL is tied to
the COMD voltage, the ERR_LOW current is the reporting current. If ERRLVL is tied to VD then the ERR_HIGH
current is the current-on pin, OUT, if a SPI timeout error occurs.
The exact value of the output current used to indicate fault is dictated by the contents of ERR_HIGH and
ERR_LOW registers.
In the case of a conflicting alarm-current setting (such as a loop error and SPI timeout error occurring
simultaneously and ERRLVL is tied high), the current-on pin, OUT, is determined by ERR_LOW current.
8.4 Device Functional Modes
The DAC161S997 reports errors in 3 different ways, by changing the OUT pin current, pulling the ERRB pin low,
and by updating the read-only register STATUS. The reporting on ERRB and OUT pin is customized by setting
the ERROR_CONFIG register.
The ERRB pin connects to a GPIO pin on the microcontroller to function as an interrupt if an error occurs.
If a Loop error and a SPI Timeout error occur simultaneously and the device is configured with conflicting error
output currents, the OUT pin current reports the Loop Error.
Frame Error
Loop Error
SPI Timeout Error
STATUS Register
Loop Reporting
Reported in FERR_STS
Not reported
Always reported
Reported in LOOP_STS and
CURR_LOOP_STS
Reported by default unless
ERR_CONFIG:MASK_LOOP_E
RR is set to 1
Reported by default unless
ERR_CONFIG:DIS_LOOP_ERR_ERRB
is set to 1
Reported in SPI_TIMEOUT_ERR
ERRB Reporting
Reported by default unless either
ERR_CONFIG:MASK_SPI_ERR
Reported by default unless
or
ERR_CONFIG:MASK_SPI_TOUT is set
ERR_CONFIG:MASK_SPI_TOU
to 1
T are set to 1
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8.5 Programming
8.5.1 Serial Interface
The 4-wire interface is compatible with SPI, QSPI, and MICROWIRE, as well as most DSPs. See the Timing
Requirements section for timing information about the read and write sequences. The serial interface is
comprised of CSB, SCLK, SDIs and SDO. The DAC161S997 supports both Mode 0 and Mode 3 of the SPI
protocol.
A bus transaction is initiated by the falling edge of CSB. When CSB is low, the input data is sampled at the SDI
pin by the rising edge of the SCLK. The output data is asserted on the SDO pin at the falling edge of SCLK.
A valid transfer requires an integer multiple of 24 SCLK cycles. If CSB is raised before the 24th rising edge of the
SCLK, the transfer aborts and a Frame Error is reported. If CSB is held low after the 24th falling edge of the
SCLK and additional SCLK edges occur, the data continues to flow through the FIFO and out the SDO pin.
When CSB transitions high, the internal controller decodes the most recent 24 bits that were received before the
rising edge of CSB. CSB must transition to high after an integer multiple of 24 clock cycles, otherwise a Frame
Error is reported and the transaction is considered invalid. When a valid number of SCLK pulses occur with CSB
low, the DAC then performs the requested operation after CSB transitions high.
Figure 8. SPI Data Format
The acquired data is shifted into an internal 24-bit shift register (MSB first) which is configured as a 24-bit deep
FIFO. As the data is being shifted into the FIFO via the SDI pin, the prior contents of the register are being
shifted out through the SDO output. While CSB is high, SDO is in a high Z-state. At the falling edge of CSB, SDO
presents the MSB of the data present in the shift register. SDO is updated on every subsequent falling edge of
SCLK.
NOTE
The first SDO transition will happen on the first falling edge AFTER the first rising edge of
SCLK when CSB is low.
The 24 bits of data contained in the FIFO are interpreted as an 8-bit COMMAND word followed by 16-bits of
DATA. The general format of the 24-bit data stream is shown in Figure 9. Complete instruction set is tabulated in
the Detailed Description section.
Figure 9. SPI Command and Data Words
10
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Programming (continued)
8.5.1.1 SPI Write
SPI write operation is used to change the state of the device. Handshaking does not occur between the master
and the slave (DAC161S997), and the master must control the communication on the following inputs: SCLK,
CSB, SDI. The format of the data transfer is described in the Serial Interface section.
A write is composed of two sections, 8-bits corresponding to a command and 16-bits of data. A command is
simply the address of the desired register to update. Note that some registers are read-only; a write to these
registers will have no effect on the device operation and the register contents will not change. The user
instruction set is shown in the Detailed Description section.
During power up or device reset, the register contents of all writable registers are set to the listed values in the
Detailed Description section.
If the DAC161S997 is used in a highly noisy environment in which SPI errors are potentially an issue, the
DAC161S997 supports a more robust protocol (see Optional Protected SPI Writes).
8.5.1.2 SPI Read
The read operation requires all 4 wires of the SPI interface, which are SCLK, SCB, SDI, and SDO. The simplest
READ operation occurs automatically during any valid transaction on the SPI bus because the SDO pin of
DAC161S997 always shifts out the contents of the internal FIFO. Therefore the data being shifted in to the FIFO
is verified by initiating another transaction and acquiring data at SDO, allowing only for the verification of FIFO
contents.
The internal registers are accessed by the user through a register read command. A register read command is
formed by setting bit 7 of the command to 1 (effectively ORing with 0x80) with the address of the desired register
to be read and sending the resulting 8 bits as the command (see Detailed Description). For example, the register
read command of the STATUS register (address 0x05) would be 0x85.
A register read requires two SPI transactions to recover the register data. The first transaction shifts in the
register read command; an 8-bits of command byte followed by 16-bits of dummy data. The register read
command transfers the contents of the internal register into the FIFO. The second transaction shifts out the FIFO
contents; an 8-bit command byte (which is a copy of previous transaction) followed by the register data. The
Register Read operation is shown in Figure 10.
Figure 10. SPI Register Read Operation
8.5.1.3 Optional Protected SPI Writes
The DAC161S997 supports an optional SPI protocol intended to provide robust support against SPI write errors.
When PROTECT_REG_WR is set to 1, all register writes require a subsequent XFER_REG command (a write of
0x00FF to XFER_REG[0x01]) to load the transferred data into the register address (see Figure 11). This
requirement provides protection against write errors in an electrically noisy environment.
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Programming (continued)
SPI Write n
Register Load
CSB
SCLK
1 2
SDI
8
9
Addr n
SDO
24
Data n
Prior Addr
1 2
8
9
XFER_REG
Prior Data
24
0x00FF
Addr n
Data n
Figure 11. Protected SPI writes
8.5.1.3.1 SPI Write Error Correction
To minimize the chance of a SPI write error, TI recommends to append a NOP command onto the end of every
register write sequence to verify that the XFER_REG is properly executed, as shown in Figure 12.
SPI Write n
Register Load
NOP
CSB
SCLK
1 2
8
9
24
1 2
8
9
24
1 2
8
9
24
SDI
Addr n
Data n
XFER_REG
0x00FF
NOP
0xXXXX
SDO
Prior Addr
Prior Data
Addr n
Data n
XFER_REG
0x00FF
Figure 12. Protected SPI writes with NOP command
The XFER_REG command combined with the automatic SDO loopback of the previous SPI write data prevents
loading of incorrect data into a register. If the loopback indicates a communication error has occurred (see
Figure 13), the CSB pin is held low and the previous write command is repeated. Although the second SPI
transaction had 48 SCLK pulses instead of 24 pulses, this is not considered a frame error. A frame error is
indicated when the number of SCLK pulses is not an integer multiple of 24.
12
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Programming (continued)
Resend Register Load
Data OK, pull CSB high to complete
operation
Register Load ± Error Detected
SPI Write n
Repeat Write n
NOP
Do Not pull CSB high
CSB
SCLK
1 2
8
SDI
Addr n
SDO
Prior Addr
9
1 2
24
Data n
8
9
XFER_REG
Communication Error
24
25 26
0x00FF
1 2
48
8
9
24
1 2
8
9
24
Addr n
Data n
XFER_REG
0x00FF
NOP
0xXXXX
XFER_REG
0X00FF
Addr n
Data n
XFER_REG
0x00FF
Error reported to MCU
Addr n
Prior Data
Data n
Figure 13. Detection of Error in Register Load
If a communication error occurs in the XFER_REG command, it is detected during the trailing NOP command
(see Figure 14). Although the register load is incomplete, the device has not changed operations. Repeat the
original data and XFER_REG command.
SPI Write n
Register Load
NOP
SPI Write n
Register Load
NOP
CSB
SCLK
1 2
SDI
8
Addr n
9
24
Data n
1 2
8
XFER_REG
9
24
0x00FF
Communication Error
SDO
Prior Addr
Prior Data
Addr n
Data n
1 2
8
Addr n
9
24
Data n
1 2
8
Addr n
9
24
Data n
1 2
8
9
XFER_REG
24
0x00FF
1 2
8
NOP
9
24
0xXXXX
Error reported to MCU
XFER_REG?
0x00FF?
Prior Addr
Prior Data
Addr n
Data n
XFER_REG
0x00FF
Figure 14. Detection of Error in Register Readback
8.6 Register Maps
Unless otherwise indicated, bits outside the register fields listed below are do not care, and will not change
device configuration. Register read operations on such do not care fields will be 0. Registers are read/write
unless indicated otherwise.
Table 1. XFER_REG (Write Only)
Address = 0x01
Bit Field
15:0
Field Name
Description
XFER[15:0]
When PROTECT_REG_WR is set to 1, then a XFER_REG command is necessary to
transfer the previous register write data into the appropriate address. Set this register
to 0x00FF to perform a XFER_REG command.
Table 2. NOP
Address = 0x02
Bit Field
Field Name
15:0
NOP[15:0]
Description
No Operation. A write to this register will not change any device configuration.
This command indicates that the SPI connection is functioning and is used to avoid
SPI_INACTIVE errors.
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Table 3. WR_MODE
Address = 0x03; Default = 0x0000
Bit Field
Field Name
Description
0: Register write data transfers to appropriate address immediately after CSB goes
high. Default value.
0
PROTECT_REG_WR
1: Enable protected register transfers: all register writes require a subsequent
XFER_REG command to finalize the loading of register data. Refer to Optional
Protected SPI Writes.
Table 4. DACCODE
Address = 0x04; Default = 0x2400, 0xE800
Bit Field
15:0
Field Name
DACCODE[15:0]
Description
16-bit natural binary word, where D15 is the MSB, which indicates the desired DAC
output code.
Note the default value of this register is based on the state of the ERR_LVL pin during
startup or reset.
Table 5. ERR_CONFIG
Address = 0x05; Default = 0x0102
Bit Field
10:8
Field Name
L_RETRY_TIME[2:0]
Description
L_RETRY_TIME sets the time interval between successive attempts to reassert the
desired DACCODE output current when a loop error is present. This has no effect if
either MASK_LOOP_ERR is set to 1 or if DIS_RETRY_LOOP is set to 1.
LOOP Retry time = (L_RETRY_TIME + 1) × 50 ms
Default value = 1 (100 ms)
7
DIS_RETRY_LOOP
0: When a loop error is occurring, periodically attempt to send desired DACCODE
output current instead of the set ERR_LOW current. The interval between attempts is
set by L_RETRY_TIMER. Default value.
1: Do not periodically reassert DACCODE output when a loop error is present;
reassert DACCODE after STATUS Register is read out.
6
MASK_LOOP_ERR
5
DIS_LOOP_ERR_ERRB
0: When a LOOP error is detected the DAC161S997 outputs the current indicated by
ERR_LOW instead of DACCCODE. Default value.
1: When a Loop Error is detected the DAC161S997 tries to maintain DACCODE
current on pin OUT.
0: When a LOOP error is detected the DAC161S997 drives ERRB pin low. Default
value.
1: When a LOOP error is detected the DAC161S997 does not drive ERRB pin low.
4
MASK_SPI_ERR
0: SPI timeout errors change the OUT pin current to an error value, which is
determined by ERRLVL pin and contents of ERR_LOW or ERR_HIGH. Note:
MASK_SPI_TOUT must be set to 0 for this to be reported. Default value.
1: SPI timeout errors do not change the OUT pin current to an error value.
SPI_TIMEOUT sets the time interval for SPI timeout error reporting. After each SPI
write command, an internal timer is reset; if no subsequent write occurs before the
timer reaches SPI timeout, a SPI timeout error is reported. SPI_ERROR reporting is
inhibited by setting MASK_SPI_TOUT.
3:1
SPI_TIMEOUT[2:0]
A NOP write is considered a valid write and resets the timer without changing the
device configuration.
SPI Timeout = (SPI_TIMEOUT + 1) × 50 ms
SPI_TIMEOUT default value = 1 (100 ms)
0
MASK_SPI_TOUT
0: SPI timeout error reporting is enabled. A SPI timeout error drives ERRB low when a
SPI Timeout error occurs. Default value.
1: SPI timeout error reporting is inhibited.
14
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Table 6. ERR_LOW
Address = 0x06; Default = 0x2400
Bit Field
Field Name
Description
Under some error conditions the output current corresponding to this value is the DAC
output, regardless of the value of DACCODE. The ERR_LOW value is used as the
upper byte of the DACCODE, while the lower byte is forced to 0x00.
15:8
ERR_LOW[7:0]
ERR_LOW must be between 0x00(0 mA) and 0x80(12 mA). The DAC161S997
ignores any value outside of that range and retains the previous value in the register.
Refer to the Error Detection And Reporting section for additional details.
The default value is 0x24, which corresponds to approximately 3.37 mA on pin OUT.
Table 7. ERR_HIGH
Address = 0x07; Default = 0xE800
Bit Field
Field Name
Description
Under some error conditions the output current corresponding to this value is the DAC
output, regardless of the value of DACCODE. The ERR_HIGH value is used as the
upper byte of the DACCODE, while the lower byte is forced to 0x00.
15:8
ERR_HIGH[7:0]
ERR_HIGH must be greater than or equal to 0x80 (12 mA). The DAC161S997 ignores
any value below 0x80 and retains the previous value in the register. Refer to the Error
Detection And Reporting section for additional details.
The default value is 0xE8, which corresponds to approximately 21.8 mA on pin OUT.
Table 8. RESET
Address = 0x08
Bit Field
15:0
Field Name
RESET[15:0]
Description
Write 0xC33C to the RESET register followed by a NOP to reset the device. All
writable registers are returned to default values.
Table 9. STATUS (Read-Only)
Address = 0x09 or 0x7F
Bit Field
Field Name
7:5
DAC_RES[2:0]
4
ERRLVL_PIN
Description
DAC resolution
On DAC161S997, returns a 111.
Returns the state of the ERRLVL pin:
1 = ERRLVL pin is tied HIGH
0 = ERRLVL pin is tied LOW
Frame-error status sticky bit
1 = A frame error has occurred since the last STATUS read.
3
FERR_STS
0 = No frame error occurred since the last STATUS read.
This error is cleared by reading the STATUS register. A frame error is caused by an
incorrect number of clocks during a register write. A register write without an integer
multiple of 24 clock cycles will cause a Frame error.
SPI time out error
1 = The SPI interface has not received a valid command within the interval set by
SPI_TIMEOUT.
2
SPI_TIMEOUT_ERR
0 = The SPI interface has received a valid command within the interval set by
SPI_TIMEOUT
If this error occurs, it is cleared with a properly formatted write command to a valid
address.
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Table 9. STATUS (Read-Only) (continued)
Address = 0x09 or 0x7F
Bit Field
Field Name
Description
Loop status sticky bit
1 = A loop error has occurred since last read of STATUS.
1
LOOP_STS
0 = No loop error has occurred since last read of STATUS.
Returns the loop error status. When the value in this register is 1, the DAC161S997 is
unable to maintain the output current set by DACCODE at some point since the last
STATUS read. This indicator clears after reading the STATUS register.
Current loop status
1 = A loop error is occurring.
0
CURR_LOOP_STS
0 = No loop error is occurring.
Returns the current Loop error status. When the value in this register is 1, the
DAC161S997 is unable to maintain the output current set by DACCODE.
16
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9 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.
9.1 Application Information
9.1.1 16-bit Dac And Loop Drive
9.1.1.1 DC Characteristics
The DAC converts the 16-bit input code in the DACCODE registers to an equivalent current output. The ΣΔ DAC
output is a current pulse which is then filtered by a third-order RC lowpass filter and boosted to produce the loop
current (ILOOP) at the device OUT pin.
LOOP+
VD
IAUX
VA
DAC
+
BASE
ILOOP
IA
-
ID
+
IDAC
IE
RE
COMA
R1 = 80k
R2 = 40
I2
OUT
DAC161S997
LOOP-
Figure 15. Loop-Powered Transmitter
Figure 15 shows the principle of operation of the DAC161S997 in the Loop-Powered Transmitter (the circuit
details are omitted for clarity). In Figure 15, ID and IA represent supply (quiescent) currents of the internal digital
and analog blocks. IAUX represents supply (quiescent) current of companion devices present in the system, such
as the voltage regulator and the digital interface. Because both the control loop formed by the amplifier and the
bipolar transistor force the voltage across R1 and R2 to be equal, under normal conditions, the ILOOP is dependent
only on IDAC through the following relationship (see Equation 1).
ILOOP = (1 + R1 / R2) IDAC
where
•
IDAC = ƒ(DACCODE)
(1)
Although ILoop has a number of component currents, ILOOP = IDAC + ID + IA + IAUX + IE, only IE is regulated by the
loop to maintain the relationship shown in Equation 1. Because only the magnitude of IE is controlled, not the
direction, there is a lower limit to ILOOP. This limit is dependent on the fixed components IA and ID, and on system
implementation through IAUX.
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Application Information (continued)
LOOP+
VD
+
-
VLOCAL
IAUX
VA
DAC
+
BASE
ILOOP
IA
-
ID
+
IDAC
IE
RE
COMA
R1 = 80k
R2 = 40
I2
OUT
DAC161S997
LOOP-
Figure 16. Self-Powered Transmitter
Figure 16 shows the variant of the transmitter where the local supply provides supply currents to the system
blocks, and not the 4-20mA loop Self-Powered Transmitter. The ame basic relationship between the ILOOP and
IDAC continues, but the component currents of ILOOP are only IDAC and IE.
9.1.1.2 DC Input-Output Transfer Function
The output current sourced by the OUT pin of the device is expressed by Equation 2.
ILOOP = 24 mA (DACCODE / 216)
(2)
The valid DACCODE range is the full 16-bit code space (0x0000 to 0xFFFF), resulting in the IDAC range of 0 to
approximately 12 μA, which, however, does not result in the ILOOP range of 0 to 24 mA. The maximum output
current sourced out of OUT pin, ILOOP, is 24 mA. The minimum output current is dependent on the system
implementation. The minimum output current is the sum of the supply currents of the DAC161S997 internal
blocks, IA, ID, and companion devices present in the system, IAUX. The last component current, IE, is theoretically
controlled down to 0, however, due to the stability considerations of the control loop, not allowing the IE to drop
below 200 μA is advised.
The graph in Figure 17 shows the DC transfer characteristic of the 4-20mA transmitter, including minimum
current limits. The minimum current limit for the Loop-Powered Transmitter is typically around 400 μA (ID+ IA +
IAUX + IE). The minimum current limit for the Self-Powered Transmitter is typically around 200 μA (IE). Typical
values for ID and IA are listed in the Electrical Characteristics table. IE depends on the BJT device used.
18
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Application Information (continued)
24.0
21.5
Programmable IERROR
ILOOP (mA)
full accuracy range
20.0
4.0
3.5
Programmable IERROR
FFFF
E500
D555
2AAA
2500
0222
MIN(ILOOP) ± Self Powered
0444
MIN(ILOOP) ± Loop Powered
0.2
0000
0.4
DACCODE (hex)
Figure 17. DAC-DC Transfer Function
9.1.1.3 Loop Interface
The DAC161S997 cannot directly interface to the typical 4 - 20 mA loop due to the excessive loop supply
voltage. The loop interface has to provide the means of stepping down the LOOP Supply to 3.6V. This can be
accomplished with either a linear regulator (LDO) or switching regulator while keeping in mind that the regulator’s
quiescent current will have direct effect on the minimum achievable ILOOP (see DC Input-Output Transfer
Function).
The second component of the loop interface is the external NPN transistor (BJT). This device is part of the
control circuit that regulates the transmitter’s output current (ILOOP). Since the BJT operates over the wide current
range, spanning at least 4 - 20 mA, it is necessary to degenerate the emitter in order to stabilize transistor’s
transconductance (gm). The degeneration resistor of 22Ω is suggested in typical applications. For circuit details,
see Figure 21.
The NPN BJT should not be replaced with an N-channel FET (Field Effect Transistor) for the following reasons:
discrete FET’s typically have high threshold voltages (VT), in the order of 1.5V to 2V, which is beyond the BASE
output maximum range; discrete FET’s present higher load capacitance which may degrade system stability
margins; and BASE output relies on the BJT’s base current for biasing.
9.1.1.4 Loop Compliance
The maximum V(LOOP+,LOOP-) potential is limited by the choice of step-down regulator, and the external BJT’s
Collector Emitter breakdown voltage. For minimum V(LOOP+, LOOP−) potential consider TROUBLE Figure 16.
Here, observe that V(LOOP+,LOOP−) ≅ min(VCE) + ILOOPRE + ILOOPR2 = min(VCE) + 0.53V + 0.96V = 3.66V, at
ILOOP = 24mA. The voltage drop across internal R2 is specified in Electrical Characteristics.
9.1.1.5 AC Characteristics
The approximate frequency dependent characteristics of the loop drive circuit can be analyzed using the circuit in
Figure 18.
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Application Information (continued)
LOOP+
RX1
DAC161S997
Gm
IAUX
+
+
BASE
A(s)
gm
CX1
CX2
-
-
ro
CX3
IDAC
RE
COMA
R1
R2
CX4
OUT
LOOP-
Figure 18. Capacitances Affecting Control Loop
Here it is assumed that the internal amplifier dominates the frequency response of the system, and it has a single
pole response. The BJT’s response, in the bandwidth of the control loop, is assumed to be frequency
independent and is characterized by the transconductance gm and the output resistance ro.
As in previous sections IDAC and IAUX represent the filtered output of the ∑Δ modulator and the quiescent current
of the companion devices.
The circuit in Figure 18 can be further simplified by omitting the on-board capacitances, whose effect will be
discussed in Stability, and by combining the amplifier, the external transistor and resistor RE into one Gm block.
The resulting circuit is shown in Figure 19.
By assuming that the BJT’s output resistance (ro) is large, the loop current ILOOP can be expressed as:
(3)
LOOP+
ILOOP
A(s)Gmve
A(s)Gm
+
+
IAUX
-
ro
ve
-
IDAC
R1
R2
ILOOP
LOOP-
Figure 19. AC Analysis Model of a Transmitter
The sum of voltage drops around the path containing R1, R2 and ve is:
(4)
An assumption is made on the response of the internal amplifier:
A(s) =
20
Ao&o
s
(5)
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Application Information (continued)
By combining the above the final expression for the ILOOP as a function of 2 inputs IDAC and IAUX is:
ILOOP = IDAC (1 +
AoGmR2&o
R1
s
)
+ IAUX
R2 s + AoGmR2&o
s + AoGmR2&o
20 log (1 +
R1
)
R2
0 dB
&
AoGmR2&o
(6)
The result above reveals that there are 2 distinct paths from the inputs IDAC and IAUX to the output ILOOP. IDAC
follows the low-pass, and the IAUX follows the high-pass path.
In both cases the corner frequency is dependent on the effective transconductance, Gm, of the external
transistor. This implies that control loop dynamics could vary with the output current ILOOP if Gm were allowed to
be just native device transconductance gm. This undesirable behavior is mitigated by the degenerating resistor
RE which stabilizes Gm as follows:
(7)
This results in the frequency response which is largely independent of the output current ILOOP:
R1
)
ILOOP = IDAC (1 +
R2
R2
&
RE o
s
+ IAUX
R2
R2
s + Ao
&o
s + Ao
&o
RE
RE
Ao
(8)
While the bandwidth of the IDAC path may not be of great consequence given the low frequency nature of the 420 mA current loop systems, the location of the pole in the IAUX path directly affects PSRR of the transmitter
circuit. This is further discussed in PSRR .
9.1.1.5.1 Step Response
The transient input-output characteristics of the DAC161S997 are dominated by the response of the RC filter at
the output of the ∑Δ DAC. Settling times due to step input are shown in Typical Characteristics.
9.1.1.5.2 Output Impedance
The output impedance is described as:
(9)
By considering the circuit in Figure 19, and setting IDAC = IAUX = 0, the following expression can be obtained:
(10)
As in AC Characteristics an assumption can be made on the frequency response of the internal amplifier, and
the effective transconductance Gm should be stabilized with external RE leading to:
(11)
The output impedance of the transmitter is a product of the external BJT's output resistance ro, and the frequency
characteristics of the internal amplifier. At low frequencies this results in a large impedance that does not
significantly affect the output current accuracy.
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Application Information (continued)
9.1.1.5.3 PSRR
Power Supply Rejection Ratio is defined as the ability of the current control loop to reject the variations in the
supply current of the companion devices, IAUX. Specifically:
(12)
It was shown in AC Characteristics that the IAUX affects ILOOP via the high-pass path whose corner frequency is
dependent on the effective Gm of the external BJT. If that dependence were not mitigated with the degenerating
resistor RE, the PSRR would be degraded at low output current ILOOP.
The typical PSRR performance of the transmitter shown in Figure 6 is shown in Typical Characteristics.
9.1.1.5.4 Stability
The current control loop's stability is affected by the impedances present in the system. Figure 18 shows the
simplified diagram of the control loop, formed by the on-board amplifier and an external BJT, and the lumped
capacitances CX1 through CX4 that model any other external elements.
CX1 typically represents a local step-down regulator, or LDO, and any other companion devices powered from the
LOOP+. This capacitance reduces the stability margins of the control loop, and therefore it should be limited.
RX1 can be used to isolate CX1 from LOOP+ node and thus remedy the stability margin reduction. If RX1 = 0, CX1
cannot exceed 10 nF. RX1 = 200Ω is recommended if it can be tolerated. Minimum RX1 = 40Ω if CX1 exceeds 10
nF.
CX3 also adversely affects stability of the loop and it must be limited to 20 pF. CX4 affects the control loop in the
same way as CX1, and it should be treated in the same way as CX1. CX2 is the only capacitance that improves
stability margins of the control loop. Its maximum size is limited only by the safety requirements.
Stability is a function of ILOOP as well. Since ILOOP is approximately equal to the collector current of the external
BJT, Gm of the BJT, and thus loop dynamics, depend on ILOOP. This dependence can be reduced by
degenerating the emitter of the BJT with a small resistance as discussed in Loop Interface. Inductance in series
with the LOOP+ and LOOP− do not significantly affect the control loop.
9.1.1.5.5 Noise and Ripple
The output of the DAC is a current pulse train. The transition density varies throughout the DAC input code range
(ILOOP range). At the extremes of the code range, the transition density is the lowest which results in low
frequency components of the DAC output passing through the RC filter. Hence, the magnitude of the ripple
present in ILOOP is the highest at the ends of the transfer characteristic of the device (see Typical
Characteristics).
It should be noted that at wide noise measurement bandwidth, it is the ripple due to the ∑Δ modulator that
dominates the noise performance of the device throughout the entire code range of the DAC. This results in the
“U” shaped noise characteristic as a function of output current. At narrow bandwidths, and particularly at midscale output currents, it is the amplifier driving the external BJT that starts to dominate as a noise source.
9.1.1.5.6 Digital Feedthrough
Digital feedthrough is indiscernible from the ripple induced by the ∑Δ modulator.
9.1.1.5.7 HART Signal Injection
The HART specification requires minimum suppression of the sensor signal in the HART signal band (1-2 kHz) of
about 60 dB. The filter in Figure 20 below meets that requirement.
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Application Information (continued)
LOOP+
DAC161S997
15 mV
VH
IDAC
15k
VH
15k
15k
virtual ground
BASE
IHART
500 nA
RE
C1
390n
6.8n
C2
C3
220n
1n
COMA
80k
40
ILOOP
1 mA
OUT
VHART
LOOP-
500 mV
Figure 20. HART Signal Injection
9.1.1.5.8 RC Filter Limitation
In an effort to speed up the transient response of the device the user can reduce the capacitances associated
with the low-pass filter at the output of the ∑Δ modulator. However, to maintain stability margins of the current
control loop it is necessary to have at least C1 = C2 = C3 = 1nF.
9.2 Typical Application
LM2936-3.3
100
OUT
IN
GND
22µ
20
100n
VD
100n
3.3µ
100n
LOOP+
VA
MSP430G2553_PW_2
BASE
PZT3904
100n
1µ
SCLK
SDI
22
SPI
SDO
DAC161S997
µC
CSB
ERRB
INT
COMD
OUT
C1
C2
C3
LOOP-
COMA
2.2n
2.2n
2.2n
Figure 21. Typical Application Schematic
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Typical Application (continued)
9.2.1 Design Requirements
The objective of the example below is to focus on the design of a robust, low-power, precise, long- term stable,
2-wire, loop-powered, 4- to 20-mA current-output temperature transmitter by making best use of signal chain,
power and computing solutions. This design uses a RTD-temperature sensor with a 4- to 20-mA current-loop
transmitter.
9.2.2 Detailed Design Procedure
The DAC161S997output is a current pulse which is then filtered by a third-order RC low-pass filter and boosted
to produce the loop current (ILOOP) at the device OUT pin. Figure 22 shows the principle of operation of the
DAC161S997 in the loop-powered transmitter. The ILOOP has a number of component currents as given in
Equation 13.
é DAC _ CODE ù
ILOOP = IDAC + ID + IA + IE = 24 mA ´ ê
ú
216
ë
û
where
•
•
•
•
IDAC = f(DAC_CODE),
ID and IA represent the supply currents of internal digital and analog blocks,
IAUX represents the supply current of companion devices present in the system,
IE is the only component which is regulated by the control loop to ensure that the actual loop current
corresponds to the DAC input code applied by the MCU.
(13)
LOOP+
VD
IAUX
VA
DAC
+
BASE
ILOOP
IA
-
ID
+
IDAC
IE
RE
COMA
24
R1 = 80k
R2 = 40
I2
OUT
DAC161S997
LOOP-
Figure 22. Loop Powered Transmitter
9.2.2.1 Reasons for Choosing a 3.9-V Zener Diode
The first and most important requirement for the proper functioning of the entire circuitry is a stable power supply.
A good design ensures that the LDO gets the sufficient input voltage (VIN_LDO) to generate a stable 3.3 V under
minimum-loop, power-supply voltage and minimum-loop current conditions.
VIN_LDO (MIN REQUIRED at 3.3 mA) = VCC + VDO(MIN) = 3.3 V + 0.1 V = 3.4 V
(14)
From the circuit implementation, it is evident that the LDO input voltage VIN_LDO is function of loop current and
collector-emitter drop (VCE ) of the bipolar junction transistor (BJT). Apply Kirchhoff ’ s Voltage Law (KVL) in the
inner loop:
V(IN_LDO) = [(RC × ILOOP) + VZENER + VCE + (RE × ILOOP)]
(15)
At minimum loop power supply voltage, BJT operation is pushed towards saturation, which means that VCE is
approximately 0 V (minimum). Due to the stability considerations of the current control loop, the minimum output
current (ILOOP(MIN)) below 200 μ A is not advised as shown in Figure 23.
V(IN_LDO) (at minimum VSUPPLY & minimum ILOOP) = [(RC × ILOOP(MIN)) + VZENER + VCE(MIN) + (RE × ILOOP(MIN))]
24
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Typical Application (continued)
To ensure stable 3.3-V LDO output under these conditions, the following relation must be fulfilled.
V(IN_LDO) (at minimum VSUPPLY & minimum ILOOP ) > VIN_LDO (MIN REQUIRED at 3.3 mA)
(17)
[(100 Ω × 200 μA)+ VZENER + 0 V + (20 Ω X 200 μA)] > 3.4 V
[VZENER + 0.024 V] > 3.4 V
From Equation 17, it is clear that without having a zener diode, a voltage greater than 3.4 V at the input of the
LDO cannot be ensured. Therefore, choose a zener diode having a zener voltage (VZENER) greater than 3.4 V
and able to handle power dissipation, PDZ = VZENER × ILOOP(MAX). In this application, the design uses a 3.9-V, 500mW zener diode.
Figure 23. DAC DC Transfer Function
9.2.2.2 Loop Compliance Voltage
To calculate the minimum-loop, compliance-voltage (loop-power supply voltage), add all voltage drops in the loop
at the maximum-expected loop current.
From a current control loop stability point of view, VCE(MIN) must stay greater than maximum VCE(SAT):
VCE(MIN) > 0.2 V
(18)
Applying Kirchhoff ’s Voltage Law in the outer loop, to find out the VCE(MIN):
VCE(MIN) = VSUPPLY(MIN) – (2 × VWIRE) - (2 × VF) - VRC(MAX) - VZENER - VRE(MAX) – VSENSE(MAX) – VLOAD(MAX)
where
•
•
•
•
•
•
•
•
2 × VWIRE is voltage drop across system wiring. If system wiring runs over a length (L) along one way, wire
resistance per unit length is ρ and current in the loop is ILOOP, then the voltage drop for one conductor can be given
as:
VWIRE(MAX) = ρ × L × ILOOP(MAX)
For example, a 24-AWG wire has resistance per unit length ( ρ ) of 0.026 Ω /ft or 0.0755 Ω /m.
VLOAD is the voltage drop caused by the internal resistance of the loop receiver. The internal resistance of the loop
receiver may vary from 50 Ω to 250 Ω .
VLOAD(MAX) = RLOAD × ILOOP(MAX)
VSENSE is the voltage drop across 40- Ω sense resistor internal to the DAC.
VF is the forward-voltage drop across the reverse polarity protection diode.
VF = 0.7 V at 30 mA forward current (from TVS data sheet, SM6T39CA Data Sheet, SM6T39CA).
(19)
Rewriting Equation 19,
VSUPPLY(MIN) > 0.2 V + (2 x ρ x L x ILOOP(MAX))+ (2 x 0.7 V) + (100 Ω x ILOOP(MAX)) + 3.9 V + (20 Ω x ILOOP(MAX)) + (40 Ω x
ILOOP(MAX)) +( RLOAD x ILOOP(MAX)) + (40 Ω x ILOOP(MAX)) +(RLOAD x ILOOP(MAX))
(20)
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Typical Application (continued)
The maximum output current that can be sourced out of the OUT pin by DAC161S997, I LOOP (MAX) , is
approximately 24 mA.
VF
-
EN
TPS7A1601
Loop
+
+
+
GND
AVDD
+
VCC =
3.3V
-
VIN_LDO
VRC
RC
Length (L)
+
VZENER
ILOOP
+
-
+
+
RWIRE
-
RWIRE
VXMTR
-
COMA
VWIRE
+
VSENSE
Loop Power
Supply
RLOAD
-
-
VRE
RE
+
±
VSUPPLY
+
VWIRE
VCE
BASE
+
DAC161S997
VLOAD
+
40
Loop
Receiver
OUT
ILOOP
+
-
VF
ILOOP
Loop-
Figure 24. Series Voltage Drops in Current Loop System
Figure 25. Specification for Loop Current Output
Next, examine what NAMUR NE43 has to say about loop current. NAMUR NE43 is an international association
of process instrumentation user companies that have worked on improving the diagnostic coverage in 4- to 20mA analog output transmitters to address associated safety issues. NAMUR NE43 provides the guideline for
signaling-failure information to the safety-interlock systems over a 4- to 20-mA loop. NAMUR NE43 recommends
using 3.8 mA to 20.5 mA as an extended measurement information range. NAMUR NE43 recommends using
loop current below 3.6 mA or above 21 mA is in the diagnostic failure information range. Choose ILOOP(MAX) = 24
mA, depending upon DAC capability and also to comply with the NAMUR NE43 recommendation as shown in
Figure 26.
Normal Range 4 to 20mA
(Measurement is valid)
A
.0m
21
A
.5 m
.0 m
A
Failure
Information
20
20
A
0m
4.0
A
0m
0m
3 .8
3 .6
0m
0.0
A
Extended Working Range 3.8 to 20.5mA
(Measurement is still valid)
A
Failure
Information
Loop Current
in mA
Figure 26. NAMUR NE43 Recommendation
26
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Typical Application (continued)
Rewriting Equation 20,
VSUPPLY(MIN) > 0.2 V + (2 x ρ x L x 24 mA) + (2 x 0.7 V) + (100 Ω x 24 mA) + 3.9 V + (20 Ω x 24 mA) + (40 Ω x 24 mA)
+( R LOAD x 24 mA) + (40 Ω x 24 mA) + (RLOAD x 24 mA)
(21)
VSUPPLY(MIN) > 2 × 0.026 Ω /ft x L x 24 mA) + (RLOAD × 24 mA) + 9.34 V
(22)
Figure 27 and Figure 28 give the loop supply voltages calculated using Equation 22 at different receiver load
resistances and system wiring lengths.
System Wiring
Length (in ft)
RLOAD
2 X 50ft
2 X 500ft
2 X 1000ft
2 X 1500ft
2 X 2000ft
50Ÿ
10.6V
11.2V
11.8V
12.4V
13.1V
100Ÿ
11.8V
12.4V
13.0V
13.6V
14.3V
150Ÿ
13.0V
13.6V
14.2V
14.8V
15.5V
200Ÿ
14.2V
14.8V
15.4V
16.0V
16.7V
250Ÿ
15.4V
16.0V
16.6V
17.2V
17.9V
VSUPPLY
VSUPPLY
Figure 27. Loop Power Supply Voltage for Different Loads and System Wiring Length in Feet
System Wiring
Length (in m)
RLOAD
2 X 15m
2 X 150m
2 X 350m
2 X 500m
2 X 700m
50Ÿ
10.6V
11.1V
11.8V
12.4V
13.1V
100Ÿ
11.8V
12.3V
13.0V
13.6V
14.3V
150Ÿ
13.0V
13.5V
14.2V
14.8V
15.5V
200Ÿ
14.2V
14.7V
15.4V
16.0V
16.7V
250Ÿ
15.4V
15.9V
16.6V
17.2V
17.9V
VSUPPLY
VSUPPLY
Figure 28. Loop Power Supply Voltage for Different Loads and System Wiring Length in Meter
However, the maximum loop compliance voltage must not exceed the absolute maximum voltage rating of any
device used in the loop. Therefore, select the device in order to meet the maximum loop compliance voltage
requirement
9.2.2.3 Selection of External BJT
DAC161S997 has been designed to use an external NPN transistor (BJT). Transistor Q1 conducts the majority of
the signal-dependent, 4-20mA loop current. Using an external transistor avoids on-chip power dissipation and
thermally-induced errors. Since the external transistor is part of a current control loop, the external transistor
characteristics are not critical. Virtually any transistor with sufficient voltage, current and, power rating may be
used. Basic requirements are as follows:
• VCEO = 40 V minimum
• β = 40 minimum
• Must be able to handle power dissipation, PD = VCE(MAX) × ILOOP(MAX)
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Typical Application (continued)
The NPN BJT should not be replaced with an N-channel Field Effect Transistor (FET) for the following reasons.
Discrete FET ’ s typically have high threshold voltages (VTH), in the order of 1.5 V to 2 V, which is beyond the
BASE output maximum range. Discrete FETs present higher load capacitance, which may degrade system
stability margins. BASE output relies on the BJT’s base current for biasing.
For further information on a complete reference design of a Small Form Factor, 2-Wire, 4- to 20-mA CurrentLoop, RTD Temperature Transmitter, refer to TIDU385A.
9.2.3 Application Curve
Unless otherwise noted, these specifications apply for VA = VD = 3.3 V, COMA = COMD = 0 V, TA= 25°C, external bipolar
transistor: 2N3904, RE = 22 Ω, C1 = C2 = C3 = 2.2 nF.
2.5
2.0
INL ( µA)
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
4
6
8 10 12 14 16 18
OUTPUT CURRENT (mA)
20
Figure 29. Linearity vs ILOOP
28
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10 Power Supply Recommendations
The DAC161S997 requires a voltage supply within 2.7 V and 3.6 V. Multilayer ceramic bypass X7R capacitors of
0.1μF between the VA and GND pins, and between the VD and GND pins are recommended. If the supply is
located more than a few inches from the DAC161S997, additional bulk capacitance may be required in addition
to the ceramic bypass capacitors. An electrolytic capacitor with a value of 10μF or 22μF is a typical choice
11 Layout
11.1 Layout Guidelines
To maximize the performance of the DAC161S997 in any application, good layout practices and proper circuit
design must be followed. A few recommendations specific to the DAC161S997 are:
• Make sure that VD and VA have decoupling capacitors local to the respective terminals.
• Minimize trace length between the C1, C2, and C3 capacitors and the DAC161S997 pins.
11.2 Layout Example
Figure 30 to Figure 32 show the DAC161S997 evaluation module (EVM) layout
Figure 30. Example PCB layout: Top Assembly Layer
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Layout Example (continued)
Figure 31. Example PCB layout: Top Layer
Figure 32. Example PCB layout: Bottom Layer
30
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12 Device and Documentation Support
12.1 Trademarks
All trademarks are the property of their respective owners.
12.2 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
12.3 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
13 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.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
DAC161S997RGHR
ACTIVE
WQFN
RGH
16
4500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 105
161S997
DAC161S997RGHT
ACTIVE
WQFN
RGH
16
250
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 105
161S997
(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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