SLAA866A December   2018  – September 2024 DAC161S997 , DAC7311 , DAC8560 , DAC8830

 

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  3.   Trademarks

Design Goals

Loop Supply Voltage DAC Output Voltage Output Current Error
12V–36V 0V–3V 4mA–20mA <1% FSR

Design Description

The loop powered current transmitter regulates the current in series loop consisting of the power supply, transmitter, and load resistance. The active circuitry in the transmitter derives power from the loop current, meaning the current consumption of all devices must be less than the zero-scale current, which can be as low as 3.5mA in some applications. A regulator steps down the loop voltage to supply the DAC, op amp and additional circuitry. The op amp biases the transistor to regulate the current flowing from Loop+ to Loop-. The circuit is commonly used in 2-wire field sensor-transmitters such as Flow Transmitters, Level Transmitters, Pressure Transmitters, and Temperature Transmitters.

Design Notes

  1. Select a single channel DAC with the required resolution and accuracy for the application. Use an op amp with low offset and low drift to minimize error.
  2. Select a low power DAC, op amp, and voltage regulator to establish a total sensor-transmitter quiescent current of less than 4mA.
  3. Minimize current flow through R1, R2, and R3 by selecting a large ratio of R3/R4 to minimize thermal drift of the resistors.
  4. Use precision low drift resistors for R1-R4, R7-R8 to minimize error.
  5. Use a voltage regulator with a wide input voltage range and low dropout voltage to allow for a wide range of loop supply voltages.

Design Steps

The output current transfer function is:

1. Select a large ratio of R3/R4:

2. Calculate R2 based on the zero-scale current (4mA), regulator voltage, and gain ratio (R3/R4).

3. Calculate R1 to set the full-scale current based on the full-scale DAC voltage and current span of 16mA.

4. Calculate the zero-scale output current based on the chosen resistance values.

5. Calculate the full-scale current based on the chosen resistor values.

DC Transfer Characteristic

Small Signal Step Response

Devices

Device Key Features Link Other Possible Devices
DACs
DAC7311 12-bit resolution, single channel, ultra-low power, 1 LSB INL, SPI, 2V to 5.5V supply 12-bit, single-channel, ultra-low power DAC in 6-pin SC70 package for battery powered applications Precision DACs (≤10 MSPS)
DAC8560 16-bit resolution, single channel, internal reference, low power, 4 LSB INL, SPI, 2V to 5.5V supply 16bit, Single Channel, 80uA, 2.0V-5.5V DAC in SC70 Package Precision DACs (≤10 MSPS)
DAC8830 16-bit resolution, single channel, ultra-low power, unbuffered output, 1 LSB INL, SPI, 2.7V to 5.5V supply 16-bit, single-channel, ultra-low power, voltage output DAC Precision DACs (≤10 MSPS)
DAC161S997 16-bit, 4-20mA current output, 100uA supply current, SPI, 2.7V to 3.3V supply 16-Bit Precision DAC With Internal Reference and 4mA-to-20mA Current Loop Drive Precision DACs (≤10 MSPS)
Amplifiers
TLV9001 Low-Power, 0.4mV Offset, Rail-to-Rail I/O, 1.8V to 5.5V supply One-channel, 1-MHz rail-to-rail input and output 1.8-V to 5.5-V operational amplifier Operational amplifiers (op amps)
OPA317 Zero-Drift, Low-Offset, Rail-to-Rail I/O, 35uA supply current max, 2.5V to 5.5V supply Low Offset, Rail-to-Rail I/O Operational Amplifier Operational amplifiers (op amps)
OPA333 microPower, Zero-Drift, Low Offset, Rail-to-Rail I/O, 1.8V to 5.5V supply Micropower, 1.8-V, 17-µA zero-drift CMOS precision operational amplifier Operational amplifiers (op amps)

Links to Key Files

Texas Instruments, Low Cost Loop-Powered 4-20mA Transmitter EMC/EMI Tested, TIPD158 reference design

Texas Instruments, 4-20mA Current Loop Transmitter, TIDA-00648 reference design

Texas Instruments, Highly-Accurate, Loop-Powered, 4mA to 20mA Field Transmitter with HART® Modem, TIDA-01504 reference design

Texas Instruments, source files for SLAA866, SLAC782 software support