CN-0270: Complete HART-Compatible 4 mA to 20 mA Solution
A complete HART-compatible 4 mA to 20 mA solution using the AD5700 low-power modem and AD5420 16-bit DAC for industrial transmitter applications.
Overview
This reference design outlines a HART-compliant 4 mA to 20 mA current loop solution utilizing the AD5700 HART modem and the AD5420 16-bit current-output DAC. The circuit is optimized for line-powered field transmitters, providing a method to modulate a 1 mA peak-to-peak frequency-shift-keyed (FSK) signal onto the analog current signal. It includes detailed analysis of HART physical layer requirements, such as output noise during silence and analog rate of change. The design features digital slew rate control and filter circuitry to ensure undistorted FSK communication at 1200 Hz and 2200 Hz while maintaining industrial-grade interference immunity.
Use Cases
- Line-powered field transmitters
- Industrial process control instrumentation
- Remote calibration and fault interrogation
- Two-way digital communication over analog current loops
- RTD, thermocouple, and ohm sensor signal conversion
Topics
Referenced Parts
Analog Devices
fully isolated output module using the AD5422 and the ADuM1401 digital isolator
Analog Devices
providing power and data isolation using the ADuM3471 PWM controller and transformer driver
| AD5700 | Analog Devices | Low Power HART® Modem |
| AD5420 | Analog Devices | 16-Bit 4 mA to 20 mA DAC |
| AD5700-1 | Analog Devices | AD5700-1 offers a 0.5% precision internal oscillator. |
| AD5410 | Analog Devices | AD5420/AD5410 and AD5700 HART Modem Connection |
| AD5421 | Analog Devices | The AD5421 and the AD5700 HART modem can be combined |
| AD5422 | Analog Devices | common variation on the circuit shown in Figure 1 is to use the AD5422 |
| AD5755-1 | Analog Devices | AD5755-1 quad voltage and current output DAC may be used. |
| ADuM1401 | Analog Devices | fully isolated output module using the AD5422 and the ADuM1401 digital isolator |
| ADuM3471 | Analog Devices | providing power and data isolation using the ADuM3471 PWM controller and transformer driver |