CN0009 Implementing a 4 mA to 20 mA Process Control Loop Using the ADI AD5662 DAC

Circuit Function and Advantages

In many process control applications, two-wire current transmitters are often used to transmit analog signals in high-noise environments. These current transmitters have a zero-scale signal current of 4 mA and a full-scale signal current of 20 mA, hence the name "4 mA to 20 mA converter." The circuit described in this article provides a low-power current transmitter with 16-bit resolution and monotonicity that operates directly from the 4 mA to 20 mA control
loop supply and consumes less than 4 mA. Transmitters that consume more than 4 mA cannot be powered directly from the loop supply and require an additional power supply.

Circuit Description

AD5662 This circuit uses a nanoDAC converter as a controller to provide a programmable output current from 4 mA to 20 mA. The loop current is sensed by measuring the voltage drop VOUT across RS. If the DAC output is 0 V, the current

where 0 ≤ D ≤ 65,535. When the digital code of the AD5662 equals 0xFFFF, this circuit provides a full-scale output current of 20.9 mA. Similarly, when the digital code of the AD5662 equals 0x0000, the output current will be 3.49 mA. The extended current range (3.49
mA to 20.9 mA) allows the user to calibrate the 4 mA to 20 mA range using software and the 16-bit resolution of the AD5662. A Schottky diode is required for this circuit to prevent the loop power-up transient from pulling the noninverting input of the AD8627 below 300 mV of the inverting input. This Schottky diode must be able to handle the full loop load of at least 20 mA.

The bias voltage for the ADR02 controller is provided by a 5 V precision reference, and no external adjustments are required for this circuit because the initial output voltage tolerance of the ADR02 is very tight and the supply current of the AD8627 AD5662 is very low. The allowable loop power supply limits for the ADR02 and ADR02 are set by the minimum input voltage (7 V) and maximum input voltage (36 V). The maximum allowable power dissipation of the 2N3904 is 625 mW at 25°C, so if the loop supply exceeds approximately 30 V, a higher power transistor must be used. The power dissipation of the 2N3904 can be reduced by adding an appropriate drop resistor in series with the collector of the 2N3904.

Common changes

This basic circuit is flexible and can be used with a variety of different references, voltage output DACs, and op amps. The accuracy of the reference, the resolution of the DAC, and the offset voltage of the amplifier need to be considered. The main requirements are that the entire circuit must operate at the loop voltage and the quiescent current must be less than 4mA (DAC code 0x0000).

[Attachment: Implementing a 4 mA to 20 mA process control loop using the AD5662 DAC]