Circuit functions and advantages

The circuit shown in Figure 1 is a 4 mA to 20 mA current loop transmitter used for communication between a process control system and its actuators. In addition to being cost-effective, this circuit is the industry's lowest power solution. 4 mA to 20 mA current loops are widely used in programmable logic controllers (PLCs) and distributed control systems (DCS) with digital or analog inputs and outputs. Current loop interfaces are popular because they enable cost-effective, interference-free data transmission over long distances. The combination of the low-power dual-channel operational amplifier AD8657 , the DAC AD5621 , and the voltage reference ADR125 can provide more power budget for higher-power devices such as microcontrollers and digital isolators. This circuit output current is 0 mA to 20 mA. The 4 mA to 20 mA range generally corresponds to the input control range of a DAC or microcontroller, while the 0 mA to 4 mA output current range is often used to diagnose fault conditions.

The 12-bit, 5 V AD5621 requires 75 μA of supply current (typ). The AD8657 is a rail-to-rail input/output dual-channel operational amplifier and is one of the lowest power amplifiers in the industry (22 μA over the entire supply voltage and input common-mode range) and the highest operating voltage. Up to 18 V. The ADR125 is a precision micropower 5 V bandgap reference that requires only 95 μA supply current. The three devices consume a total of 192 μA of supply current (typ).

Circuit description

For industrial and process control modules, 4 mA to 20 mA current loop transmitters are used as a means of communication between the control unit and the actuator. The 12-bit DAC AD5621 is located in the control unit and generates an output voltage VDAC between 0 V and 5 V based on the input code. The code is set via the SPI interface. The ideal relationship between input code and output voltage can be expressed as:

V _DAC  = V _REF  × (D/2 ¹⁴ ) (1)

Among them: V _REF is the output voltage of ADR125 and the supply voltage of AD5621. D is the decimal equivalent of the binary code loaded into the AD5621.

The DAC output voltage sets the current through sense resistor R _SENSE :

I _SENSE  = V _DAC /R _SENSE  (2)

The current through R _SENSE varies from 0 mA to 2 mA as a function of V _DAC . This current creates a voltage across R1 and sets the voltage at the non-inverting input of the AD8657 amplifier (A2). A2 The AD8657 closes the loop and pulls the voltage at the inverting input to the same voltage as the non-inverting input. Therefore, the current flowing through R1 is mirrored to R2 by a factor of 10. This can be expressed by Equation 3:

I _OUT  = I _R2  = (V _DAC /R _SENSE  ) × ( R1/R2) (3)

The V _DAC range is 0 V to 5 V, so this circuit produces a current output ranging from 0 mA to 20 mA.

AD5621 is a 12-bit DAC, belonging to the nano DAC series, operating with the 5 V output voltage of the reference voltage source ADR125. It has an on-chip precision output buffer that provides rail-to-rail output swing, resulting in a very high dynamic output range. At a supply voltage of 5 V, the AD5621 consumes 75 μA of supply current (typ).

Additionally, this circuit solution requires a rail-to-rail input amplifier. The AD8657 dual-channel op amp is an excellent choice because of its low power consumption and rail-to-rail characteristics. The op amp operates with a supply current of 22 μA (typ) over the specified supply voltage and input common-mode voltage range. It also provides excellent noise per unit current and bandwidth performance. The AD8657 is one of the lowest power amplifiers and operates with supply voltages up to 18 V.

The ADR125 is a precision, micropower, low dropout (LDO) voltage reference. At an input voltage of 18 V, quiescent current is only 95 μA (typ). The LDO reference is preferred because it allows more voltage drop in the loop wires from the control unit to the actuator. To maintain stability, a small 0.1 μF capacitor is required at the output of the ADR125. In addition, connecting a 0.1 μF to 10 μF capacitor in parallel can improve the load transient response performance. Although the input capacitor is not required, it is recommended. A 1 μF to 10 μF capacitor can be connected in series to the input to improve transient response when the supply voltage changes suddenly. Adding a 0.1 μF capacitor in parallel also helps reduce power supply noise.

A bypass capacitor is also required (not shown in Figure 1). In this example, each dual op amp should have a 10 μF tantalum capacitor in parallel with a 0.1 μF ceramic capacitor on each supply pin. For detailed instructions on proper decoupling techniques, please refer to Tutorial MT-101 .

This circuit solution outputs current from 0 mA to 20 mA. Figure 2 shows the circuit output current measured in a 250 Ω load resistor. Figure 3 shows the output current error graph.

Figure 3 shows a plot of the output current error as a percentage of full-scale range. The total worst-case error is approximately 0.35%, measured over the output range between code 256 and code 16,128.

Figure 4 shows the calibrated output current error plot. After removing the gain and offset errors in Figure 3, the accuracy is better than 0.05%, measured over the output range between code 256 and code 16,128.

The data in Figures 3 and 4 show large errors at zero scale and full scale because the AD5641 DAC output buffer limits when the output signal is within 10 mV of any supply rail. Therefore, the linearity specification excludes the area between code 0 and code 255 and code 16,129 and code 16,384. The corresponding DAC voltage outputs are approximately 0 V to 80 mV and 4.92 V to 5.00 V; the reference current outputs are 0 mA to 0.32 mA and 19.68 mA to 20.00 mA.

The test data was obtained using the test board shown in Figure 6. Complete documentation for this system is located in the CN-0179 Design Support Package .