The circuit shown in Figure 1 provides 16-bit fully isolated ±10 V and 4 mA to 20 mA outputs, making it suitable for programmable logic controllers (PLCs) and distributed control systems (DCS).

The circuit uses digital isolation, PWM controlled power conditioning circuitry and associated feedback isolation. An external transformer allows power to be transferred across the isolation barrier while the entire circuit operates from a single +5 V supply on the primary side. This solution is superior to isolated power modules, which are often bulky and may have poor output regulation.

Digital isolators perform better than optical isolators, especially where multi-channel isolation is required. The integrated design isolates the circuitry from the local system controller to prevent ground loops and ensures immunity from external events often encountered in harsh industrial environments.

The AD5422 is a fully integrated, fully programmable 16-bit voltage and current output DAC with the following programmable ranges: 4 mA to 20 mA, 0 mA to 20 mA, 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V. Voltage output headroom is typically 1 V, and current outputs require approximately 2.5 V headroom. This means that with a 15 V supply, a 20 mA current output can drive a load of approximately 600 Ω.

The ADuM347x are quad-channel digital isolators with integrated PWM controller and low-impedance transformer drivers (X1 and X2). An isolated DC/DC converter requires only the following additional components: a transformer and a simple full-wave diode rectifier. The device can deliver up to 2 W of regulated isolation power from a 5.0 V or 3.3 V input supply, eliminating the need for a separate isolated DC/DC converter. 

i Coupler chip-scale transformer technology is used to isolate logic signals; the integrated transformer driver with isolated secondary side control function can improve the efficiency of isolated DC/DC converters. The internal oscillator frequency can be adjusted from 200 kHz to 1 MHz, determined by the value of R _OC . When R _OC = 100 kΩ, the switching frequency is 500 kHz.

The ADuM3471 regulates from the 15 V positive supply. Regulatory feedback comes from the voltage divider network (R1, R2, R3). The resistor is selected based on the following requirement: When the output voltage is 15 V, the feedback voltage is 1.25 V. The feedback voltage is compared to the ADuM3471 internal feedback setpoint voltage of 1.25 V. Regulation is achieved by changing the duty cycle of the PWM signal driving an external transformer.

The negative supply is less tightly regulated and can be as high as −23 V for light loads, which is still within the maximum operating voltage of −26.3 V. For nominal loads above 1 kΩ, the additional power dissipation due to the larger unregulated negative supply voltage is not an issue. In applications requiring higher compliance voltages or very low power consumption, other power supply designs should be considered.

The circuit was tested using the ADR445, a 5 V, high-accuracy, low-drift (3 ppm/°C maximum grade B) external reference. The circuit has a total system error of less than 0.1% over the industrial temperature range (−40°C to +85°C).

AD5422 integrates a high-precision reference voltage source with a maximum temperature drift of 10 ppm/°C. If this reference is used instead of an external reference, only 0.065% additional error will be generated over the entire industrial temperature range.

Test data and results

The AD5422 differential nonlinearity (DNL) is tested to ensure system accuracy is not compromised by switching power supplies. Figure 2 shows DNL over the ±10V range. This result shows that the DNL error is less than 0.5 LSB.

In addition, the average output noise over a certain period of time was tested and measured, as shown in Figure 3. The total drift is approximately 75 µV, corresponding to only 0.25 LSB of noise.

The actual error data of the circuit are shown in Figures 4 and 5. The total error (% FSR) in output current and voltage is calculated by dividing the difference between the ideal and measured output by the FSR and multiplying the result by 100. As shown in Figures 4 and 5, errors below 0.5% FSR are achieved in both current and voltage output modes.

If the V _OUT pin must drive a large capacitive load of up to 1µF, a 3.9 nF capacitor can be connected between the AD5422's V _OUT pin and the C _COMP pin by connecting a jumper to the P4 pin on the board . However, increasing this capacitance reduces the bandwidth of the output amplifier, thereby increasing settling time.