How to design a microcontroller controlled isolated 16-bit output module?

Traditional solutions use discrete designs to convert digital signals from microcontrollers into analog signals or provide different analog outputs and achieve electrical isolation. However, discrete designs have many disadvantages compared to integrated solutions. For example, the large number of components leads to very complex systems, large circuit board size, and high cost. Other features such as short-circuit withstand capability and even fault diagnosis further highlight these shortcomings.

A better solution is to integrate as many functions as possible on a single chip, such as the high-precision 16-bit DAC AD5422 from ADI. In addition to digital-to-analog conversion, it also provides fully integrated programmable current source and programmable voltage output to meet the needs of industrial process control applications.

Figure 1. Simplified example circuit for isolated control of an analog output stage using the AD5422 and ADuM1401.

Figure 1 shows an example circuit for an analog output stage that can fully isolate a control output module. It is particularly suitable for PLC and DCS modules in process control applications that require a 4 mA to 20 mA standard current output and a unipolar or bipolar output voltage range. Here the AD5422 is used in combination with the ADuM1401 quad-channel digital isolation module.

The output of the 16-bit DAC AD5422 can be configured via a serial peripheral interface (SPI). The module also integrates diagnostic functions, which are useful in industrial environments. The required insulation resistance between the microcontroller and the DAC can be implemented using the ADuM1401, whose four channels are used to implement the SPI connection with the AD5422: three channels (LATCH, SCLK, and SDIN) transmit data, and the fourth channel (SDO) receives data.

Especially in industrial applications, reliable outputs that are immune to high interference voltages must be provided. Reliability requirements are defined in standards such as IEC 61000, which specifies requirements regarding electromagnetic compatibility (EMC), for example. In order to comply with these standards, additional external protection circuits are required at the outputs. One possible protection circuit is shown in Figure 2.

Figure 2. IEC 61000-compliant protection circuit for the AD5422 output.

The current output (I _OUT ) can be set to either a 4 mA to 20 mA or 0 mA to 20 mA range. The voltage output is provided via a separate VOUT pin that can be configured to have a 0 V to 5 V, 0 V to 10 V, ±5 V, or ±10 V range. All voltage ranges have an overrange of 10%. Both analog outputs are short-circuit and open-circuit protected and can drive capacitive loads of 1 μF and inductive loads of 50 mH.

The AD5422 requires an analog power supply (AV _DD ) of 10.8 V to 40 V. For the digital supply voltage (DV _CC ), 2.7 V to 5.5 V is required. Alternatively, DV _CC can also be used as a power supply pin for other components in the system or as a termination for a pull-up resistor. For this purpose, the DV _CC _SELECT pin should be left floating and the internal 4.5 V LDO regulator voltage is applied to the DV _CC pin. The maximum available supply current is 5 mA. In the circuit shown, DV _CC is used to power the electrically isolated side of the ADuM1401.

A high accuracy rotation result can be obtained from the 16-bit DAC using the ADR4550 external reference. This is a high accuracy, low power, low noise reference with a maximum initial accuracy of 0.02%, excellent temperature stability, and low output noise.

The circuit shown in this article is particularly suitable for output modules of PLC or DCS modules that provide both current and voltage outputs and must comply with EMC standards such as IEC 61000.