Control the output module in complete isolation from the microcontroller

Whether in buildings or on production floors, programmable controllers are now needed everywhere to regulate processes, machines and systems. This relates to the Programmable Logic controller (PLC) or distributed control system (DCS) module to which the device is connected. To control these devices, PLC and DCS modules usually have an output module with a current output, a voltage output, or a combination of both. The industrial control module covers the standard analog output voltage and current range of ±5 V, ±10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. Especially in industry, electrical isolation of microcontrollers and output peripherals is often required.

Classic solutions offer discrete designs that convert digital signals from microcontrollers into analog signals or provide different analog outputs and achieve current isolation. However, separate designs present a number of disadvantages compared to integrated solutions. For example, a large number of components can result in high system complexity, large circuit board size, and high cost. Additional features such as short-circuit capability and even fault diagnosis highlight these shortcomings.

A better solution is to integrate as much functionality as possible on a single chip, as is done with Inc. (ADI), for example. In addition to digital-to-analog conversion, it provides a fully integrated programmable current source and programmable voltage output to meet the requirements of industrial process control applications.

Figure 1 shows an example circuit for fully isolated control of the analog output stage of the output module. It is particularly suitable for PLC and DCS modules in process control applications requiring 4 mA to 20 mA standard current output and unipolar or bipolar output voltage ranges. This circuit is based on ADI's high precision 16 bit DACAD5422 and ADuM1401 four channel digital isolation modules.

The output of a 16-bit DAC can be configured via the serial Peripheral interface (SPI). The module also has integrated diagnostic capabilities and can be used in industrial environments. The insulation resistance required between the microcontroller and the DAC is provided by the isolation module, whose four channels are used to connect to the SPI of the DAC: three channels (latches, SCLK, and SDIN) for sending data, and a fourth channel (SDO) for receiving data.

Especially in industrial applications it is necessary to provide a robust output that can withstand high interference voltages. Standards such as IEC 61000 set out requirements for robustness, such as electromagnetic compatibility (EMC). To comply with these standards, additional external protection circuits must be installed at the output. One possibility for the protection circuit is shown in Figure 2.

                                   

Figure 1. Simplified example circuit for isolation control of analog output stages using AD5422 and ADuM1401.

                    

Figure 2. The AD5422 output protection circuit conforming to IEC 61000 standard.

Current output (outside I) can be selectively programmed in the range of 4 mA to 20 mA or 0 mA to 20 mA. The voltage output is provided with an external pin via a separate V and can be configured in a voltage range of 0 V to 5 V, 0 V to 10 V, ±5 V, or ±10 V. The overrange of all voltage ranges is 10%. Both analog outputs have short and open circuit protection and can drive capacitive loads up to 1 μF and inductive loads up to 50 mH.

The DAC requires an analog power supply (AVDD) in the range of 10.8V to 40V. For digital supply voltage (DV CC), 2.7V to 5.5V is required. Alternatively, a DV CC can be used as a power pin or pull-up resistor for other components in the system. For this, the DV CC _SELECT pin should float and the internal 4.5V LDO regulator voltage should be applied to the DV CC pin. The maximum available supply current is 5 mA. In the circuit shown, the DV CC is used to power the electrical isolation side of the ADuM1401.

High precision conversion results are obtained from a 16-bit DAC using the ADR4550 external reference voltage. It is a high-precision, low-power, low-noise reference voltage source with a maximum initial accuracy of 0.02%, excellent temperature stability and low output noise. The circuit shown here is particularly suitable for the output module of a PLC or DCS module that provides current and voltage output and must conform to EMC standards such as IEC 61000.