AN-1517: 16-Bit, Fully Isolated, Voltage Output Module Using the AD5662 DAC, ADuM1401 Digital Isolator, and External Amplifiers
AN-1517: 16-Bit, Fully Isolated, Voltage Output Module Using the AD5662 DAC, ADuM1401 Digital Isolator, and External Amplifiers
Circuit Function and Benefits
The circuit shown in Figure 1 provides a complete solution for an industrial control output module. This design is suitable for process control programmable logic controllers (PLCs) and distributed control systems (DCSs) that require bipolar output voltage ranges. The AD5662 nanoDAC® is a 5 V, 16-bit digitalto-analog controller (DAC) in a SOT-23 package. The ADuM1401 4-channel digital isolator provides all the necessary signal isolation between the microcontroller and the DAC.
Circuit Description
For industrial control modules, analog output voltage ranges are typically ±5 V, ±10 V, 0 V to 5 V, or 0 V to 10 V. The AD5662 provides a 0 V to 5 V output, which passes through two gain and offset stages to provide 16-bit resolution in each of the previously mentioned ranges. Jumpers (shown in Figure 1 as switches) are used to switch between output ranges. The OP2177 was chosen for this design, primarily due to its low noise and offset performance, as well as bipolar voltage capability.
The ADR02 was chosen as the reference for this circuit. The ADR02 has excellent parts per million drift specifications at 9 ppm/°C maximum. It is also often used in industrial applications due to its high input range of up to 36 V.
The ADuM1401 is a 4-channel digital isolator based on Analog Devices iCoupler® technology. It is used to provide isolation between the AD5662 and the system microcontroller, with an isolation rating of 2.5 kV rms. Three wires are used to connect the standard serial peripheral interface (SPI) connections to the AD5662: SYNC, SCLK, and DIN.
Figure 2 shows an output error plot (integral nonlinearity) of the output of the circuit when the AD5662 is used with the ADR02 external reference. Results are shown in percent fullscale range (%FSR) as a function of input code.
See “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems,” (Slattery et. al., Analog Dialogue, April 2009) for more discussion of external protection technique.
References
MT-015 Tutorial, Basic DAC Architectures II: Binary DACs. Analog Devices.
MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs. Analog Devices.