This circuit provides a complete solution for an industrial control output module. This design is suitable for process control programmable logic controllers (PLCs) and distributed control system (DCS) modules that require standard 4 mA-to-20 mA current outputs and unipolar or bipolar output voltage ranges. The AD5422 16-bit DAC is software configurable to provide all necessary outputs and has many integrated diagnostic features useful in an industrial environment. The ADuM1401 provides all the necessary signal isolation between the microcontroller and the DAC. The circuit also includes standard external protection and has been tested and verified to be fully compliant with IEC 61000 specifications.
For industrial control modules, standard analog output voltage and current ranges include ±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. The AD5422 is a precision, fully integrated 16-bit DAC offering a programmable current source and programmable voltage output designed to meet the requirements of industrial process control applications.
The output current range of the circuit shown in Figure 1 is programmable from 4 mA to 20 mA or 0 mA to 20 mA and has an overrange function of 0 mA to 24 mA. Voltage output is provided from a separate pin that can be configured to provide 0 V to 5 V, 0 V to 10 V, ±5 V, and ±10 V output ranges. An overrange feature of 10% is available on all ranges. Analog outputs are short- and open-circuit protected and can drive capacitive loads of 1 μF and inductive loads of 1 H.
Figure 1: 16 -Bit Industrial Control Output Module, Integrated Diagnostics, Output Protection (Simplified Schematic)
By default, the DVCC pin on the AD5422 accepts a power supply of 2.7 V to 5.5 V. Alternatively, the DVCC SELECT pin can be used to connect an internal 4.5 V power supply to the DVCC pin for use as a digital power supply for other devices in the system or as a termination for pull-up resistors. Maximum current available from the DVCC pin in this mode is 5 mA. In this design, the DVCCoutput is used to supply the field side of the ADuM1401 digital isolator.
The ADuM1401 is a quad-channel digital isolator based on Analog Devices, Inc., iCoupler® technology. It is used to provide isolation between the AD5422 and the system microcontroller, with an isolation rating of 2.5 kV rms. All four wires are used to connect the standard SPI interface to the AD5422: three wires transmit (LATCH, SCLK, and SDIN) and one wire receives (SDO).
The AD5422 has an on-board 10 ppm/°C reference. For high performance over temperature, this design uses an external ADR445 5 V reference. This device has 0.04% maximum accuracy error and a 3 ppm/°C maximum temperature drift. This drift contributes approximately 0.02% error across the industrial temperature range.
Figure 2 shows a plot of the output error of the AD5422 when used with the ADR435 external reference. The data is shown as output voltage error in %FSR (full-scale range) across a nominal 0 V to 10 V output range.
Figure 2: INL Accuracy Plot for 0 V to 10 V Output Range
This circuit is from a portion of the PLC demo system. The PLC demo system has been successfully tested to the IEC 61000 standards shown in Table 1 (see Colm Slattery, Derrick Hartmann, and Li Ke, “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems,” Analog Dialogue (April 2009) for more discussion of external protection techniques.
Table 1. Conformance to IEC Specifications1
|EN55022||Radiated emission Class A, 3 meter anechoic chamber||Passed and met -6 dB margin.|
|EN & IEC 61000-4-2||Electrostatic discharge (ESD), ±8 kV VCD||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively -8 ppm, 10 ppm, and 13 ppm when there is interference.|
|Electrostatic discharge (ESD) ±8 kV HCD||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively -8 ppm, 10 ppm, and 13 ppm when there is interference|
|EN & IEC 61000-4-3||Radiated immunity 80 MHz to 1 GHz 18 V/m, vertical antenna polarization||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 0.05%, 0.004%, and -0.13%. Performance automatically resorted to less than or equal to 0.05% after interference. Class B.|
|Radiated immunity 80 MHz to 1 GHz 18 V/m, horizontal antenna polarization||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively -0.09%, 0.003%, and -0.02%. Performance automatically resorted to less than or equal to 0.05% after interference. Class B.|
|EN & IEC 61000-4-4||Electrically fast transient (EFT) ±4 kV power port||Passed Class B.|
|Electrically fast transient (EFT) ±2 kV analog I/O ports||Passed Class B.|
|EN & IEC 61000-4-5||Power line surge, ±2 kV||No board or part damage occurred, no performance degrade, passed with Class A.|
|EN & IEC 61000-4-6||Immunity test on power cord, 10 V/m for 30 minutes||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 9.3%, 11%, and 3.4%. Passed Class B.|
|Immunity test on I/O cable, 10 V/m for 30 minutes||Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 4.5%, 4.7%, and 1.4%. Performance automatically resorted to less than or equal to 0.05% when interfernce stopped.|