CN0064: 16-Bit Fully Isolated 4 mA to 20 mA Output Module Using the AD5662 DAC, ADuM1401 Digital Isolator, and External Amplifiers

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OVERVIEW

Circuit Note PDF, 11/2010 (pdf, 114 kB)
Benefits & Features
  • 4mA to 20mA Output Module
  • Digitally Isolated 16-Bit Industrial Control Output Module
  • Error < 0.025% of FSR Over All Codes
Products Used
    Applications: 
  • Programmable Logic Controllers/ Distributed Control Systems
  • Field Instruments/ Smart Transmitters

CIRCUIT FUNCTION AND BENEFITS

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 and transmitters that require a 4 mA to 20 mA current output range. The AD5662 nanoDAC® is a 5 V16-bit DAC in a SOT-23 package. The ADuM1401 four-channel digital isolator provides all the necessary signal isolation between the microcontroller and the DAC.

Figure 1. 16-Bit Isolated 4 mA to 20 mA Industrial Control Output Module (Simplified Schematic)

CIRCUIT DESCRIPTION

For industrial control modules, analog output current ranges are typically 4 mA to 20 mA or 0 mA to 20 mA. The AD5662 provides a 0 V to 5 V output, which sets the current through the sense resistor RS and, therefore, the current through R1.This current is mirrored (10×) to R2, using the second half of the AD822 op amp. The AD822 amplifier was chosen for its high performance, high voltage operation. The first stage of the current mirror needs to operate at 0 V input to ensure there is no dead band when the DAC operates at zero-scale output. This requires that the negative supply to the AD822 be at least −1 V so that there is sufficient headroom on the output of the first stage AD822. The two diodes in the second stage, which are in series with the positive power supply, ensure that the output voltage of the second stage does not go to the positive supply rail of the AD822, which can be as high as 35 V. The output of the FET is also protected by a series diode. Note that this series diode will increase the power dissipation of the circuit, which may be an issue in some applications.

The ADR02 has drift specs of 9 ppm/°C maximum. It is also often used in industrial applications due to its high input voltage range to 36 V.

The ADuM1401 is a quad-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 ( (SYNC, SCLK, and DIN), which connect the standard SPI connections to the AD5662.

Figure 2. INL Accuracy Plot for 24 mA Full-Scale Output Range

Figure 2 shows an output error plot (integral nonlinearity) of the current output of the circuit into RL when the AD5662 is used with the ADR02 external reference. Results are shown in % FSR (full scale range) as a function of input code. The ADR02 was chosen as the reference for this circuit.

See Colm Slattery, Derrick Hartmann, Li Ke, “PLC Evaluation Board SimplifiesDesign of Industrial Process Control Systems,” Analog Dialogue, 43-04, April 2009, for more discussion of external protection techniques.

SAMPLE PRODUCTS USED IN THIS CIRCUIT

Product Description Available Product Models to Sample
AD5662 2.7-5.5V, 16-Bit nanoDAC® Converter in a Sot-23 AD5662ARJZ-1500RL7 AD5662BRJZ-2500RL7 AD5662BRJZ-1500RL7 AD5662BRMZ-1 AD5662ARMZ-1 AD5662BRJZ-1REEL7
AD822 Single-Supply, Rail-to-Rail Low Power FET-Input Dual Op Amp

To obtain samples of this part, please contact ADI

ADR02 Ultracompact, Precision 5.0 V Voltage Reference ADR02AUJZ-REEL7 ADR02AKSZ-REEL7 ADR02ARZ
ADUM1401 Quad-Channel Digital Isolator (3/1 Channel Directionality) ADUM1401WTRWZ ADUM1401CRW ADUM1401ARWZ ADUM1401WSRWZ ADUM1401CRWZ ADUM1401BRWZ
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