CN0067: Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC, the ADuM5401 Digital Isolator, and a High Performance In-Amp

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OVERVIEW

Circuit Note PDF, 11/2010 (pdf, 271 kB)
Benefits & Features
  • Industrial Control Input Module
  • 4mA to 20mA Current Inputs and Unipolar Input Voltage
  • 24-Bits of Digitally Isolated Resolution
    Applications: 
  • Programmable Logic Controllers/ Distributed Control Systems
Design Resources
Device Drivers
Software, such as C code and/or FPGA code, used to communicate with a component's digital interface.

CIRCUIT FUNCTION AND BENEFITS

This circuit provides a complete solution for an industrial control input module. This design is suitable for process control programmable logic controllers (PLCs) and distributed control system (DCS) modules that must digitize standard 4 mA to 20 mA current inputs and unipolar input voltage ranges. The AD8220 in-amp is used to level shift the bipolar signals to provide a 0 V to 5 V input signal to the AD7793 ADC. The ADuM5401 provides all the necessary signal isolation and power between the microcontroller and the ADC. The circuit also includes standard external protection and has been tested and verified to be fully compliant with IEC 61000 specifications.

Figure 1. 24-Bit Isolated Industrial Control Voltage Input Module (Simplified Schematic)

CIRCUIT DESCRIPTION

For industrial control modules, analog input voltage and current ranges include 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. A resistor divider (R2-R3) on the input is used to attenuate high voltage inputs so that they match the maximum input range of the AD8220 (−0.1 V to +2.9 V when operating on a single +5 V supply). A good variation of this circuit is to use the AD8226, which has a wider maximum input range (−0.1 V to +4.1V). The AD8220 is used to level shift the input signal and provide gain so that it matches the input range of the AD7793. This circuit is specifically designed for unipolar analog inputs. Other boards have been designed for bipolar operation, which requires bipolar supplies (see Scott Wayne,“iCoupler® Digital Isolators Protect RS-232, RS-485, and CAN Buses in Industrial, Instrumentation, and Computer Applications,” Analog Dialogue, October 2005).

The AD8220 also has the excellent CMRR needed in these applications. Jumpers (shown here as switches for clarity) are used to switch between current and voltage ranges in hardware and to set the gain of the AD8220. For example, when the input is set to receive a 4 mA-to-20 mA current, the switches are configured to provide a 250 Ω load resistor (R1) on the input, providing a full-scale voltage of 5 V.

The AD8220 output is biased with a common-mode signal connected to the REF pin. This reference signal is generated from the same reference as the AD7793, divided down to provide a 0.42 V bias voltage that is buffered using the AD8601 and connected to the REF pin of the AD8220. The REF input to the AD8220 should always be driven with a low impedance, heavily decoupled source.

The ADR441 is the chosen reference for this circuit. The ADR441 has excellent accuracy specifications of 0.04% and drift specifications of 3 ppm/°C maximum.

The ADuM5401 is a quad-channel isolator with integrated isoPower technology, based on Analog Devices, Inc., iCoupler® technology. It is used to provide isolation between the field side and the system microcontroller, with an isolation rating of 2.5 kV rms. The ADuM5401 also has an integrated dc-to-dc converter that can provide 500 mW of regulated isolated power at either 5 V or 3.3 V. This design uses 5 V provided from the ADuM5401 to supply all the analog circuitry on the input module. Four wires are used: three for transmit (CS, , SCLK, and DIN) and one for receive (DOUT), which connect to the standard SPI interface.

The AD7793 is specified for an AVDD of 2.7 V to 5.25 V under normal operation. For loads greater than 10 mA, the output voltage of the ADuM5401 is 4.75 V to 5.25 V, although for lighter loads, it is 4.7 V to 5.4 V. For this reason, the user can either ensure that the isolated circuit draws more than 10 mA during operation or place a diode in series with the AVDD supply of the AD7793 to ensure that the AVDD voltage is within the specification. The circuit shown in Figure 1 draws ~4.8 mA.

Figure 2 shows a noise plot of the input module when a 2.5 V input signal is connected using the 0 V to +10 V input range. The measured noise on the output is in peak-to-peak resolution. The 19.5 bit peak-to-peak noise-free code resolution means there are 4.5 bits of noise flicker on the output of the ADC with respect to the ADC’s 24-bit resolution. Therefore, the ADC’s noise-free code resolution is 19.5 bits. In terms of contributing to the overall system error, the measured noise of 4.5 bits contributes ~0.00014% error with respect to the full-scale range of the ADC. The integral nonlinearity error of the AD7793 is 0.0015%.

Figure 2. Input Module Histogram, 4.7 Hz Update Rate, Gain =1, Input = 2.5 V Reference

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, 43-04, April 2009, for more discussion of external protection techniques.

Table 1. Conformance to IEC Specifications1

Test Item Description Result
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.

1 A sample was tested during initial release of the PLC Demo system (V07) and met the test compliances listed in this table. These results should be viewed as typical data taken at 25°C. For these tests, the DAC outputs were connected to the ADC inputs, that is, DAC_CH2 to ADC_CH2, DAC_CH3 to ADC_CH3, and DAC_CH4 to ADC_CH4. The DAC outputs were set to 5 V, 6 V, and 10 mA, respectively.

SAMPLE PRODUCTS USED IN THIS CIRCUIT

Product Description Available Product Models to Sample
AD7793 3-Channel, Low Noise, Low Power, 24-Bit Sigma Delta ADC with On-Chip In-Amp and Reference AD7793BRUZ
AD8220 JFET Input Instrumentation Amplifier with Rail-to-Rail Output AD8220BRMZ AD8220ARMZ
AD8601 DigiTrim™ RRIO Single Op Amp with Very Low Offset Voltage AD8601ARTZ-REEL7
ADR441 Ultralow Noise, LDO XFET® 2.5V Voltage Reference w/Current Sink and Source ADR441ARMZ ADR441ARZ
ADUM5401 Quad-Channel Isolators with Integrated DC/DC Converter (3/1 channel directionality) ADUM5401CRWZ ADUM5401WCRWZ ADUM5401ARWZ ADUM5401WCRWZ-1
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