CN0066: Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC and the ADuM5401 Digital Isolator

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OVERVIEW

Circuit Note PDF, 11/2010 (pdf, )
Benefits & Features
  • Fully Isolated Input Module
  • 24-Bit Sigma-Delta ADC
  • Single Supply Input Circuit
  • Programmable Gain Allows Detection of Small Signals
Products Used
    Applications: 
  • Programmable Logic Controllers/ Distributed Control Systems
  • Temperature Controllers
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 a single-supply input circuit design requiring isolation. The AD7793 is a 24-bit Σ-Δ ADC. The ADC includes an on-chip PGA and can, therefore, accept small signal inputs from sensors directly. The PGA gains can be set for 1, 2, 4, 8, 16, 32, 64, or 128. The ADuM5401 provides all the necessary signal isolation and power between the microcontroller and the input.

Figure 1. 24-Bit Isolated Single Supply Input Circuit (Simplified Schematic)

CIRCUIT DESCRIPTION

Sensor outputs are commonly small signal, such as thermocouple or RTD. This design allows these small signal inputs to be directly connected to the AD7793 input, which has an internal PGA with a maximum gain of 128. The design can be used as a complete solution to a non-loop-powered SMART transmitter.

The ADR441 is the chosen reference for this circuit. The ADR441 has excellent accuracy specs of 0.04% and a drift 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, which can provide 500 mW of regulated isolated power at either 5 V or 3.3 V. This design uses the 5 V provided from the ADuM5401 to supply all the analog circuitry on the input module. All four data wires are used: three for transmit (CS, SCLK, DIN) and one for receive (DOUT), which connects 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.

Figure 2. Input Module Histogram AD7793, 512 Samples, 4.17 Hz Data Rate, Gain = 1, Input = 2.5 V Reference

Figure 2 shows a histogram plot of the AD7793 output performance when a 2.5 reference is connected to its inputs. The measured noise on the output is measured in peak-to-peak resolution. The 20.4-bit peak-to-peak resolution figure means there are 3.6 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 20.4 bits. In terms of contribution to the overall system error, the measured noise of 3.6 bits contributes ~0.00007% error with respect to the full-scale range of the ADC. The integral nonlinearity error of the AD7793 is 0.0015%.

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 System,” Analog Dialogue, 43-04, April 2009,” 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. The ADC channels correspond to the circuit as in CN0067.

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
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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