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Currently, we have a new requirement from one of our customer on ADC interface. They want the ADC circuit which should support 3 and 4 wire configurable input, and it should support 4-20mA input. The sampling rate is 125MSPS with the 16bit of resolution. Since this is my first analog design, I'm a bit confused to selecting a suitable ADC part to meet the above requirement. I would like to get some clarification on my below query.

  1. To support 3 and 4 wire input, is there any specific ADC IC, or can we use any of the IC which is available in manufacturing site.?
  2. Is there any specific part that is supporting 4-20mA current input, or can we use the standard or high speed ADC for this application?
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  • \$\begingroup\$ by 3 and 4 wire input, do you mean Kelvin impedance measurements, as sometimes used for RTDs? \$\endgroup\$
    – Hearth
    Sep 22, 2023 at 18:51

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For 3- or 4-wire measurement you can use two differential ADCs.

For 4-20 mA, first you'll need a shunt resistor to convert the current to voltage, then you can use either:-

  • a differential amplifier to amplify this voltage to more meaningful range then feed it to a single-ended or differential ADC, or
  • a differential ADC directly (This may require a relatively larger shunt).

So, all you need is an ADC having multiple differential inputs and required sampling rate. A differential amplifier for 4-20 mA may or may not be needed.

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  • \$\begingroup\$ Is there any standard shunt resistor value for measuring the 4-20mA current? or we can use customized resistor value based on the maximum ADC capability? \$\endgroup\$ Sep 22, 2023 at 13:48
  • \$\begingroup\$ @Chitharanjan there's no specific/standard resistance value. It depends on a few factors such as the signal source's maximum voltage. Keep it low to stay away from the source's limits, and high to be more noise-proof. 250R sounds like a good value as it gives a 1~5V range. If you use 150R the range will be 0.6~3V which is quite an acceptable value for 3.3V systems. In both cases, it might still be worth using a differential amplifier (with unity gain) to convert this differential voltage into single-ended while eliminating the noise. \$\endgroup\$ Sep 22, 2023 at 14:06
  • \$\begingroup\$ thanks for the detailed explanation, Also I would like to know can I use ADC3564 ADC IC from Texas Instrument for 4-20mA loop current input application by using input shunt resistor? since ADC3564 will convert analog voltage to digital form. \$\endgroup\$ Sep 25, 2023 at 8:18
  • \$\begingroup\$ @Chitharanjan that's a 14-bit ADC but design spec requires 16-bit. \$\endgroup\$ Sep 25, 2023 at 8:43
  • \$\begingroup\$ Yes we are OK with 14Bit resolution with Lesser sampling rate (<125MSPS). Can you please confirm whether I can use ADC3564 or AMC3306 for reading 4-20mA Current by using input shunt resistor? \$\endgroup\$ Sep 25, 2023 at 10:02

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