3 wire and 4 wire analog input

Question

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?

Answer 1

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.