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I am currently designing a 4-20mA current loop with external sensors which work at 12V DC. I saw a differential application on the datasheet of the ADS1115 ADC, but the simple shunt resistor I believe would be OK in my situation. I would like your opinion on the GND side of the shunt resistors, is a 100 ohm (R25) resistor enough?

On a MAX31865ATP there is an analog ground for the analog part of the schematic.

Should I also isolate my 4-20mA or doesn't it matter, because I only need to measure the current passing through the known shunt resistors?

keep in mind that (R21 to R24) are the same part

**keep in mind that (R21 to R24) are the same part

Also, is my assumption of the 10 ohm shunt good, providing that the ADC reads from -0.3V to 0.3V? Or should I put a bigger resistor and extend to the input limits of my ADC?

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  • \$\begingroup\$ You should add more details. First of all, what do you mean with "ADC reads from -0.3V to 0.3V"? Can you provide some details on how this sensor is working? Why do you think that a common 100 Ohms resistor (R25) is necessary? \$\endgroup\$ – Stefan Wyss Oct 7 '18 at 18:44
  • \$\begingroup\$ Thanks for the observation, the adc reads from GND - 0,3 to VCC + 0,3 . So I would definitely increase the shunt resistor value. I just added a 100Ohm resistor to the GND I really don't know why, I was affected by the analog ground of the max31865. I think that the adc works like a volt meter and it needs the "negative lead", should I just put the commons directly on the ground? \$\endgroup\$ – geocheats2 Oct 8 '18 at 4:07
  • \$\begingroup\$ Yes, you should put the commons directly to GND. Use 240 Ohms resistors to make use of the full input range of the ADC. \$\endgroup\$ – Stefan Wyss Oct 8 '18 at 4:29

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