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I'm designing a device that needs to take a wide range of analog readings and I'm trying to keep it to one microcontroller with multiple input ADCs. I need to measure voltages between 0 and 12V as well as currents. I also need to measure the voltage drop over a P-Channel high side FET with precision (it should be somewhere around 150mV).

I know I could measure the upper and lower voltages and subtract them but the tolerances on the resistors would not provide accurate enough results.

I'm wondering if there is a way to isolate the voltage differential and amplify it with respect to ground?

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One way to do this is to make a small isolated supply, like maybe a flyback converter. Transformers aimed at POE (power over ethernet) are usually well suited for such small isolated supplies.

Once you have a isolated supply, you can run a small microcontroller with A/D and serial port. There can be analog circuitry in front of the A/D to amplify or condition the signal as necessary. The micro converts the signal to digital, which it then sends out serial over a opto-isolator. The main micro then reads these values over its UART intead of it A/D.

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