I have a 5 V system and I want to monitor the current in different parts of the system. I am planning to place a small value resistor on the positive power rail and feed the rail voltage and the voltage after the sense resistor to the microcontroller ADC peripheral. The maximum current is 0.2 A, and the sense resistor I am thinking of is 0.5 Ω.
I always see people use a differential op-amp to amplify the signal. Can I not use the op-amp and directly read the voltage using the ADC?
Some microcontrollers may be able to read a differential of 100 mV with 5V common mode, but with 5V reference and a 10 bit ADC, that is only about 5 mV/count, so 100 mV would be just 20 counts or a precision of 5%. A 12 bit ADC would provide close to 1%.
There are many high side current monitors that can provide gain of 20, 50, and 100, and are much better than most differential amplifiers.
Something like this:
simulate this circuit – Schematic created using CircuitLab
Sure, you can do that.
200 mA through 0.5 Ω is 100 mV, but your ADC reads 0 to 5 V so only 2% of its range is usable. So, for example on an Arduino Uno, instead of 1024 counts you only get 21 counts for 0 to 200 mA, 1023 being 0 mA and 1002 being 200 mA.
