Hi Bob, Thank you for your answer. That is what I expected. Unfortunately, I am struggling to get good results. My testing circuit is a little bit different now. I am using AIN1 - AIN2 to measure VB (with gain 2) and AIN0 - AIN1 to measure VAB (with gain 4). R3 is actually 9.927K and Vcc is 0.5V. Measurement of VB is as expected to the 3rd decimal place. Unfortunately, I don't have an instrument to exactly measure VAB so I calculate R3 from VB and VAB (=IB) to check the readings. Performing more measurements I get values in the range: 8.189K - 10.280K My test circuit is on a breadboard so I think that values are good enough. I have also tried to measure VA and VB separately (AIN0-AIN2 and AIN1-AIN2) and calculate the difference but I get worse results. Any suggestion to get better results will be helpful. By the way, my objective is not to calculate the value of R3. In the final system, R3 will be replaced by a non linear device and I have to measure its current at different exciting voltages. Since VA could be as large as 15V in the final system, I need a voltage divider to bring the voltages down to required ranges as shown below. Problem is that the readings are now very far from the expected values. Calculated R3 is in the range: 0.120K - 0.121K Voltage divider resistors are 1% but not perfectly equal so swapping them I get completely different values (even VAB < 0!). I tested with different voltage divider resistors values (up to 1M) getting similar results. Could you please help me sorting this problem out? Do I need a different ADC device? Thank you in advance.
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