I developed this circuit based on TI's SBOA336 app note:
Low(Microamp), High-Side, Current-Sensing Circuit with Current-Sensing Amplifier... Instead of theINA190A1 I used a INA225 as it was available at JCLPCB, and an ADS1114 ADC for evaluating changes to the temperature monitor I've been working on. This works, with a few unexpected
features I can't explain;
1. Large offset voltage for the input of INA225 (it follows the gain setting of the INA225. I have an offset of about 37mV, which translate into 0.97V at g = 25, 1.87V at g = 50 and 3.75 at g = 100. Initially, I thought this might be due to the input offset of the voltage follower for the analog output (poor op-amp choice), so I cut the trace to this stage without any change.
2. I've done testing with a 1k shunt, and loads of 4.7M, 2.7M and 1M with a 5 volt supply. If I subtract the open-circuit output from the load current measurement, the ratios for current for the different resistors track, and the ratio for the same resistors at the different gain settings track. For example, I get a reading of ~16.6mV/ µA, where I expect close to 25mV/ µA with a 1k shunt resistor.
I did look at the output with a cheapie 'scope. No evidence of anything oscillating.
Any suggestions as to what I'm missing?