Hi, thanks in advance for your time and expertise.
I am using an INA122 to measure input and output voltage in an GaN inverter.
When I measure the sensor response with the scope when the circuit is not switching (no noise), voltage measured in the RC output filter is clean and perfectly linear.
When the circuit is in operation, but no connector is attached to the INA122 except output RC filter, measure is also OK.
When the sensor is connected to the FPGA ADC, DC offset value appears at the output. This DC offset error is reduced when the noise generated by the switching is also reduced. The response approaches then the previously measured one (almost ideal, orange in the picture).
The load of the INA122 is an RC filter, and two twisted cables going to the ADC of the FPGA.
Why is there a DC offset in the output? How is the noise affecting the INA ? How can I fix it? I've tried using a common-mode choke, but this did not fix the problem.
Thanks,
Iñigo
Sensor Input voltage is 0-400V which at INA122 input its 0-200mV and with G=5 means 0-1V output for the ADC.
In orange, measured sensor response when inverter not switching.
In blue, measured response when inverter switching. The "3 staged trend" of the data points I think is just a comsecuence of the operating points, rather than INA Gain since noise makes measure approach ideal response.
In red: Measure moves towards good value when operating point is varied so that less switching noise is generated.