We are using F280025C launchpad for the prototyping purposes. One of our use case is sensing AC Current signal through a CT. This signal is level shifted using internal 1.65V volts which is available on VREFHI pin (in internal supply mode). Finally this signal is processed to give a RMS value indication of the AC Current being sensed. The problem is subtracting the DC level shift internally from the sensed ADC values.
We thought that since internal 1.65V is being sensed then the ADC value corresponding to that will be stable at 2047 or 2046. But while probing pins on the launchpad, we observed that probing the same VREFHI pin gave 2047 on one instance and 2040 on other instance, and sometimes drifts (not by huge amount) in the middle of operation. Also, the 3V3 pins on the launchpad, give ADC values which are lower than 4095 (showing values around 4080 or 4070). We need a proper estimate of this 1.65V in order to calculate RMS values.
I have tried changing Acquisition window from 8 to 20 to 50. The ADC value for VREFHI came correctly (at around 2048), but value 3V3 was still 4080 which doesn't make sense.
Please suggest a way forward.
