Gabonator! You mentioned that genarator gives signal above zero only. Connect generator and a probe via capacitor.
I missed the reason why device is so unlinear. But i see few experiments with it.(I have not device itself, so it is only imaginations).
- Battery test. Take AAA(AA) battery and try to measure voltage. it must be about 1.5 volts in direct polarity and -1.5 in reversed. if it is so(omitting details that battery has not exactly 1.5v), then linearity in point 1.5/-1.5 looks good.
- Test with sawtooth signal - connect generator and probe via capacitor, the waveform must be really linear and visually symmetric relative to zero level. if it is so…you really do not need any calibration.
anyway you can develop a calibration application based on measurements of “sawtooth via capacitor” measurement data.
regards.