ADCIN & 16F870 - how to avoid non linear conversion?

[CuriousOne]

Set scope to 50mV resolution and AC input. Found an 120mv max noise pulses, with 3.35hz following frequency.

[HenrikOlsson]

Where?
On the power rail, at the output of the voltage reference, at the ADC input or all of the above?

If the noise is only present at the ADC input of the PIC then the problem is probably too high source impedance (that 10k pot you're using) and the 3.35Hz you're seeing is how often the ADC is sampling the input. Or is this with a low impedance source?

[CuriousOne]

Yes it was with 10k pot, measured at ADC input. no other tests so far.

Actually, I went with ADC route because I was not able to make BH1750 light sensor to work with picbasic, and since I badly need light measurement, I've decided to do a circuit with photoresistor and ADC, but it appears to have it's own bunch of problems.

[CuriousOne]

Yes, these pulses were from ADC. If turn ADC off, pulses are gone.

Problem is solved, but it was weird.

Since PIC has two Vss pins, I was using one Vss for GND, and to other Vss pin I've tied LCD gnd pin. So current going trough the PIC was causing the troubles. After I've added jumper to feed Vss directly to LCD, problem had gone and linearity improved.