>what if padaanin ko muna sa sa op-amp (or diff amp)?
Yes, it is needed. See my post above.
>di kaya mas malaki ang error ko?
No. Opamps should not affect. Voltage offset though affects at lower inputs near 0V. But you could correct this by using a negative voltage supply or floating ground (1/2Vcc) and subtract the 1/2Vcc in software.
>may opamp ba na kayang mag-operate down to 3.3V supply?
Yes there are. We are using mostly 3V opamp. LMV358 is the one we use.
>sa ngayon naka-set yung TAD ng ADC sa 3.2us
, ok na kaya yun?
>2us daw ang minimum ng pic16LF73
according sa datasheet..
Yes, TAD is minimum 1.6uS in 877. Maybe same as your chip.
BTW, you can use the smallest TAD to be fast. This does not affect your input impedance. But the 120pF sampling cap does affect in AC reading.