I don't have the answer for you, and I'm ceretainly no expert, but I have used the PICs ADCs before and found them to be very accurate,
Something's for sure not right with your readings though.
I take it you are running at 5V.?

It's a 10-bit ADC so you should have 1024 values.
So 0 being 0 is the only one that looks good to me.

Your 5 volts should be 1023.

Vss reference is sent to ground?
Are you using Vref?

Did you look at the voltages of these pins to make sure everything is as should be?

>The numbers I am getting from the ADC are
>
>V       ADC
>0       0
>1.67    257
>2.5     257
>3.33    514
>5       771

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