Noise. Probably from Vref. Also from stray lead noise. Possibly too
high a source impedance.

I put a good bypass cap on the PIC power leads, and also a small value
cap (100pf) right at the AN0 pin to minimize the effects of charge
dumping
of the multiplexor.

The quickest way to find your problem is to use a good scope. Be sure
you
ground the scope probe with a SHORT lead, right at the PIC gnd, or
you'll
be chasing ground noise rather than your real noise source.

Also, make sure that your A/D conversion is complete. Reading before the
conversion is complete can give you a partial result wherein the LSB's
are still set high.


Jeethu Rao wrote:
>
> I just wanted to test the accuracy of the PIC16f877 ADC. So, I wrote a simple routine to perform a A/D convertion and to read the ADRESH and ADRESL regs, convert them to ASCII and then transmit them through the UART to a terminal Program on a PC. I used only the AN0 input. And I did'nt opt for external voltage refs.And Finally te results were right justified
>
> But I was getting readings between 00 and 12 (Decimal). Is this normal or Is it because I'm not using External Refs ? I connected the AN0 pin to Ground and I get a perfect 00 on the ADRESL Reg. The track connected to the AN0 pin is quite long. So is it because of some stray capacitance or RF Interferance ?
>
> Can someone suggest a fix for this problem ?
>
> Jeethu Rao
>
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