18F2455 and ADC conversion with VREF on AN3

[NicolasC]

I'm using AN0-AN2 on 18F2455 with a 10 bits conversion.
As my input will be 0V-0.6V, I use setup_adc_ports(AN0-AN2|VSS_VREF) and input a trimmer midpoint on AN3.
The AN0-AN1 conversion is fine, but on AN2, I suspect the conversion is still done with a 0V-5V scale because on the port I can reach only 10% of the full scale value.
Does this have something to do with the fact that AN2 may be configured as VREF- ?

Regards

[Ttelmah]

First comment, if you intend to drop Vref to 0.6v, it won't work for anything approaching the accuracy you want. (Vref Min, for 3.3v operation, is 1.8v, and for 5v operation, 3v). Beware also, that the Vref source, needs quite a low output impedance (<2.5KR). Using a direct potentiometer, is not a formula for good accuracy.
Now, the same ADC, is used whatever pin is being read. There is only an input 'multiplexer' on the front. So, you need to check what voltage is actually present on your AN2 pin. I'd suspect that the problem is that you haven't disabled the comparator module, so the comparator Vref is being drawn on this pin, loading it down significantly.
setup_comparator(NC_NC_NC_NC);

Best Wishes