18F4585 adc-in Pull-up ????

[BigWumpus]

Hello,
I have a problem !

I use 2 adc-ports AN0 and AN1
On AN0 there is an table signal 1,27V.
On AN1 there is a stable signal 2,7V.

At each port I use a resistor (22k or 10k) to protect the port and 10nF in order to filter any spikes.

I measured the signals and found out:
At port AN0 the signal has 1,27V and "in front of" the resistor 1,27V too.
But at AN1 i measure 2,8V at the port-pin and "in front of" the resistor 2,7V.
AN1 seems to have an pull-up....

what the f... ?

[sougata]

Hi,

Please note that the ADC results are dependent on the supply accuracy. Since your Vref is derived from VDD (I hope). If you require accurate results try using a temperature compensated Vref. May be a TL434.
Also since you are using a low pass assure you have enough sampling time.

[Bruce]

Use smaller resistor values on your input filter. Recommended max is 2.5K.