Has anyone applied the recommendation in "EIE/AN93017 Using the analog-to-digital converter of the 8XC552 microcontroller" application note of using a RC filter before the ADC of the 80c552. It says there a filter with a time constant >= 500us should be put before the ADC. Doesn't this filter alter the signals? And what values did you choose for the RC? If a resistance bigger the 1k is used, there may be a 1mV voltage drop accross it, as the input current of the ADC may be 1uA.
Then, the capacitor of the RC low pass filter is in parallel with the sampling capacitor of the ADC... Isn't this a problem?

Thank you!