Hi all,

I have an application running on a 16F872 @ 20 MHz that
(among other things) reads and converts data from three
analog channels connected to RA0, RA1, and RA3. The ADC
clock is selected as Fosc/32, so Tad = 1.6 us (Tad is
the bit conversion time).

Each channel is connected as follows:

                         50 Ohm
PIC's RAx ---------+---------/\/\/\/\----------- actual source
                    |
                    /
                    \ 10K
                    /
                    \
                    |
                   -+-
                   ///

So the impedance seen from the PIC should be somewhere between
50 Ohm and 10 K.


Software side of things:

1. I select the channel.

2. Then I am allow for an acquisition time of 22 us (the datasheet
    mentions 20 us for a 10 K source)

3. Then I start the conversion (set the 'GO' bit)

4. Once the conversion is over, I wait _at least_ 11 us (the
    datasheet also mentions that a delay of 2 x Tad needs to be
    honoured here)

5. Go back to step 1, but with the next channel.


Now my problem is the following: I just set one of the three channels
(let's say RA0) to a 4V source, and leave the other two (RA1 and RA3
in this example) unused (i.e. just connected to GND through the 10K
resistor). The first channel (4 V) is read OK, but for the next one
I read I get a non-zero value (about $008). Looks like the S & H
capacitor is not completely discharged.

I've checked the followin:

- Set RA1 to 4 V instead of RA0. In this case it is RA3 the one
   that reads a non-zero value.

- Replace the 10K resistor with a 1K resistor; in this case I
   measure about $004 instead of $008 in the 'next' channel.


Does someone have a clue? What's happening here?

Thanks in advance,
G.

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