Hello all.
I need to read 50Hz signal with PIC16F877. Actually, 10-bit accuracy is
enough, but the signal level is unknown, therefore PIC has to adjust to the
signal. The level changes relatively slowly and there is some time allowed
for adjusting. In this case four gain bands (1:2, 1:4, 1:8, 1:16) will do
the job.

Now, the problem is that I don't want to use variable gain amps, because
they are too expensive and hard to get here, in Ukraine. Another way is to
use analog switches which will change the gain of a usual op amp. Can the
PIC's pins be used as analog switches, connected to ground? The datasheet
says that ports use FETS and that the leakage current is as small sa 1uA.

I would connect four resistors to PIC and made the pins go low or tri-state.
Will the circuit work?

         __
-------| + \
         |       \
         |         >------------- To RA0/AN0
         |        /          |
     ---|--  /              |
     |        ____      |
     |-----[_____]----
---------
---------
     |----------------
     |      |      |     |
    [ ]    [ ]   [ ]   [ ]
    [ ]    [ ]   [ ]   [ ]
     |      |      |     |
     |      |      |     ----------    RA1
     |      |      ---------------    RA2
     |      ---------------------   RA3
     --------------------------    RA5

Thanks in advance.