It was a basic variable gain op-amp circuit (ascii schematics, ACK!  I
swear, I'm going to write a VGA Dos circuit renderer, if this keeps
up...)

Looked functional to me, though I'd want a little more flexibility than
just 4 gain bands (as in, the jump from 1:8 to 1:16 is pretty large; how
about using any 1,2,3 or 4 pins at once to give more gain choices?)
Careful resistor choices should let this happen...  When all 4 resistors
are grounded, you have 1:2 (with my plan here.)  Just another option <G>

  Mark

Dr. Imre Bartfai wrote:
>
> Hi,
>
> sorry, the circuit cannot be read. OTOH, you can try digital potentiometer
> from Xicor or from DALLAS...
>
> I hope this helps.
>
> Imre
>
> On Tue, 26 Oct 1999, Nikolai Golovchenko wrote:
>
> > 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.
> >
> >