Saw you message on the pic-list. Might be off topic , but
try this program for ascii schematics. Can't remember where I got it from.
Cheers
Rob <<sch-draw.exe>>

> -----Original Message-----
> From: Mark Willis [SMTP:mwillis@FOXINTERNET.NET]
> Sent: 26 October 1999 10:13
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Variable gain amp using PIC - possible?
>
> 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.
> > >
> > >

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