Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!husc6!psuvax1!psuvm.bitnet!art100
From: ART100@PSUVM.BITNET (Andy Tefft)
Newsgroups: sci.electronics
Subject: Re: Op amp gain control
Message-ID: <72478ART100@PSUVM>
Date: 20 Feb 89 23:32:23 GMT
Organization: Penn State University - Center for Academic Computing
Lines: 39

Thanks to everyone who answered my question about controlling op amp gain
digitally.  I didn't expect to get so many answers!  Here's a short summary of
some of the suggestions I received (sorry I can't attribute these to specific
people, but they know who they are anyway):

Some people suggested an analog multiplexer, with a different resistor on each
line.  Good idea, possible for intermediate testing, but in the final version
we will want more resolution.

Many suggested a discrete FET.  We want to stay away from discrete
transistors.  I also don't think we could get the range we want out of this.

One idea which I can't remember the details of was a sampling-type circuit
which varied the frequency of a square wave which was multiplied by the
output.  Varying the frequency would change the average value of the signal.
Unfortunately I'm kind of worried this would kill an audio signal.

Other suggestions included an optocoupler or phototransistor/led combination.
This was one of my first ideas but I really didn't want to do it.

Another common suggestion that I considered was using a Xicor EEPOT.  I was
unable to locate any of these on short notice, and also found out that it
wouldn't quite fit our needs.  We'd like to be able to read off and display an
indication of how much gain we have.  Short of a non-volatile counter or
something of that nature, we couldn't think of a great way to do this.
Besides, I never got any indication that the EEPOT could be reset to some
preset value.

I think we're settling on the most common suggestion, a multiplying DAC.  I
didn't even know such beasts existed...  Anyway there are apparently several
different ways this can be incorporated into our design, and we'll try a
bunch.  We can play around with varying the values fed into this to give us
different ranges, and it's still easy to display or reset the value of the
gain, unlike the EEPOT.

Well, I tried to thank everyone personally but I know some didn't make it
through, so if you sent me a suggestion, thanks.  I'll post later to let
everyone know how this works out.

Andy