USIng PGA and POT getting variable gain

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USIng PGA and POT getting variable gain

Postby kunalnitw » Fri Jun 12, 2009 12:00 am

I want variable gain non inverting OPAMp using psoc. But when i am seeing data sheet of PGA i am seeing that i have to use a fixed gain.Anyone suggest how i can make variable gain amplifier using opamp.That i am using for amplifying Audio output from audio pc jack.
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Re: USIng PGA and POT getting variable gain

Postby danadak » Sat Jun 13, 2009 9:32 am

Many approaches, some better than other :

OK : Use A/D & MDAC8 modules, basically digitize pot voltage value and use as
one of the inputs to MDAC8. You are limited in db range to ~ 256: 1 or a little
less than 50 db, and thats not counting noise analysis, etc..

Better (depending on design veiwpoint) : Use AMPINV and pot strapped across
it to control gain. Note POT can be across amp, at input with a fixed R across amp,
or pot used as both feedback and input resistors. All approaches have noise consider-
ations, and are not rail to rail on output side. The latter impact cuts down on output
swing allowed, or Vinmax for a given gain. This is a minimal solution, parts and PSOC
resources.

Third : Could digitize pot, feed to PWM, and use PWM output to modulate an external VCA,
like a transconductance amp. These are cheap and have wide dynamic range and can be made
into VCA easliy. This could yield much greater resolution, set by A/D if PWM >= 16 bits, so
~ 90+ db, but practically noise would control and you would have to do the analysis to determine.
60 - 70 db realizable ?, maybe, not sure.

Regards, Dana.
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Re: USIng PGA and POT getting variable gain

Postby mtellez » Wed Apr 23, 2014 7:35 pm

So there are no work a rounds for this using only PSOC peripherals? I'm trying to dynamically amplify/attenuate a sine wave without having to mechanically turn a pot. I also dont want to digitize the signal. The PGA is only in a few discrete levels, no bueno. Offchip VCA's are expensive. What about daisy chaining PGAs? There has to be some creative fix here.
Thanks!
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Re: USIng PGA and POT getting variable gain

Postby danadak » Thu Apr 24, 2014 2:15 am

Try using "faux" OpAmp" -

http://www.cypress.com/?docID=32375

Regards, Dana.
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Re: USIng PGA and POT getting variable gain

Postby EricS » Thu Apr 24, 2014 2:48 am

One solution could be to use a digital potentiometer like those http://ca.mouser.com/Semiconductors/Digital-Potentiometer-ICs/_/N-4c498.

This kind of device usally use SPI to set the required resistance.

As for noise analasys I can't help you about it.

Good luck!
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