DPRG mailing list archives: [DPRG] Analog Gurus needed

Wed Nov 29 15:51:46 CST 2006

Thanks for all the replies guys...

The national app note is a nice compilation.. Thanks Kip -- and
National...

I am trying to create an instrumentation amplifier using a single supply
quad op amp at 3.3 volts.
I can make the classic instrumentation amp using 3 op-amps, and I have
one left over to use for the offset.
The sensor I'm using has a tolerance of +/- 2mV, and I don't want the
differential output to ever go negative, so I wanted to add some offset
(5mV) to the positive side before amplification. I don't want gain here
because multiple stages of gain will yield a non linear response. It
looks like I might be able to use the non-inverting summing circuit from
the national app note. I'll have to build it and give it a try.

Thanks for the input...
Rick

-----Original Message-----
From: Kipton Moravec [mailto:kip at kdream.com] 
Sent: Tuesday, November 28, 2006 5:38 PM
To: Kenneth Maxon
Cc: dprglist; Rick Bickle
Subject: RE: [DPRG] Analog Gurus needed

Keith you are backwards.

He wants a difference amplifier and then offset the result +5 mv.

I would probably use a second op-amp after the difference to provide the
offset.

It would be configured as a non-inverting summing amplifier. The best
reference is from National. AN-31
http://www.national.com/an/AN/AN-31.pdf

In practice I would probably use a package that has 2 op-amps and make a
difference amplifier.

Kip

On Tue, 2006-11-28 at 17:18 -0600, Kenneth Maxon wrote:
> Here to you will need a negative rail for your op amp.  If inverted 
> output can not be used, a follow on inverter can be used.  If all you 
> are trying to do is remove an offset from a signal then make all the 
> resistors the same value, say ~10K.  Op-amp selection will depend on 
> the speed of the signal you are trying to monitor and other factors...
> 
> http://www.users.qwest.net/~kmaxon/difference_amp.gif
> 
> -Kenneth
> 
> -----Original Message-----
> From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
> Behalf Of Jeff Koenig
> Sent: Tuesday, November 28, 2006 3:20 PM
> To: Rick Bickle
> Cc: dprglist at dprg.org
> Subject: Re: [DPRG] Analog Gurus needed
> 
> 
> Hello Rick,
> 
> Will a Summing amplifier work?
> 
> http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm
> 
> It inverts, but the gain is 1.  You can add another unity-gain 
> inverting stage if you need to.
> 
> Do you have a negative rail for your voltage supply, or only a 
> positive supply and ground?
> 
> -Jeff
> 
> On 11/27/06, Rick Bickle <rbickle at intconsys.com> wrote:
> >
> >
> >
> > Hey all,
> >
> > Here's a question for all you analog gurus out there.
> >
> > I am trying to design an amplifier to take a differential output 
> > signal of
> 0
> > to 10mV, to a ground referenced signal with a range of 5mV to 3.3V.
> >
> > The tolerance of the input is +/- 2mV, which means that the input 
> > could go negative at minimum readings.
> > An instrumentation amp will handle the differential to ground 
> > referenced conversion, but I need to add a positive offset of 5mV or

> > so before I amplify the differential signal.
> >
> > How can I add 5mV to the input signal without adding gain?
> >
> > Your help is much appreciated.
> >
> > Thanks,
> > Rick
> > _______________________________________________
> > DPRGlist mailing list
> > DPRGlist at dprg.org
> > http://list.dprg.org/mailman/listinfo/dprglist
> >
> >
> >
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--
Kipton Moravec <kip at kdream.com>