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[DPRG] Analog Gurus needed

Subject: [DPRG] Analog Gurus needed
From: Kenneth Maxon kmaxon at qwest.net
Date: Tue Nov 28 17:52:51 CST 2006

Hello Kip & Rik,

The circuit provided is a classic difference amp shown as the 2'nd example
down the left side of the first page of the reference in your link.   Sorry
for my hurried wording, I should have added a bit more detail.  The
difference amp is used in both stages, to buffer the differential signal as
need, and in the 2'nd stage to add the offset.  In the 2'nd stage the offset
is a constant DC voltage as that shown in the drawing provided.  Yes, my
brain jumped on that first answer a bit to quick.  Thank you for catching my

If you keep these around for reference, Natl's Application Note 20 is also
worth keeping around.


-----Original Message-----
From: Kipton Moravec [mailto:kip at kdream.com]
Sent: Tuesday, November 28, 2006 5:38 PM
To: Kenneth Maxon
Cc: dprglist; rbickle at intconsys.com
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

It would be configured as a non-inverting summing amplifier. The best
reference is from National. AN-31

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


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
> 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
> 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
> > 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>

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