You want to look at differential amplifiers. For any accuracy, you
should use the classic three op-amp design. You can buy these prepackaged as
'instrumentation amplifiers' I like the INA125 from Burr Brown (now TI), as
it includes a voltage reference. There are many other varieties. It is
difficult to cost-effectively match the performance of an integrated
instrumentation amplifier with a discrete design.

        Also, the circuit you chose is a current amplifier, not a
differential voltage amplifier. Look one page back (pg 15). That page shows
the classic three opamp design.

-Phil


> -----Original Message-----
> From: A.J. Tufgar [mailto:tufgaraj@MUSS.CIS.MCMASTER.CA]
> Sent: Wednesday, July 17, 2002 1:23 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: Suitable op-amp bridge amp.
>
>
> Hello all,
>           I was wondering if I could have some second opinions on an
> amplifier design I'm using.
>
> Basically I have a wheatstone bridge with input resistance of
> 5Kohm and
> output resistance of 3Kohm.  I have a 3.3V source applied and when
> pressure is applied The output is between .1-.5 mV (that I
> care about).
>
> All I did was connect the sensors outputs to the diff inputs
> on a LM324
> and put a 2.1Mohm between output and V-.  Seems to work ok.
> The output
> is centered around 97mV.
>
> I'm worried about it for a few reasons though, I used the
> schematic out
> of the lm324 datasheet (pg 16) and it wasn't nearly sensitive enough.
> So I removed the resistor beween V+ and ground now it seems to work
> fine, as stated above, but there is some nasty drift over
> time of maybe
> 10mV.
>
> I've been trying a lot of differential amplifier designs(mostly right
> out of the datasheet or textbooks), but none seem to be sensitive
> enough.
>
> Anyone have a more sensitive amplifier for this range?  Or can help me
> correct the drift?
>
> Thanks,
> Aaron
>
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