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'[EE]: Suitable op-amp bridge amp.'
2002\07\17@132415 by A.J. Tufgar

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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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2002\07\17@134018 by Eisermann, Phil [Ridg/CO]

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       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 removed}

2002\07\17@140929 by A.J. Tufgar

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Thanks Phil,
           I tried this design, but if doesn't seem to work with my
sensor.  It centers around 42mV, but when I apply a slight pressure the
output drops to about 40 mV then if I apply alittle more it jumps to
like 200mV.  No idea why.

Aaron

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2002\07\17@163739 by Eisermann, Phil [Ridg/CO]

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       Are you using single or dual supplies for the op amps? You said the
output is 0.1V to 0.5V. I assumed this was the differential voltage. What is
the voltage of the sensor at one leg of the bridge? At the other? It might
be possible that this voltage is very close to the negative supply. What
kind of pressure sensor are we talking about?

-Phil



> {Original Message removed}

2002\07\17@165354 by Francisco Ares

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The LM324 has a poor offset temperature drift, you may try to look to a
better IC. This voltage applys directly to your input voltage as it
increases proportionaly with gain, i.e., if you have 400 mV of signal
swing and a drift of, say, 1mV/degree of temperature offset drift, in 10
degrees of temperature difference from the time you adjust your sensor
to the time the hole circuit stabilizes in temperature, you will have a
10 mV drift, aparently over time. AFAIK, Texas TLC27L4 has an
exceptionaly low drift, but there are many others.

By the way, a good and true diferential amplifier uses three opamps:



              |\
 S-   --------|+\
              |  >----+--R3--+--R4---+
           +--|-/     |      |       |
           |  |/      R1     |       |
           |          |      |       |
           +----------+      |  |\   |
                      |      +--|-\  |
                      R2        |  >-+- Eo
                      |      +--|+/
           +----------+      |  |/
           |          |      |
           |  |\      R1     |
           +--|-\     |      |
              |  >----+--R3--+--R4---+
  S+  --------|+/                    |
              |/                    GND

and the gain is

  Eo              2 x R1      R4
---------  = ( 1 + ------ ) x ----
(S+ - S-)            R2        R3

Instead of conecting the ground to the bottom R4, you may conect this
terminal to the output of the forth opamp of the package connected as a
folower of a trimmer conected between Vcc and GND (or -Vcc), for
example, for zero or offset adjust.

Hope this helps.
Francisco




A.J. Tufgar wrote:

{Quote hidden}

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2002\07\17@175959 by A.J. Tufgar

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Phil you missed the m in front of the voltage.  :)  It's about 0.1 to
0.5 millivolts or 100 - 500 microvolts.

I'm using a single supply and the voltage is approx half way between
Ground and Vcc I think 1.6V.  The pressure sensor is a gage type, 0.3
psi air.  Again 5kohm input and 3Kohm output.

Thanks Franciso for your help also and again to Phil.

Again I've had trouble with the three op-amp instrumental setup, no idea
why.  For some reason a slight change in pressure makes the voltage go
lower, but increasing the pressure further will then cause the opamp
output to go pass the initial value and increase drastically.  Any idea
why?

Thanks,
Aaron

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2002\07\17@182935 by Tom Messenger

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I did not notice what type of opamp you tried for the 3amp classic circuit,
Aaron, but *IF* you are using a LM324 type, remember that it has it's input
common mode range restricted to about 1.5 volts down from the positive
rail. It's output also only goes about that high. This might have been
thwarting you. If so, a rail-to-rail type of CMOS opamp might work out for
you.

At 05:59 PM 7/17/02 -0400, you wrote:
{Quote hidden}

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2002\07\17@185242 by A.J. Tufgar

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Thanks a bunch Tom!
Blast, So it is the lm324 that's my nemesis!  I'd overvoltage it in
retaliation but my benchtop supply only goes to 30V and the LM324 will
take a 32V supply, foiled again.  I'll have to save it for another
project.  :)

Oh well slowly learning.  I think I'll try an LM6134 instead.

Thanks to all,
Aaron

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2002\07\18@135745 by Peter L. Peres

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On Wed, 17 Jul 2002, A.J. Tufgar wrote:

>Thanks Phil,
>            I tried this design, but if doesn't seem to work with my
>sensor.  It centers around 42mV, but when I apply a slight pressure the
>output drops to about 40 mV then if I apply alittle more it jumps to
>like 200mV.  No idea why.

One of your inputs exceeds the cmmr of the opamp. Get a rail to rail
opamp.

Peter

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2002\07\18@135800 by Peter L. Peres

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Get a rail-to-rail output opamp from Microchip, Linear tech, etc. What's
wrong with LM10 again ?

Peter

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