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I am working on a project, and am trying to amplify the voltage across wheatstone bridge. Here is the diagram:

Wheatstone bridge to amplifier

where \$R_1 = 1,000 \Omega\$

\$R_3 = 10,000 \Omega\$

and \$R_2\$ is a potentiometer, set so that the voltage across the bridge is 10 mV

The gain of the amplifier is simply \$R_3/R_1 = 10\$, so I expect an output voltage at \$V_{out}\$ of 1V.

I do not see 1V out, I see somewhere around 1.5V, and I am certain that whatever the problem is has something to do with the fact that the voltage across my bridge doesn't stay at 10mV when connected to the amplifier. When I connect the bridge to the two leftmost op amps, and check the voltage across the bridge again, it is around 45mV.

Why is this? I've checked my amplifier circuit with classmates and rebuilt it several times, I can't tell what might be wrong. What might be causing such an issue?

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  • \$\begingroup\$ You have proper +/-V supply rails to the opamp, not just +5 and GND? Or is it a rail-to-rail opamp? \$\endgroup\$
    – user16324
    May 27, 2016 at 21:51
  • \$\begingroup\$ It's an LM324N opamp, one lead takes a positive voltage and one goes to ground, which is how I have it configured. \$\endgroup\$ May 27, 2016 at 21:54
  • \$\begingroup\$ @jphollowed Not to belabor Brian's point, but is the correct one going to ground (pin 11) and the correct one (pin 4) going to +5? (numbers refer to DIP or SOIC packages). If so, are the grounds the same, and what is the voltage on each input wrt ground? \$\endgroup\$ May 27, 2016 at 22:10
  • \$\begingroup\$ @SpehroPefhany Yea the are correct \$\endgroup\$ May 27, 2016 at 22:12
  • \$\begingroup\$ Have you accounted for the non-ideality of the your op-amps? Those are BJT input stage op-amps and thus will have input bias currents as well as having offsets on the outputs. You should be able to systematically determine where the difference is coming from by changing up connections and shorting leads together and seeing how things diverge from expectation. \$\endgroup\$ May 27, 2016 at 22:19

2 Answers 2

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I'm not sure why you expect 1V out, with a 10 mV input and a gain of 10. Personally, I would expect something closer to 100 mV, but in any case, 1.5 V is obviously wrong.

The LM324 should be capable of driving the output to 0V. However, what it can't do is drive the output negative. Are you sure your bridge is unbalanced in the direction that will drive the output positive — left side higher than right side?

Some opamps have the property that they invert the output when the input stage is driven too hard into saturation. I don't recall offhand if the LM324 has this problem, but it could explain what you're seeing.

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  • \$\begingroup\$ Thanks for the reply. I fixed it. You're right trust i should not have expected a 1 v output, but rather a 0.1v. I discovered that the op amp for some reason will not output below 0.5v, I'm the amplifier configuration i am using. I changed my resistance ratio by using some 220 ohm resistors, and now have a gain of 40, and changed my input voltage to ~30mV. I also made the op amp and bridge share a common ground, which they weren't doing originally \$\endgroup\$ May 28, 2016 at 5:36
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The LM324 inputs draw a bias current, albeit very small. Unless the bridge supply and amplifiers have a connection (your diagram shows one floating with respect to the other) then the bias current will pull the inputs right up to the top rail, out of the permitted input range, and your amplifiers will stop working as amplifiers.

As the bridge outputs are nominally 2.5v, and that is a valid input level for LM324s, connecting the opamp and bridge grounds should be sufficient.

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