# Unexpected results when trying to amplify voltage across a wheatstone bridge

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

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?

• You have proper +/-V supply rails to the opamp, not just +5 and GND? Or is it a rail-to-rail opamp?
– user16324
May 27, 2016 at 21:51
• It's an LM324N opamp, one lead takes a positive voltage and one goes to ground, which is how I have it configured. May 27, 2016 at 21:54
• @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? May 27, 2016 at 22:10
• @SpehroPefhany Yea the are correct May 27, 2016 at 22:12
• 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. May 27, 2016 at 22:19