# Wheatstone bridge imbalance voltage calculation

I have a simple force measuring circuit which consists of a strain gauge SG1 and an amplifier with a Wheatstone bridge.

The variable resistor RV1 is set to 200 Ω. If the strain gauge is fully deflected, the circuit gives an output of 50 mV.

How would I go about calculating the imbalance voltage at the bridge?

I found the voltage that goes into the op-amp's pin3:

$$R_{FSR}=\dfrac{1}{\dfrac{1}{1320}+\dfrac{1}{1320}}=660\ \Omega$$

$$V=\dfrac{680}{680+660}\cdot9=4.57\ \mathrm{V}$$

Any help will be appreciated.

• I am having a hard time understanding what you are asking here and an even harder time figuring out why you have the op-amp wired that way. It should be like this bing.com/images/… Commented Nov 30, 2017 at 1:23
• there is nothing wrong with the circuit. Commented Nov 30, 2017 at 1:29
• Sure.. its fine till you hook any significant load on the output.. Commented Nov 30, 2017 at 1:47
• The Supply sensitivity is too high and you need a differential Amp on the output, when you should have it here. Change all 1k2 to 120 and add an LDO for 15mA into 120 Ohms or 1.8V then use a rail to rail Op Amp with >10K load using Trevor's link. You should get 500mV then full scale then and less CM error and gain error. Commented Nov 30, 2017 at 1:53
• Your gain is ~200 so the full scale input is 250uV making it very prone to noise and errors. So there are a few things wrong Commented Nov 30, 2017 at 1:59