I'm trying to amplify the voltage of my load cell (Wheatstone bridge I believe), but my calculated values are not the same as my experimental values.
The load cell outputs a differential voltage of 0.1mV - 5mV (measured with a voltmeter), and I want to boost it to 0V - 5V (initially, then from 0.3V - 3.3V).
The OP Amps I'm using are MCP6273 "170 μA, 2 MHz Rail-to-Rail Op Amp".
First stage is amplifying the voltage - I put a 2.5KG mass on the load cell, and it output a voltage of 2.5mV differential voltage as expected. However, when I measured the differential voltage between the outputs of the first stage op-amp's (all resistors removed, unity gain) I get a differential voltage of 7.8mV. Why?
Output of second stage is complete wrong. R1 and Rgain are 1Kohm each. R2 is 470 ohm, R3 is 100k ohm. This should give me a gain of 638 (i.e. 63.8mv - 3.19V at the output). However, even with no load (i.e. 0mV) the output is 3.3V. At 2.5KG (i.e. 2.5mV) it hits the 5V rail.
- I have checked the connections multiple times
- I have tried simply buffering the input (i.e without R1/Rgain/R1)
- All the op amps are identical
What am I doing wrong?