Electronic – Why is the instrumentation amplifier’s output voltage completely wrong


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".


  1. 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?

  2. 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?

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Best Answer

The offset voltage of each of the op-amps you're using can be as much as +/-3mV at room temperature. So, the difference between two outputs could be as much as 6mV different from the inputs with unity gain. You're seeing 5.4mV which is large, but within specifications and therefore plausible.

Since you don't have much gain in the first stage (only 3) you also have to consider the offset voltage in the second stage. In any case, 638 times your measured differential input offset voltage of 5.4mV + 2.5mV signal is almost 5V.

You can either use better op-amps (such as autozero or 'zero drift' types) or null out the offset voltage by some means (trimpot or reduce the gain and do it digitally). You should also consider the drift of the op-amps you're using which is not guaranteed, but is fairly reasonable typically (+/-1.7uV/K).