I need to amplify 1mV signal from a wheatstone bridge (no DS available). 5kg max weight; output is 1mV/Kg @ 5V. The resistances are 880+880 and 749+749.
Currently I only have LM324 and LM358. And can procure quickly OP177 (others will take a week, which I dont have)
I have tried a standard difference amp (gain 470, 100 and then 1). I have tried the 02 op amp difference amp and finally an instrumentation amp all constructed from a single LM324 chip. The op-amp configurations are from TI SLOA034 http://www.ti.com/lit/an/sloa034/sloa034.pdf. I have used 10k 1% resistors (for unity gain) and 470K, 1K as and when required.
Simulation on Proteus (below) (and the feedback on LM324 on stackexchange) leads me to the conclusion that that it is the opamp that can't perform.
My questions are:
Have I understood the problem correctly (op-amp to blame)?
What parameter limits the the performance of LM324 in such applications?
What is the theory behind the problem?
What is the eventual solution?
- Can I use OP177 to fix the problem?
The problem is depicted below in the picture (on the real circuit it behaves almost similar - the o/p volts are in the range of 0.3 - 0.5V the i/p difference is 1 - 2 mV). In words it is as follows:
- The S- input is at 2.500V. The o/p of the amp is reasonably correct +17mV.
- The S+ input is at 2.515V. The o/p of the amp is reasonably correct -13mV.
- The input at the non-inv pin of the final amp is 1.266V. [Correct]
- The input at the inv pin of the final amp is 1.488V. [Incorrect - since both inv and non-inv should be at the same potential, op-amp golden rule. Meaning the o/p is not correct]
- The output of the final amp is at 0.4896V. Incorrect, as per the eqn of an instrumentation amp it should be 03 times the i/p difference i.e 3 x 15mV = 45mV.
Hope it is all clear now.