How to choose the right opamp?

  1. sreejuviswan
    For one of my projects I am using 2 stage amplification. the first stage of amplification is done by a LMV324 opamp under the difference input configuration. for The second stage i need a programmable gain amplifier hence i tried using MCP6s21 PGA. this PGA has amplification gain setting of +1, +2, +4,+5,+8,+10,+16 or +32. However when i analyse the output signal the amplification does not happen as expected for a gain set at +8 the amplification is only twice the input signal and for +16 and +32 setting there is only noise in the output.

    My signal frequencies are not more than 2 KHz and the datasheet claims that this PGA could handle upto 18 MHz.

    Why is the opamp performing so poorly? is it because the opamp has poor performance or because i have not chosen the right opamp for my application? what are the opamp parameters i should look into for choosing the right amplifier?
  2. Tim_W
    Before you question the programmable gain amp, is the output of the LMV324 valid? Have you checked the common-mode voltage of the signal? The LMV324 will take a few tenths below the negative supply, but only to within 0.8V of the positive supply.
    PGA - is the reference tied to Vss? Given a single-supply difference amplifier as the input, you've got a signal referenced to 0V (Vss), so the reference would need to be tied to 0V.

    Good luck
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