Amplifying +/-25mv to 0-3.3v cascaded op-amps

Discussion in 'The Projects Forum' started by Marco Lopes, Oct 28, 2015.

  1. Marco Lopes

    Thread Starter New Member

    Oct 28, 2015
    2
    0
    Hello,

    I'm trying to amplify a 25mvp, 40Khz sin wave to 0-3.3v in order to feed it to an ADC.

    I've aimed for 2 cascaded inverting op-amps with 11 and 6 gain, respectively, with a dc bias given by the v+ on the first stage.

    Although I'm getting a weird answer from the second stage.

    On the leftmost osciloscope you can see the input signal , on the rightmost one you can see the first stage opamp output after the buffer and the second stage output below.

    LM6134 are rail to rail single supply able, 5v common voltage range opamps.

    Can you please help me solving this problem?

    Thank you.


    upload_2015-10-28_20-59-3.png
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    All of your stages are DC coupled, so any DC errors caused by offset voltages, bias currents, and component tolerances along the way will be amplified along with the signal, probably to the point that an output stage saturates. Better to DC bias all opamp inputs to 1.65 V, and AC couple the signal throughout.

    Does the input common mode range extend below the negative rail?

    ak
     
  3. Marco Lopes

    Thread Starter New Member

    Oct 28, 2015
    2
    0
    AnalogKid thank you very much for your advice.

    I just found out I was making the most stupid mistake, I was trying to use 2 inverting amplifiers. So, in the second stage the DC bias, and consequently all the signal, would in fact go below the negative rail.

    I've solved it simply by implementing a non-inverting amplifier as the second stage.

    Although the AC coupling and DC biasing on the 1st stage immediately to 1.65v are probably good ideas.

    Thank you once again.
     
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