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PWM to voltage conversion - OpAmp offset.

Discussion in 'General Electronics Discussion' started by vojtik, Mar 7, 2012.

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  1. vojtik


    Mar 7, 2012
    Hi all.
    I'm strugling with PWM to voltage converter. PWM duty 0-100%, ampl. 5V, freq.300Hz-3kHz. PWM signal is produced as High Side Driver (pull down resistor at converter's side). Current capability up to 2mA .

    Please help me to debug the following circuit which is supposed to convert PWM to voltage (there can be also 0-5V voltage at the input to the circuit- in this case the circuit must serve as voltage follower ).


    The circuit should be working well (tested by a friend of mine) with Opamp LMC662CN used with unipolar supply ( Anyway, I'm not able to make a test as I dont have this OpAmp.

    I was forced to used OpAmp TLC271, supplied with unipolar 12V. This is not a rail2rail kind but should it mind when high Vdd used?!
    In simulation works quite fine.
    Reality differs. The voltage following is ok.
    PWM to voltage conversion is NOK. the difference is huge (50%PWM is not 2.5V but about 2.63V, 10% PWM is about 0.7V and not the expected half volt). Vcc always 5V+/- 0.002V.
    I'm already desperate and dont understand why there is the offset. ANy thoughts why LMC662 works and TLC271 doesn't? Any important parameter I overlooked? Please could any of you propose an alternative OpAmp which would work and explain why.
    Thanks a lot.
  2. duke37


    Jan 9, 2011
    The LMC662CN has a specified maximum offset of 4mV.
    The TLC271 has an offset adjustment. Did you set this?
  3. vojtik


    Mar 7, 2012
    No I did not. N1 and N2 are not connected at all. The bias is set to low (connected to Vdd).

    One would expect max offset bellow 10mV right?
  4. vojtik


    Mar 7, 2012
    Guys, I checked once again the board. It's correct. Also tried to play with the C1,C2 and resistor values according to different designs in Microchip FilterLab.
    Used a different piece of TLC271...and no success. No more ideas why this doesn't work.
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