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Inverting op amp attenuator help

Discussion in 'General Electronics Discussion' started by george2525, Nov 17, 2016.

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  1. george2525


    Jan 30, 2015

    Im having trouble with the real version of the simulated inverting op amp below

    I need to attenuate 0.1-3.1 volts input by 1/50

    As shown the output for 0.1V input should be about 2mV

    the simulated version is close enough

    What I actually get is close to 3mV

    if I increase the input to 0.2V the output is (0.2/50)+1mv = 5mV

    But I need the output to double!

    I have seen this exact ratio and op amp model used in other circuits to attenuate voltages of similar value and noone ever mentions an offset of any kind. the simulated version works well enough.

    Ive seen things on op amp biasing and adding extra resistors etc but i dont know what to do.

    I need the feedback resistor to be 1K or 2K and the input must be inverted and attenuated by around 1/50

    I have tried the TL072 and LM324 op amps. I cant use anything that cannot run off +/-6V

    im sure the TL072 is appropriate anyway

    Any ideas about how I can get a nice attenuated output that is proportional to the input without any weird 1mV shift???



    Attached Files:

  2. OBW0549


    Jul 5, 2016
    Your problem is caused by the input offset voltage of the opamp. If you look on the data sheets, Vos for the LM324 can be as much as 7 millivolts, and for the TL072 it can be up to 10 millivolts. These will translate to a direct offset error in your output.

    To fix the problem, use a better opamp, such as a LT1007 (25 μV), OP177 (25 μV), or a MAX44246 (5 μV). And there are plenty of other suitable parts besides those three.
  3. george2525


    Jan 30, 2015

    Thanks but how do I find out the minimum supply voltages for the LT1007 and OP177?

    The datasheets dont say anything!
  4. AnalogKid


    Jun 10, 2015
    Yes they do. LT1007: Look at the power supply rejection ratio voltage range and three of the parameters plots.

    OBW0549 likes this.
  5. george2525


    Jan 30, 2015
    OK thanks guys. I ordered a OP177. hopefully that will help
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