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Bias current and other non linearity

Discussion in 'General Electronics Discussion' started by luke67, Feb 9, 2013.

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  1. luke67

    luke67

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    Mar 18, 2012
    Hi! Do you know if there's a way to calculate bias current effects and other non linearity effects (like offset voltage) of this analogic circuit ? Is there any software that can do that ? Thank you all!
     

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    A simulator, as good as it may be, should not be your only tool to analyze such a circuit and the side effects. You should understand what the error sources are and what they do to your circuit. Otherwise you will have a hard time compensating the errors.
    Add voltage and current sources that mimic the effects (e.g. bias current, offset) and set these sources to the appropriate values from the OpAmp's datasheet. Then compute the circuit's response.
    I admit it's not the easy way, but it will give you an understanding which effects are relevant and what you can do to counter them.


    But since today so many people like to rely on tools:eek: , here's how:
    Get a SPICE type simulator (e.g. LTSPICE).
    Get a SPICE model for the OpAmp
    Incorporate the model into your SPICE design (the manual or help of your SPICE tool will tell you how to do this).
    Run a simulation.
    Since you apparently have never before used SPICE, learn the usage using some simple circuits before starting on an OpAmp circuit.

    And hope, that the OpAmp model is good enough to incorporate all features you want to simulate. Chnaces are that not all features are modeled.
    And don't trust the simulation results blindly. The least you can do is to replace the specific OpAmp model by an ideal OpAmp model and compare the simulation results. If they are too close to each other, this may be a hint that the model of the real OpAmp is not very accurate.

    By the way: bias currents and offset voltages are not nonlinearities. Being more or less constant they are perfectly linear, just unwanted.
     
    Last edited: Feb 9, 2013
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