common mode

Discussion in 'General Electronics Chat' started by lost in analog, Sep 23, 2005.

  1. lost in analog

    Thread Starter New Member

    Sep 23, 2005
    If we have a perfect opamp and we input the same signal on both the positive and the negative terminal, the output should be zero. But in reality this never happens. What would be the best way to get as close as posible to a common voltage gain of 0?.
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Post a copy of your schematic and I will be more that happy to assist you with your common mode problem.

  3. Brandon

    Senior Member

    Dec 14, 2004
    What are you trying to do?

    The fact you don't have exactly zero doesn't mean a thing. If your under 0.7-0.5 volts most micros and logic gates see that as off.
  4. Gorgon

    AAC Fanatic!

    Aug 14, 2005
    Common mode error adjustment is normally done with a small voltage adjustment of the common ( '0v' reference voltage) input.

    To do this you just connect a potmeter between + and - supply(or better, proper reference voltages), and connect the viper with a relative high value resistor to a low value resistor(Rx). This point is your new reference. The other end of the low value resistor is connected to the(old) common ref. point. You can now adjust the voltage over Rx, 'off' to both sides of the old common point. The ratio between the two resistors and the voltage swing will give the amount of adjustment. A general rule is to keep it as low as possible.

    Short the two inputs on the opamp and adjust for optimal 'zero' output.

    I suppose there are several ways to do this, and I don't know if its optimal or the best.

    TOK ;)