Common Mode Range Question

Discussion in 'General Electronics Chat' started by Engr, Oct 28, 2010.

  1. Engr

    Thread Starter Member

    Mar 17, 2010
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    Hi everyone. I am a bit confuse about the Common Mode Range of an Op-Amp. Can anyone tell me what this a Common Mode Range and how to test an Op-Amp to know its Common Mode Range? By the way, the configuration of the Op-Amp is like a comparator, there is no feedback.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Consult the datasheet for the opamp in question to determine the common-mode range.

    Input Voltage Range (Operating Common-Mode Range)
    This is simply the range of voltage that you can send to the input terminals while ensuring that the op amp behaves as you expect it to. If you exceed the input voltage range, the amplifier could do some unexpected things.

    Opamps should not generally be operated open-loop. If you need a comparator, use a comparator instead of an opamp. Opamps are generally internally compensated, which gives them a relatively slow output slew rate. Comparators are generally not compensated, so they have very fast slew rates, but make for poor amplifiers.
     
  3. Engr

    Thread Starter Member

    Mar 17, 2010
    114
    5
    Actually this is not an Op-Amp device, this is a device that has a comparator circuit inside it. The input and output pins of this comparator inside the device is also accessible. Looking into its datasheet, it has the specs of the Common Mode Range (Gnd+1 -to- Vs-1). I am not sure how to test the device for its Common Mode Range specs. I am looking for a methodology on how the test for this kind of specs is done.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    OK, so...

    1) Set one of the inputs to Vs-1.001v
    2) Start the other input to Vs-1.002v and raise it to Vs-1v while monitoring the output. If it changes states, the Vs-1v part is working.
    3) Set one of the inputs to Gnd+1.001v.
    4) Start the other input to 1v, and monitor the output while raising it to 1.002v. If the output changes states, then it passes the common-mode range test.
     
  5. Engr

    Thread Starter Member

    Mar 17, 2010
    114
    5
    Thanks for the help. May I know which input will I raise, is it the V+ or V-? Another question too, why is the Gnd+1 part has one input set to Gnd+1.001v and the other input to be raised started at 1v?
     
    Last edited: Oct 30, 2010
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    656
    Many comparators have input offset voltages greater than a millivolt. Check your datasheet. You might have to modify SgtWookie's instructions to accommodate the specs.
    Why don't you post a link to the datasheet?
     
  7. Engr

    Thread Starter Member

    Mar 17, 2010
    114
    5
    No datasheet available yet on the internet,this is a new device....
     
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