Measuring Voltage Using Op Amps?

Discussion in 'General Electronics Chat' started by crazyengineer, Jul 5, 2011.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Okay, so I want to do a little experiment involving measuring the potential across a component. So here's a schematic I drew in or-cad

    [​IMG]

    Ideally, I was expecting the output of the omp amp to be 5 volts. However, here's the result I got after running a simulation

    [​IMG]

    Can anyone explain what happened to the other .9 volts? Did the omp amp truncate the voltage because it was close to the saturation voltage?
     
    Last edited: Jul 5, 2011
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Without trying to figure out what the circuit is doing, I can tell you that op amps do not like to output a voltage that is the same as the power supply voltage

    Try raising your power supply inputs on the op amp to 9 volts and see what happens.
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    If the output load on an LM324 is >=10k, it can go nearly to ground, but won't get within about a volt of the positive rail; best to assume not within 1.5v. Like Kermit mentioned, raising the positive supply voltage should effect a cure.
     
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  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The (+) and (-) op-amp inputs are backwards. The feedback goes to the inverting (-) input.
     
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  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Well spotted!

    Along with the other useful comments, it might also be worth demonstrating the operation as a true difference amplifier - by inserting an additional resistor from the 5V source V4 positive terminal to the top of R6 and another from the bottom of R6 to the common rail. Still take the measurement across R6 as you have it. This will test the ability of the amplifier configuration to amplify only the potential difference across R6.
     
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  6. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
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    With these in mind I finally got it to properly measure 5 volts...well 4.998volts but that's close enough

    [​IMG]

    [​IMG]

    Thanks everyone for the help!

    I'll make sure to do that during my next or cad simulation with this schematic
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Actually, this circuit is cheating, it isn't measuring the voltage on the resistor, it is measuring the +5V power supply! Try removing the R6 test resistor and see the voltage does not change.

    Now to really see my point, put R6 back in along with another 2K resistor in the top leg where the "5.000V" marker is. You might expect to see approximately 2.5V out of the amp, but you will see much less.
     
  8. crazyengineer

    Thread Starter Member

    Dec 29, 2010
    156
    2
    I didn't remove r6, did a test with a 1/2vcc voltage divider. Here's what I got.

    [​IMG]
     
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