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LM124: Vcm impact Vout issue

Benjamin Zhou
Expert 4000 points
Part Number: LM124
Other Parts Discussed in Thread: OPA830,

Hi Team, 

   Customer used LM124 to build differential amplifier circuit and reported the output is higher than expected. Customer used single power source, and test as below:

（1）           Vcm=1.8V，Vid=130mV，Au=4， Vout=603mV；

（2）           Vcm=67mV，Vid=134mV，Au=4， Vout=546mV；

Even the (1) with larger error, but while decreased Vid to 0, the Vout still with >500mV and seems related to Vcm.

 

And the simulated result is quite different from real schematic.  could you kindly help on below items:

  1. Give comments that under Figure 1 simulated model, reasons for that the bigger difference between the simulation and real schematic.

  2. Provide the open loop Gain and phase margin figure for LM124 which datasheet doesn’t include;

More details shown as attached file.LM124 Issue.docx


Best Regards

Benjamin

  • 0
    Guru 140200 points
    Hi Benjamin,

    have you checked this behaviour with another fresh LM124?

    What are the tolerances of resistors R1...R4? For a good common mode rejection these resistors must have very small tolerances.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hello Kai,

    All four of LM124s appeared the problem.The circuit works properly when i use AD8042 instead of LM124.Keep other components unchanged.The same is true for OPA830.

    The tolerances of resistors are 1%,these's not enough to produce such a result.AD8042 and OPA830 tests can prove it.

    Thank you very much if you can share the open loop Gain and phase margin figure for LM124 with us.

    Hao

  • 0 in reply to hao xu
    Guru 140200 points

    Hi Hao,

    is the LM124 additionally sinking load current? Have you noticed figure 1 of datasheet? With a common mode input voltage of 2V a current of 2V / (2k + 8k2) = 196µA is flowing into the output of LM124. This could explain the output offset. Please keep in mind that the LM124 is no rail-to-rail OPAmp.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru*** 143096 points

    Hello Hao,

    The LM124 has two sink drivers as can be seen in the datasheet device schematic. One supports near GND voltage but can only sink tens of microamps.  The other is the PNP output transistor that can sink  more but needs 1 diode voltage diode of headroom to run.

    Here is sample data for this device family.

    Notice that VOL varies with sinking current. VCM in application will affect the amount of current that need to be sink.

    The dotted line represents 300k resistor to a common mode of 5V and VOL is very good

    The solid black line represents 16.2k resistor to a common mode of 5V and VOL is much higher

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Hello,expert

    Your analysis is reasonable and consistent with my experiment.

    Output inflows more than one hundred  microamp ,when I expect the output to reach 500 millivolts, yet not realized.

    On the other hand,If the output resistance is set to 2k resistor, the amplifier circuit works properly.

    In addtion,Is lm124 unit-gain stable?There is no the open loop Gain and phase margin figure in the datasheet.

    Regards,

    Hao

  • 0 in reply to kai klaas69
    Prodigy 40 points
    Hi Kai,
    Just as you said,No proper sinking load is placed.Understand this ,I can continue to deal with this problem.
    thank you!
    Hao
  • 0 in reply to hao xu
    Guru 140200 points
    Hi Hao,

    you wrote:

    "On the other hand,If the output resistance is set to 2k resistor, the amplifier circuit works properly."

    Yes, in this case the 2k resistor can absorb a big portion of the load current.

    Yes, the LM124 is unity gain stable: An open loop gain plot can be found in the datasheet of LM124-N (figure 5). See also figure 8 for the performance of unity gain follower. And see figure 4 and 7 in the datasheet of LM124.

    Kai
  • 0 in reply to kai klaas69
    TI__Guru*** 143096 points
    Hao,

    Try changing the [2k, 8.2k] resistors to [20k, 82.5k] or [24.3k, 100k] for J2 being 1.9V so the required sink current will intersect the chart data below 200mV.
  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Ronald，

    Theoretically, this method should be effective.  I would plan to set the sink current below 20 microampere.

    Thank you very much for your generous help

    Regards,

    Hao