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LMP8350: different feedback values

John Kolano
Prodigy 30 points
Part Number: LMP8350
Other Parts Discussed in Thread: LMH6552

There was an oversight in our design and need to change the gain for the differential opamp but the only way to get the correct gain is to have 2 slightly different values for the feedback (RF resistors in app note). We are using the op amp for dc only in single to differential mode. The signal is going to an isolated piece of equipment (no GND reference, only differential from the opamp is cabled) and we only care about the voltage across the differential outputs. It appears to work ok but need to know if there are any surprises we might encounter by doing this.

thanks,

John

  • 0
    TI__Genius 15975 points

    Hi John,

    Is it possible for you to attach a brief schematic of your circuit? If you are using it for dc only, then probably you might be okay.

    However, the one thing I would be wary of is to make sure the input common mode requirements are met if it is connected to an ADC. If you have two different Rf values, you could end up with two different single-ended output voltages whose common mode voltage [Vcm = (Vo+ + Vo-)/2] could shift outside the ADC input common mode voltage requirements if the difference between the two single-ended output voltage is high.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Prodigy 30 points
    Hi Rohit,
    We are bypassing the VOCM with a .1uF to GND.
    John
  • 0 in reply to John Kolano
    Prodigy 30 points
    ...the bypass cap is the only connection.
  • 0 in reply to John Kolano
    TI__Genius 15975 points

    Hi John,

    I meant to be wary or make sure that the shifted common mode at the LMP8350 output does not cause any issue at the detector/ADC.

    For example, I have used an FDA (LMH6552) which is similar to the LMP8350 with different Rf values in the feedback network. The different Rf values in the feedback network result in different single-ended gains for each paths. So, even though you are taking the difference of the two single-ended outputs, the common-mode of the difference signal will deviate from center (or 0V) and shift high or low depending upon how the difference is taken. If you are using a detector such as an ADC at the output of this FDA to make your measurement at the other end of the cable, I would recommend that you are well within its input common mode requirements as specified in its datasheet.

    Best Regards,

    Rohit