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LM258A: Opamp Gain Error

Jayashree vilva
Prodigy 20 points
Part Number: LM258A

I m using lm258A as differential amplifier for current sensing. Op-amp gain is more than calculated value .

For 24 A, 2 mohm ,

Vd=0.048,Gain = 20

Vout = 0.048*20=0.96V .

But i m getting Vout as 3V @ 24 A in practical case.

All the resistors are 1 % tolerance .Please find the attached circuit

.Please give solution to solve the more op-amp gain 

  • 0
    Prodigy 20 points

    Part Number: LM258A

    I m using opamp as differential amplifier for current sensing .Opamp gain is more than calculated valueThe resistors used in the circuit are 1% tolerance .

    I m using 2mohm,24A

    Vd =0.048 v ,Gain = 20

    Vout = 0.96V but I got Vout = 3V .

    Huge gain variation found in testing.

    Pls give solution for this opamp Gain error.

  • 0
    TI__Guru*** 145826 points

    Jayashree,

    There many factors that create small differences. Due to the large error and very small sense resistor, I suggest looking at the kelvin connection on the sense resistor very carefully.

    If that looks OK, test results at 0A, 8A, 16A too.

     

  • 0
    Guru 140200 points

    Hi Jayashree,

    keep in mind that a solder joint can already have a resistance of 1mR. So it plays an enormeous role where exactly you "touch" the shunt.

    Also, I would use +/-0.1% toleranced resistors in the differential amplifier. A better resistor match results in a much better common mode rejection :-)

    Can you show your layout and a photo of your setup?

    Kai