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OPA2836 Performance Variation due to Feedback resistor

Hi,

In one of my Applications, using OPA2836IRUNT (1st channel) as a Differential to Signal Ended Amplifier with gain of 20, Rf= 10K, RG= 0.5K with 0.01% tolerance.

I'm not seeing an expected output rather than getting Vout= Vout +/- 10mV if I use Rf with 0.01% tolerance. But, when I relax the tolerance to 0.5%, I am able to get the expected output without any error.

Could anyone help me what makes the difference in operation of these cases briefly?

  • Hello Vignesh,

    What is connected to the non-inverting input (do both inputs see the same effective resistance)? How many combinations of resistors did you try?

    I suspect that you got an accidental offset where the resistor error of the 0.5% resistor exactly offset the other circuit errors. In order to fully understand this you would have to measure each resistor before soldering it to the board.

    I would not expect universal improvement with 0.5% resistors, but it is possible that choosing a slightly different value of Rg (or Rf) will offset some systematic error in the circuit.

    Regards,
    Loren
  • Hi Loren,

    Thanks for your reply.

    My Vin+ is connected to non-inv input with 10K_0.01% (Same part as Rf) grounded and shunted with a cap of 22pf , Vin- is connected to inv-input fed back with output Rf=10K_0.01% shunted with cap 22pf.

    For input resistance Rg, we are using 2 parallel combination of 1k_0.01% for both the inputs. I tried changing only Rf with less tolerance once and it worked well.

    Regards,

    Vignesh.

  • Vignesh,
    Please provide an accurate circuit schematic along with the a screenshot of the test results. How are you driving the amplifiers differential inputs (what amplifier are you using, what is its frequency and signal swing)? Degrading the resistor tolerance should not make the amplifier "start working".

    -Samir