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OPA129 Input Bias Currents using Tina/Pspice Model

GregGable
Expert 3640 points


I downloaded the OPA129 Tina Reference Design and notice that there is ~16pA of input bias current.  I measured using the Ampere Meter.  I was expecting to measure < 100fF.  Can you help explain this?

Thanks,

Greg

  • 0
    TI__Mastermind 27715 points

    Hello Greg,

    I took a look inside the model netlist for the OPA129. It was built by the Analog eLab in 2011 and seems to be a very basic model, and unfortunately it does not document which parameters are actually simulated. I can only find notes that show sections for output current through the supply, input capacitance, and input voltage and current noise. A pair of JFETs is used at the input to model the other DC parameters, but it's likely that this was not tuned properly.

    I've made a note of this issue in my op amps SPICE model SharePoint site. When we update the OPA129 model with my new SPICE model architecture, the input bias current and input offset current, as well as all the other parameters, will be corrected.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps