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Original question:

OPA2205: Input bias current, input offset current and super-beta inputs

OPA2205: Input bias current from OPAX205A Pspice model significantly different than OPA2205 datasheet.

Montgomery McGuire
Prodigy 10 points
Part Number: OPA2205
Other Parts Discussed in Thread: TINA-TI

Hello,

I was using the OPAX205A Pspice model found in sbomb98a.zip and the input bias current seems to be much larger than stated in the OPA2205 datasheet. Instead of the expected 0.1nA, the model produced around 50nA, increasing the total offset significantly when used in a typical circuit.

I looked inside of the model, and on lines 157 and 158, what seems to be the input bias currents are defined to be 50nA for each input terminal:

I_IBN         MID ESDN DC 50N 

I_IBP         ESDP MID DC 50N 

I edited these lines to set the values to 100pA each, and the model now produces the expected (typical datasheet) value of 0.1nA input bias current. However, I did not model the input offset current, or do a clever estimation of the input bias current differences between input terminals, so if there is a more realistic value to use for each input terminal’s bias current, I would appreciate knowing those values.

Thanks!

  • 0
    Guru 140200 points

    Hi Montgomery,

    you could set both input bias currents slightly differently, according to figure 6-20 of datasheet of OPA2205, e.g. at 25°C. The difference of both input bias currents would be the input offset current.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 10 points

    That makes sense, but I'm hoping that TI can fold the fix back into the Pspice model as well. Someone must have slipped some orders of magnitude somewhere, and as is, it makes this otherwise fine part look like a mess if you only look at the model. The application is a non-inverting integrator for a DC servo circuit, and with the edited parameters, it's a very good performer. I guess I'm going to set them to +50pA and -50pA to get the expected 100pA offset. Thanks!

  • 0 in reply to Montgomery McGuire
    Prodigy 10 points

    Testing this out, I see that the polarity of the positive and negative input bias currents is already reversed by the way the current source nodes were chosen, so editing both original declarations of "50N" to "50P" solves the problem completely, providing a +50pA bias and a -50pA bias, and an offset of 100pA.

    Given that this is just a one character exponent error, I'm convinced that this is probably what the model designers intended originally - I'll use that value and ask TI here what they think, and if appropriate, could they fold that into the published model.

  • +1 in reply to Montgomery McGuire
    Guru 140200 points

    Hi Montgomery,

    here's the behaviour of the TINA-TI model:

    Again +50nA and -50nA input bias current :-)

    Kai

  • +1
    TI__Genius 10850 points

    Thanks Kai.

    Hi Montgomery,

    Thanks for your post. I will share this with our model expert to get it updated. Looks like you have it resolved on your end, I will close this thread now.