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Spice model of OP470  /  Problem with the input bias current

Hi,

While testing the model of the OP470 Op-Amp which we plan to use on a MIL PCB, I found a problem during the transient analysis.

The datasheet of this component tells us that the input bias current should be around 10nA

But in the simulations I have a short between the / inputs and kilo-amps flowing through them.

Joined you have a picture of the setup.

Could you help me ?

Best regards,

Altay

  • Hi Altay, 

    I tried to do an operating point simulation on the OP470. For the circuit we use to test bias current, the amplifier should still be in a closed loop configuration. See below. The bias current will be read through R1:

    The spice model is modeled according to the specification. 

    Thanks, 

    Kris

  • Hi Kris,

    Thanks for the reply.

    I should just avoid using it as a comparator or a Schmidt trigger. Correct ?

    Altay

  • Hi Altay, 

    Yes, in general, operational amplifiers are not designed to work well as comparators. The OP470 specifically has back to back diodes across the inputs. When used as comparators, large differential voltages appear across the inputs of the amplifiers, and you might end up drawing large amounts of current at the inputs, which eventually might ruin the device. Also, because of the input architecture, the datasheet specifies an absolute maximum rating of +/-1V differential input voltage. Using the amplifier as a comparator would very likely violate this rating.

    You can check this application note if you want to read up on why we generally do not use op amps as comparators. 

    Perhaps you could provide your requirements and we can see if we have a suitable comparator for you. 

    Thanks, 

    Kris

  • Thank you for your answer.

    I did completely miss the fact that there were some diodes in there.

    First time I see that.

    :-)