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OPA727 input bias current specifications

Tim Hoeppner
Prodigy 110 points
Other Parts Discussed in Thread: OPA727

Hello,

I'm a bit confused regarding input bias current specifications for the OPA727 op-amp. Referring to the datasheet, the "ELECTRICAL CHARACTERISTICS" specify 85pA typ and 500pA max @ 25C, what common-mode voltage is this specified at?

Also, I have a few questions regarding figure 7. "input bias current vs common-mode voltage":

1) The valid common-mode input range is specified at V- to V+-2.5V or -6V to 3.5V in this case, is there something special about the test performed in Figure 7 that lets it extend past the valid range?

2) The region of bias current when the common-mode voltage is 0V has a very small temperature coefficient as per the graph in Figure 7, would this cross-over region always be at the halfway point of the total supply voltage? For instance, if I used V+=6V and V-=-4V, would the cross-over region be at 1V?

Thank you very much for your time,

Tim Hoeppner

  • 0
    TI__Guru 63390 points

    Tim,

    The typical PDS value of IB is measured with Vcm in the middle between the supply voltages.  However, I believe there is an error in the PDS graph (Fig 7) showing IB to be highly dependent on Vcm - typically the slope of IB in CMOS amplifiers is very small since it represents the common-mode input impedance: RINcm = [delta_Vcm/delta_IB] = ~1E+12. 

    In the OPA727 PDS table below the input impedance is specified to be 1E+11 so 12V change in Vcm should result in only 120pA change of IB.

    We have just measured IB of OPA727 for two supply voltages: +/-2V and +/-6V and the results are shown below.  Unlike the PDS graph, they do NOT show the unusually large increase in IB as one changes Vcm away from the mid-supply; [IB expressed in pA]

  • 0 in reply to Marek Lis
    Prodigy 110 points

    Marek,

    Your response is very helpful thank you. I'm still wondering why your tests include common-mode voltage which extend past the valid common-mode range? I don't know enough about the internal workings of the op-amp to know what are the side effects are for doing so. The datasheet specifies a common-mode voltage range of (V-) to (V+-2.5).

    In the first graph with +/-2V supply, shouldn't you only test from -2V to -0.5V?

    And in the second graph with +/-6V supply, shouldn't you only test from -6V to +3.5V?

    Thanks,

    Tim

  • 0 in reply to Tim Hoeppner
    TI__Guru 63390 points

    Tim,

    We characterize the IB across the entire supply voltage range just to show what will happen if the customer exceeds the input common-mode voltage range, which happens quite often.  In the case of the CMOS op amps, like OPA727, IB is not effected since the input current is dominated by the reverse-biased ESD protection diodes.  However, in the case of the bipolar transistor op amps, IB increases dramatically once the input common-mode range is violated, which together with the use of high value input resistors may lead to lock-up condition requiring turning-off power supply in order to recover.

  • 0 in reply to Marek Lis
    Prodigy 110 points

    Wow, you have been very helpful, thanks.