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LMP2011 input bias current - temperature dependence and datasheet inconsistency

Jacob Bernstein
Prodigy 10 points
Other Parts Discussed in Thread: LMP2011, INA114, OPA376, LMP7721, LMP7701, OPA277, OPA192

Hi, 

I'm designing a voltage-to-current converter (e.g. based on Figure 13 in the INA114 datasheet), trying to achieve 100pA accuracy, so I need an op-amp with low offset voltage and input bias current.  I have a couple questions about the LMP2011:

*On page 7, the datasheet lists the Input current as -3pA, and the Input offset current as 6pA, but Figure 6 on page 9 shows an Input bias current of ~125 pA at a 2.5V common mode voltage. Is there an inconsistency here? Or is this reporting the AC value (as explained in section 7.4.1), while the earlier chart lists the DC value?

*Section 7.4.1 says that at high temperatures, the input currents become much larger, with 0.5nA typical. Does a chart exist for input current vs. temperature? I don't think I see one on the datasheet, and my application is very sensitive to this variable.

Thanks,

Jake Bernstein

Synthetic Neurobiology Group

MIT Media Lab

  • 0
    TI__Guru 60786 points

    Hello Jacob,


    The LMP2011 should not be used for high source impedance (>1Mohm) applications.

    The chopping action of the input stage causes charge injection into the inputs, which creates a pulsating current into, and out of, the inputs.

    These "dynamic" currents tend to integrate against the source impedance and the input capacitance, causing a unpredictable change in what looks like the 'bias current". The "bias current" will differ circuit to circuit.

    I am not familiar with the high impedance performance of the OPA3xx Zero drift series, which uses a higher chopping frequency, so their reperesentitve can comment if they are suitable.

    You should really be using a low offset, non chopper device for the best sub-pA performance. The LMP7721 is such a low offset, low bias current device. Also see the LMP7701, OPA376, OPA192 or OPA277 series.

    Regards,