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Original question:

OPA188: Input bias current

OPA188: current noise

Erich Lippmann
Prodigy 30 points
Part Number: OPA188
Other Parts Discussed in Thread: OPA187,

Does anybody know if the current noise specs for the OPA188 and OPA187 are correct?  I am asking because the data sheets say 160 fA/√Hz for the OPA187 but only 7 fA/√Hz for the OPA188.

Such a huge difference would be strange as both have the same architecture.

  • 0
    TI__Prodigy 715 points

    Hi Erich, 

    Being a lower power part there are differences internally such as input transistor current densities, sizes of switches, frequency of switching, etc that play a role in Ib and hence Ib noise.  Given that the Ib noise correlates with the Ib here, I don't have any reason to believe it is inaccurate.  

    Current noise of course is a concern as input impedance rises.  Be mindful that on chopper/autozero amplifiers it is best practice to match the input impedance on both inputs to minimize the effect of the chopped current spikes.

    Thanks,

    Scott

  • 0 in reply to scottgulas
    Prodigy 30 points

    Hi Scott,

    the reason I do not believe the specs are correct is that Ib noise does NOT correlate with Ib - the OPA187 is specified at an Ib of 100pA whereas the OPA has a typical Ib of 160 pA.  Given this I find it hard to believe that current noise differs by a factor of more than 20.  Are there any specs as to the frequency dependence if the noise current density of these devices?

    Thanks,

    Erich

  • 0 in reply to Erich Lippmann
    TI__Prodigy 715 points

    Hi Erich,

    You are correct.  I apologize I misconstrued the data.  

    The OPA187 typical Ib is 100pA but max is 350pA.  The OPA187 typical Ib noise is 160fA/rtHz

    The OPA188 typical Ib is 160pA and max 1400pA.  The OPA188 typical Ib noise is 7fA/rtHz.

    I talked to the design and characterization teams...  Current noise that low is just about theoretically unmeasureable and hence comes from simulation only.  A typical CMOS input amplifier has single digit fA/rtHz current noise density.  Anything beyond that is likely a function of the bias current spikes from chopping.  The OPA187 and OPA188 were developed at different times and it could be real differences in transistor sizing, current densities, chopping frequency, etc -- or -- it could be differences in transistor models, simulation setup, data interpretation, etc.  At the end of the day a simulation to this level is probably only worth so much.    

    What I would guide you on is that if current noise is a concern for you, it means you must have a high input impedance.  On choppers it is important to match the input impedance as closely as possible to minimize the effects of commutation of non-symmetrical input current spikes as voltage offset/noise.  The errors from mismatched impedances can far exceed the effect of bias current noise.

    In the shared architecture of the OPA18x devices, the chopping frequency is approximately 1/3 of the bandwidth.  Hence the OPA187 chops at approximately 550kHz/3=183kHz and the OPA188 chops at approximately 667kHz.  There will be short duration current spikes at these frequencies.  

    In cases of high source impedance where it is difficult to match the input impedance (e.g. photodiode application), we sometimes suggest to consider an e-Trimmed or laser trimmed CMOS or JFET part instead of a chopper.  

    Thanks,

    Scott

  • 0 in reply to scottgulas
    Prodigy 30 points

    Hi Scoitt,

    thanks for this in-depth answer.  In my application I need both - zero 1/f noise and high input impedance so if the 7fA/rtHz of the OPA188 are real, that part is ideal except for the higher power consumption.  So I guess I will have to try to measure current noise for both OpAms myself.

    Cheers,

    Erich