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OPA657: How to measure the ultra low input current noise like OPA656/657?

user4382324
Prodigy 50 points
Part Number: OPA657
Other Parts Discussed in Thread: OPA656

I am using OPA656/657 in liquid nitrogen temperature. And wanting to measure the input current noise of OPA656/657. From their datasheet, their input current noises are as low as 1.3 fA/sqrt(Hz). It means that I have to put a resistor as large as 10G in its input stage to make inRs override sqrt(4kTRs).

From their datasheet, I find the common mode input capacitance of OPA656/657 is 2.8pF/4.5pF. The impedances of these capacitance is far small than 10G above a frequency like 1kHz. I guess thess capacitance will bypass the input current noise of OPA656/657 and make the measurement fails.

I wonder if this method can work. But if not, can anyone tell me how manufacturer like TI measure such low input current noise?

  • 0
    TI__Genius 15975 points
    Hi,

    The input current noise as low as 1.3 fA/rt(Hz) is extremely difficult to measure, and would require special setup involving Teflon PCB and Faraday Cage to get rid of the background noise. As you have noted, it would require a 10G (practically difficult to get) value resistor to overcome the 4kTRs noise which could be BW limited due to the input CM capacitance. Also, a lot of averaging is involved in this measurement along with repetition of each frequency for getting the accurate value, making the ROI extremely low for most practical purposes. I don't think the reported input current noise of 1.3 fA/rt(Hz) is measured, rather it is a simulated curve published in the datasheet.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 50 points

    Yeah, I guess you are correct.

    I looked up the datasheet, the typical value of the input bias current Ib of OPA656/657 at 25°C is 2 picoampere. Using shot noise fomula in=2(q*Ib)^0.5, the calculated input current density is 0.8fA/sqrt(Hz). Anyway, this value is somewhat in agree with 1.3fA/sqrt(Hz) from datasheet.