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OPA320　Current noise deviation

Shinichi Inoue
Guru 19671 points
Other Parts Discussed in Thread: OPA320

Dear Specialists

My customer is considering OPA320.

He want to know the deviation of current noise.

According to the datasheet, It is 0.6 fA typ.

But max and min value is not described.

How much is it over temperature range(-40deg to 125deg)

What percent is apart from the typical value?

Best regards,

Shinichi

  • 0
    TI__Guru 60570 points

    Shinichi,

    The input bias current, IB, in OPA320 is dominated by a reverse-biased leakage current of ESD protection diodes and thus roughly doubles every 10 deg C.   Over the entire specified temperature range IB can reach 600pA - see below.  Since the input current noise is proportional to square-root function of the input current (Inoise=sq-rt[2*q*IB]), input current noise can reach magnitude of up to Inoise=(sq-rt[2*1.6E-19*600e-12] =~14fA/rt-Hz

  • 0 in reply to Marek Lis
    Guru 19671 points
    Dear Marek

    Thank you for your reply.

    I have one question.
    At the datasheet, typical input current noise is 0.6fA/root Hz.

    On the other hand Ib typical is 0.2pA
    Typical Ib is used to the equation Inoise=(sq-rt[2*1.6E-19*0.2e-12]

    The result is 0.25fA/root-Hz

    How can I think about it?

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    Guru 19671 points

    Dear Marek or other 

    How is the condition?

    Could you explain the difference between datasheet spec and calculation value?

    The customer wants to know about it.

    I appreciate your great help.

    Best regards,

    Shinichi

     

  • 0 in reply to Shinichi Inoue
    TI__Guru 60570 points

    Shinichi,

    OPA320 Input Current Noise Density calculated value of 0.25fA/rt-Hz is a DC value based on the typical IB=0.2pA.  At f=1kHz, the differential input capacitance causes IB to increase above 1pA, resulting in the higher value of the input current noise of around 0.6fA/rt-Hz - see graph below.

  • 0 in reply to Marek Lis
    Guru 19671 points

    Dear Marek

    Thank you for your reply.

    It makes sense to me now.

    It's depended on the difference of Ib between the frequency.

    I appreciate your great help.

    Best regards,

    Shinichi