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OPA657: NA current measurement for the Transimpedance Amplifiers

Mickey Zhang
Genius 12080 points

Part Number: OPA657
Other Parts Discussed in Thread: LOG112,

Hi team,

The customer needs to measure the NA current. I recommend Log112. But the customer feels it is too expensive.

So, he would like to use the transimpedance amplifier to measure the NA current. 

Q1. Can the transimpedance amplifiers  measure the NA current? If they can, can you recommend a suitable 

 transimpedance amplifier? Is OPA657 OK?  Would you send me a TINA circuit for this application?

If they cannot, how much current can the transimpedance amplifiers measure? pA current?

Q2. For the figure 34 of OPA657 datasheet, how to determine the values of CD CPCB and CF?

I remember there is a blog to explain these parameters.

Q3. If the transimpedance amplifiers measure the NA current, such as OPA657,and we know he Logarithmic Amplifiers,

such as log112,can also measure the NA current. What are they differences for the NA current measurement application?

  • 0
    TI__Mastermind 31400 points

    Hi Mickey,

    Q1. Yes, transimpedance amplifiers (TIA) can measure current in the nA but you will need a lot of gain in the system. 

    Q2. Here is a good app note for transimpedance amplifier considerations.  Cd is the capacitance of the photodiode so that depends on which photodiode you use, Cpcb is the capacitance of the PCB board which can vary but a good estimate is 0.3pF.  Finally Cf depends on a number of factors including stability and bandwidth.  

    Q3. The advantage of the TIAs compared to the logarithmic amplifiers is TIAs have higher bandwidths.

    I would have to have a better idea of what your system requirements are to see if the OPA657 would be the best fit. Here is a TINA circuit for an similar TIA application.

     

    Regards,

    Robert Clifton

    OPA657 Transimpedance Amplifier.TSC