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OPA857: Output noise

na na78
Guru 16770 points
Part Number: OPA857

Hi

We have question of OPA857.

1.
We want to calculate output noise.

There is Input Referred current noise on the datasheet.

So, is it right way to calclate output noise using Rf x input referred current noise?

Or what kind of noise should we consider for output noise and how to calculate it in OPA857?

2.
Brickwall filter bandwidth = 135MHz
Closed-Loop transimpedance Bandwidth = 125MHz (5kohm gain).

The input-referred current noise is obtained using 135MHz brickwall filter.
How to get the input-referred current noise using normal bandwidth (125MHz)?
Could you please tell me a way to calculate?

BestRegards

  • 0
    TI__Genius 15975 points
    Hi,

    I would recommend going through the Precision Labs video: Op amp Noise 3 in-order to calculate the output noise of the OPA857. It provides detailed steps on calculating the op-amp output noise.

    training.ti.com/ti-precision-labs-op-amps-noise-3

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Guru 16770 points

    Hi Rohit

    Thank you for your reply.

    I'm checking the noise chapter.

    As for OPA857, how to get broad bandnoise and 1/f voltage noise?
    (The graph of voltage noise density seems not to be on datasheet)


    BestRegards

  • 0 in reply to na na78
    Guru 16770 points
    Hi

    I hope you could reply.

    BestRegards
  • 0 in reply to na na78
    TI__Genius 15975 points

    Hi,

    Sorry for the delay in reply.

    1. Yes. The output noise calculation is basically Rf x input referred current noise. In the OPA857 datasheet, the input referred current noise should already takes into account the broadband and 1/f noise, as it is specified in RMS for a particular Brickwall filter BW.

    2. For changing the noise numbers to a different BW, you could use the below equation:

    Input-referred noise (125MHz) in Vrms = Input-referred noise (135MHz) x sqrt(125MHz/135MHz)  in Vrms

    So, the input-referred noise (125MHz) in Vrms = 25 nVrms x sqrt(125/135) = 24 nVrms

    Even though the closed-loop trans-impedance BW is 125MHz, I would still consider the Brickwall filter BW of 135MHz for output noise calculation. This is because the Brickwall filter represents the total integrated equivalent noise power BW that is present at the OPA857 output for closed-loop TIA BW of 125MHz.

    Best Regards,

    Rohit