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OPA857: Help selecting the correct topology (TIA + FDA +VGAMP)?

Javcel
Intellectual 470 points
Part Number: OPA857
Other Parts Discussed in Thread: THS4131, , THS4541, LMH6554, TINA-TI, OPA656, OPA659, OPA657, LMH6629

Hello,

I am currently working on developing an amplifier comprised of different stages. The idea is to convert to voltage the current (High frequency signal) coming from two sensors and amplify the differential signal out of them. The first stage must be a Transimpedance amplifier, this is clear, but after the first stage I am not very sure what is the best way to do it.

I have attached a block diagram of the proposed solution.

The second stage (THS4131) is meant to be used as a buffer and also to compensate the differences between  the sensors when there is no excitation.

The last stage is to adjust the differential signal to the input of ADC.

I have a couple of questions:

-Is this topology correct or should I better use a diff to single ended amplifier in stage two to compensate each of the branches independently, then do the differential and finally the variable gain? Any comments about this topology in general or how could it be improved?

- I am a bit confused with the differential output of the OPA857... Should the OUT- output of the two OPA857 connected together and to the Vocm input of the THS4131?

Looking forward to hear your opinion.

Kind regards,

  • 0
    TI__Genius 15975 points

    Hi Javcel,

    To answer your questions:

    1. I think the topology you have selected should be good. However, for the fully differential amplifier, I don't think the THS4131 would be adequate due to its lower bandwidth (BW) compared to the OPA857. I would recommend to switch to the THS4541 or LMH6554, which are higher BW parts compared to the THS4131. I have attached a TINA-TI schematic showing the OPA857 in difference operation with the THS4541 which you can use to build on your circuit.

    OPA857_TIA_Diff.TSC

    2. I would recommend to leave the OUTN of OPA857 as open or connected via a 500-ohms to GND.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 470 points

    Hi Rohit,

    Thanks a lot for your reply and your remarks.

    I have chosen the  THS4131 because its low noise, which is a key parameter in our setup. The max signal frequency should be around 50 MHz, so I think the THS4131 should be sufficient for this range of frequencies.

    The OPA857 works very well in the test we have done so far, but there is something that concerns me. In the datasheet its written that the trans-impedance gain error can be up to 15%. We need to have the gain in both branches as close as possible so that the common mode rejection is the highest possible. Should I consider switching to standalone OPAMP as TIA? for instance OPA656?

    Kind regards,

  • 0 in reply to Javcel
    TI__Genius 15975 points
    Hi Javcel,

    If you are concerned about the trans-impedance gain error, then I would suggest switching to standalone op-amp like the OPA657 or OPA659 which will provide better trans-impedance gain accuracy. I would recommend to use less than 1% tolerance resistors for your application if good common mode rejection is desired. You could also use the LMH6629 for your application to provide a low-noise input stage.

    In your application, the THS4131 is second stage after the TIA. So, the noise requirements for the second stage are a bit relaxed compared to the first stage. As a result, you should be able to use the THS4541 or LMH6554 instead of the THS4131 which have better distortion & large signal BW performance at 50MHz.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Intellectual 470 points
    Hi Rohit,

    Apologies for the delay.

    I will try your suggestions.

    Thanks you very much for your help

    Regards,