• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 49 subscribers
  • Views 3048 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA857: differential TIA

xyz xyz
Intellectual 1000 points
Part Number: OPA857
Other Parts Discussed in Thread: OPA657, , LMH6552, LMH6629

Dear
I want to build a TIA with a gain of 1K and my desired band is 50MHz.

which one is better?

  • 1- single ended TIA with OPA657 + ADA4950 differential driver?

  • 2- differential TIA with OPA857 + ADA4950 differential driver? (if I want to use OPA857, how should I connect OPA857 to ADA4950?)

Best Regards

  • 0
    TI__Genius 15975 points

    Hi,

    Usually a single-ended TIA with OPA657 + Differential driver is better for a gain of 1K and 50MHz bandwidth. Just out of curiosity, is there a reason that you are going with the ADA4950 for the differential driver? You might be able to use the LMH6552 instead which has better overall noise and distortion performance than the ADA4950. If you are thinking of programmable gain being an option with the ADA4950, then it is still feasible using the LMH6552 by external switches which you would still need for the ADA4950.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 1000 points

    Spice model of LMH6652 is not available so I can't simulate the circuit.
    is it necessery to make an impadance matching between single-ended TIA (with OPA657) and above differential driver? one of my main problem is I can't calculate the output impedance of first stage! :)
     

    even I can't download these files:
    www.ti.com/lit/slyt677
    www.ti.com/lit/an/slyt326/slyt326.pdf
    www.ti.com/lit/slda033
     
     thanks

  • 0 in reply to xyz xyz
    TI__Genius 15975 points

    Hi,

    The Spice model for the LMH6552 can be found here:

    For downloading the files, I would recommend copying the link for these files on a web browser like Internet Explorer or Chorme and googling it.

    In-order to achieve flat frequency response, it is recommended to make appropriate impedance matching. From Figure 26 in the OPA657 datasheet, the output impedance of the first stage is <1ohm for your application of 10kHz-20MHz. As this value is negligible enough, I would recommend to use 0-ohms in your output impedance calculation of the first stage.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 1000 points

    I have an another question :)
    the current is 100uAp (200uApp) so I need a TIA with a gain of 10Kohm.

    if I have a series parasitic capacitance through the input line, is there any solution to reduce the effect of this capacitnace?

    Iout >--------||----------->TIA input

     
    thanks

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

    I think the LMH6629 would be a good candidate for a TIA with 10kohm of gain.

    If you have a series parasitic capacitance, then you would just be blocking the DC content of the signal. You could design the TIA circuit by basically AC coupling the input. It would be better if you could draw up a block diagram of what you are trying to do in-order to better understand your problem.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Intellectual 1000 points

    Yes.
    www.ti.com/.../sboa122.pdf
    I have an another question! :)
    I want to use OPA857 as a TIA but there is a problem! the input current doesn't come from a photo-diode it comes from a current source.
    OPA857 has an internal self biasing circuit for the photo-diodes.
    I checked the datasheet and the file you mentioned in another post some days ago.
    even I couldn't find any circuit to realize this usage. (I mean I want to use the OPA857 as low noise simple 20-kΩ Transimpedance)

    another problem is:
    the TIA should be mounted on a sensor and then I should send the output signal on the cables.
    coax cable is not suitable for differential output and I think I should use STP cables.
    but you know I should make impedance matching between the cable and the drivers to make the reflection minimally.
    I checked the cables. most of them are CAT6 and CAT7 cable :| all of I know is these cables is good for network usages.
    problem into problem...