ADA4817 - Transimpedance Configuration Evaluation

Hello,

I purchased the evaluation board for the ADA4817 amplifier (model ADA4817-1ACP-EBZ) and have set it up in a transimpedance configuration. My problem is that  I'm not sure on how I measure its frequency response. I'll be using a VNA for this purpose which in my understanding, sources a voltage and subsequently measures a voltage. Is there an easy way to transform the source voltage into a small current? (which I can use as an input to the transimpedance amplifier).

I've attached the evaluation board user guide. The values I'm using are:

R1 = 0Ohm

R3 = 0Ohm

R5 = 10kOhm

If what I'm asking is not easy to implement what can I do to verify my amplifier works? Any help would be greatly appreciated!

Thanks,

George

UG-084.pdf
  • Hello George,

    Could you please upload a schematic, using the basic schematic of UG-084, with your settings

    drawn in by hand? That would help a lot.

    Assuming that you use the standard transimpedance configuration (IN+ of opamp on GND),

    you have a virtual ground on IN- of the opamp. In that case you just have to add a series resistor between

    your voltage source and the -IN input connector. You can use R1* for it.

    Best regards,

    Reinhard

  • Hi Reinhard,

    Thanks for getting back to me so quickly. I've attached a schematic of my configuration. I haven't added any of the components that I have crossed out.

    I followed what you said and connected a 1kOhm between the VNA's source port and -IN. I set the source power at -35dBm and managed to measure a frequency response (S21 parameter), please see attached.

    It seems like the frequency response I'm getting is quite different from my simulations. Is it because of the parasitic capacitance at the input node? Also, I haven't added the 10uF bypass capacitors yet do you think that's a big issue?

    Best,

    George

  • 0
    •  Analog Employees 
    on Sep 26, 2014 12:25 PM

    HI George,

    The frequency response is very bad. The configuration you made by inserting 1kohms between the VNA source and -IN forms an ordinary voltage amplifier with a gain of -10 and not a transimpedance amplifier. Can you do the following for me and let's see if we can get better results.

    • Put a 50 ohms termination in R2.
    • Change the value of the R1 to 100k. Add Caps connected to the R1 and ground. The value of this caps should be the same with the value of the capacitance on your photodiode.
    • Put a 50 ohms series resistorin R9
    • Input Source should be +10dBm.
    • Get the frequency response and upload it here.

    Best regards,

    Emman

  • Could anyone help or have some suggestions?

    Thank you,

    George

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:46 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin