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OPA657: TIA Design for the Detector (100 MHz Bandwidth Requirement)

Chou Chu
Prodigy 10 points

Part Number: OPA657
Other Parts Discussed in Thread: LMH6629, OPA607, OPA2607

Tool/software:

I am currently designing a TIA circuit for a  detector, and my requirements are as follows:

  1. The signal current is approximately 80–300 pA, detector capacitance CD is about 100 pF (with 24 V bias), and the dark current is around 15 nA.
  2. I need a bandwidth f-3dB at 100 MHz (based on other papers that implemented a similar design using the ADA4851 op amp).
  3. I would like the maximum output voltage to fall within 1 V to 1.5 V.

At the moment, I am considering using either the OPA657 or LMH6629 (but I’m also open to other recommendations). My questions are:

  1. How should I design the first-stage TIA circuit ?  Given the influence of dark current, should I use AC coupling between the detector and the TIA?
  2. Considering my bandwidth requirement, do I need to use a smaller feedback resistance in the TIA and then add subsequent amplification stages to gradually achieve the desired output level? If so, what would be an appropriate value for the feedback resistance?
  • 0
    TI__Expert 3206 points

    Hi ,

    1. Is the signal current 80-300pA or nA? The dark current of 15nA and signal current 80-300pA will need an ultra-low noise TIA to detect such currents. Also, the given maximum output voltage would require very high gain which LMH6629 would also fall short. You may have to use 2 stages to achieve the same.
    2. what is the max input frequency?
    3. Given the Bandwidth requirement similar to ADA4851, LMH6629 or OPA657 would be overkill.

    We have Photodiode Circuit Design Tool where you can get the TIA suggestion and choose the right one for the application.
    We also have LMH34400 which comes with ambient light/dark current cancellation.

    Thanks

    Vikas J

  • 0 in reply to Vikas Jeevannavar
    Prodigy 10 points

    Hi ,

    Thank you for your reply !

    Sorry, let me update my question with more details:

    1. My signal current is 5.5 nA, and the dark current is 20 nA, 

    2. The maximum input frequency is 100 kHz.

    3. I’m targeting an output voltage range between 1 V and 1.5 V.

    I’m working on designing a two-stage TIA (transimpedance amplifier) circuit. However, I’m not sure how to select the value of the feedback resistor for the first stage, so that the second stage amplifier can properly amplify the signal.

    Any suggestions or guidance on how to determine the optimal feedback resistor value for this setup would be greatly appreciated.

  • 0 in reply to Chou Chu
    TI__Mastermind 31825 points

    Hello Chou Chu,

      Thank you for the additional details! Vikas is out on travel, I will be helping here while he is out. 

       Is your signal current max 5.5nA? If so, I would recommend a FET-input type. LMH6629 and the ADI part are bipolar, but OPA657 that you have chosen is a FET-input. The reason why FET-input structure would be the best choice for your application is due to the high feedback resistor (gain) needed. This means, current noise will play a dominant role for your total output noise, and FET-input amplifiers exhibit lower current error effects, while bipolar input amplifier exhibit higher current error effects, but lower voltage. The OPA657 might be overkill for your design due to your lower bandwidth requirements, but still have a pretty high gain and input capacitance. 

       For example, OPA607 using the Photodiode Circuit Design Tool Vikas shared earlier, OPA607 is a pretty good choice for max SNR: 

       You would then have to follow this up with a high gain Bipolar input VFB (10V/V - 20V/V) in order to achieve your 1V to 1.5V output requirements. Or, to save board space, you could use the dual OPA2607.

       Or, it could be possible in one stage if you use a higher bandwidth part, but it is a pretty large feedback resistor and could saturate your amplifier easily if you have a wide input current range:

       Then, for both cases, you would AC couple at the output of the amplifier. I would not recommend AC coupling at input since this requires a more complicated setup for transimpedance setups compared to a usual amplifier application. You can read more about this at this e2e thread: link1 and link2.

    Thank you,

    Sima