• State
  • Replies 2 replies
  • Subscribers 47 subscribers
  • Views 85 views
  • Users 0 members are here
Support feedback
Options
Options
Related

OPA847: Large overshoot and oscillation problem on my TIA amp.

Joe G
Prodigy 10 points
Part Number: OPA847
Other Parts Discussed in Thread: DEM-OPA-SO-1A, TINA-TI, , LMH6629, LMH32401

Tool/software:

Hello,

I’m a beginner with transimpedance amplifier design.

APD:

- Photocurrent: pulses of a few µA, width on the order of 10 ns

- Intrinsic capacitance: ~22 pF

For now, I’m only at the simulation stage, working on developing a stable TIA circuit without actually using my APD. So instead, I’m using a function generator to produce 1 MHz square pulses, and right after the input I place a 1 MΩ resistor, which gives me a current of 1 µA.

I estimated the total parasitic capacitances at the input to be about 28 pF.

I used the TIA_calc Excel calculator to check my calculations.
I need a bandwidth of 40 MHz, so a feedback resistor of 10 kΩ and a feedback capacitor of 0.5 pF seem appropriate (I subtracted 0.2 pF from my actual Cf value to account for the internal capacitance of the resistor, which is 0.2 pF).

I built this on a DEM-OPA-SO-1A board suited for TIAs, and I do get a square signal.

The big issue is that I have a very large overshoot regardless of the setup (on the order of several hundred percent) and when I try to set a feedback Cf (0.2,0.3,0.5pF) it goes straight to oscillation.

Also, if I try to adjust the Cf values, my system starts oscillating badly, and I don’t really understand why...

I’m attaching my KiCad schematic and the Tina-TI file (I’m not very familiar with this software, I did my best), along with oscilloscope screenshots of my output.

Thanks.

opa847_10kR.TSC

  • 0
    TI__Mastermind 28475 points

    Hello Joe,

      Thank you for using our calc tools and for all this helpful information!

      You are correct in the debugging method for TIAs. The reason it is oscillating is because I believe you forgot to include the PD input capacitance you used to calculate your design, the 22pF cap should be right at the input of the amplifier VIN-. This input capacitance to ground is used to apply the pole/zero creation + interaction with the feedback resistor/feedback capacitance in order to achieve a stable system since the PD is not being used for initial testing. It will shape your noise gain in the higher frequencies to achieve at least a gain of 12 V/V for amplifier stability. Once PD is used at the input for your final design, this capacitance will be at the input of the VIN- and work in a similar way in the system, 

      Also, you are at the right thought process on testing without a PD, but the best way we found in testing with a function generate is via this method: 

       This will help achieve proper 50ohm impedance matching at the source to minimize power reflections. 

    • At higher frequencies, the capacitors short out and the network analyzer see its desired 50ohm termination.
    • The caps act like a current divider sending a small portion of the current through the series 50ohm into the series cap that connects to the circuit under test.
    • The apparent source impedance looking back into the circuit becomes the series capacitor at high frequency

      Here is an e2e thread on this same topic with more details. The links in the response do not work, but the summary I gave after the response is the same information: e2e.ti.com/.../opa818-testing-tia-without-photodiode.

      Also, as a simpler quick method, you can use the same values as below from LMH32401EVM, which shows how to test with a voltage source if no current source is available. The LMH32401 as integrated feedback capacitance to keep amplifier stable, this will need to be added for LMH6629.

       Overall, you should try adding the input capacitor of 22pF to ground at the IN- node of the amplifier. Then, you can look into the second part for optimizing testing with a function generator. 

    Thank you,

    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 10 points

    Hello Sima, and thank you a lot for your reply,

    I’m not sure I fully understood everything, so please excuse me for asking somewhat basic questions.

    I tried adding a 22 pF capacitor between In– and GND as you suggested, but during testing on my bench I ended up with a signal that oscillates (see attached updated schematic and signal) and the overshoot grow a lot.
    Perhaps I misunderstood your message and the capacitor should only be added when testing with the photodiode? What I did was based on what is shown in the TI OPA847 documentation for a transimpedance configuration. Should I understand that these example circuits are not necessarily complete?

    Regarding the simulation setup you described, thank you very much — I haven’t yet had the time to implement it but I will give it a try.

    One 2 more questions: what is the purpose of the series resistor Rs_in that is sometimes added on the PCB before the In– input? And how to choose the capa on the In+ (C1 and C2 in my case), i have not found anything. It was the documentation values.

    Thanks!