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OPA855: EVB_OPA855_Oscillation problem

shin takahashi
Prodigy 10 points

Part Number: OPA855

I am evaluating the OPA855 on an evaluation board. I configured a TIA connected to a large-capacitance photodiode.

Please refer to the attached document for details.

EVB_OPA855_Oscillation problem.xlsx 

The passband is 2.5 MHz to 12.5 MHz. 

1.Oscillation was observed with the initial constant values. 

2.The distance from the PD to the IN- pin was shortened. It has subsided considerably, but oscillation is still observed. 

3.The resistor for preventing ringing was changed to 0 ohm. Oscillation is still observed. 

I believe that oscillation stability has been ensured for the low-frequency range. I suspected that the length from PD to IN- was a factor, so I shortened it, but the oscillation in the high-frequency range (1 GHz and above) persists. What seems to be the problem? Please let me know if I am using it incorrectly or if there are any solutions. 

 

  • 0
    TI__Expert 3050 points

    Hi Takahashi-san,

    Thanks for your documentation for the oscillation debug it is very helpful. 

    The main cause of an oscillation like this the distance from In- to the Cpd. That length has some parasitic inductance which resonates with the Cpd to create this 1GHz+ oscillation.

    My suggested fix is as follows:

    1. Shorten the length to Cpd as you've done. As close as physically possible. How close have you done?
    2. The 10ohm for R4 actually helps dampen that resonance. I would keep it there and possibly increase it to 50ohms. 
    3. Adding some additional capacitance between the inputs as the R4 resistor isolates the Cpd required to make the device stable. In your case, an additional 100pF in-between the In- and In+ pins would be my suggestion. Placing it between the pads of R2/R1 on the EVM to keep it close to the pins is ideal. Something like this drawing below.

    Thanks,

    Evan

  • 0 in reply to Evan Kirkbride
    Prodigy 10 points

    Hi Evan-san,

    Thank you for your answer.

    Regarding the point 1, it is actually difficult to shorten the distance.

    I will try your advice 3. I'll let you know the results.

    Takahashi

  • 0 in reply to shin takahashi
    Prodigy 10 points

    Hi Evan-san,

    Under the initial conditions (Case 1), I added a 100pF capacitor (Cng) between IN- and IN+. While the severe oscillation has subsided, peak oscillation remains around 4.8 GHz.

    Is there anything else I can do?

    EVB_OPA855_Oscillation problem_2.xlsx

    Takahashi

  • 0 in reply to shin takahashi
    TI__Expert 3050 points

    Hi Takahasi-san,

    Looks like the oscillation moved from ~3.2GHz to 4.8GHz. The parasitic inductances from that MLCC Cng capacitor could be continuing to pick up a resonance. 

    Are you using an actual photodiode for Cpd or are you using a capacitor to simulate a photodiode capacitance? If it is a simulated capacitor, then you could use the ground plane on the top layer or R3 to place the capacitor nearer to the device. 

    Otherwise we can lower Cng and Cf to keep the device stable and push out any resonance from any ESL. These would be my recommendations:

    • Using a 0201 size capacitor for Cng
    • Lower Cf to 1pF and Cng to 10pF
    • As low ESL for Cng as possible. I've had success with these Passive Plus 0201N series: https://www.digikey.com/en/products/detail/passive-plus/0201N100FW500-T500/29715992 
    • We could increase Rf to better lower Cf and Cng as well. Is there a reason you chose 82ohms? It's on the lower end of what I've seen, and makes the noise gain shaping a little more difficult at these frequencies.
    • There are potential fixes in layout as well but we can try to get it stable up with what you have.

    These simulations might give you a better idea of the general thought process: 

    1. Here the parasitic inductance (1nH) is creating a resonance with the Cpd.



    2. Adding the 10ohm series resistance dampens that resonance, but the Noise Gain (NG) suffers. 

    3. Adding the capacitor between the inputs helps bring the NG back to intercept the AOL at a good point. 



    However it's important that this new capacitor doesn't introduce a similar issue with any ESL. You can see the magnitude of ESL that is problematic, so I think my suggestions will help.

    Thanks,

    Evan

  • 0 in reply to Evan Kirkbride
    Prodigy 10 points

    Hi Evan-san,

    Thank you for your answer.

    I used a 0402 size capacitor for Cng.

    And used a capacitor to simulate a photodiode capacitance. But similar to the actual photodiode arrangement, the dummy capacitors are placed with spacing. L occurs, but if possible, I’d like to improve the situation under the current conditions.

    I will try 0201 size, following your advice 

    And I will try to use lower Cf to 1pF and Cng to 10pF. Is low-frequency stability maintained even if Cf is set to 1 pF?

    Below 15MHz, Rf is set to 82Ω to minimize noise gain as much as possible.

    I'll let you know the results.


    Takahashi

  • 0 in reply to shin takahashi
    Prodigy 10 points

    Hi Evan-san,

    I tried.

    EVB_OPA855_Oscillation problem_3.xlsx

    5.Cf : 10pF, Cng : 100pF 

    6. Cf : 1pF, Cng : 10pF 

    I used 0201 size capacitors for both Cf and Cng. When Cng was set to 100 pF (as in item 5), the oscillation ceased. Switching to the 0201 size seems to have been effective. Thank you for your advice.

    However, under condition 6, oscillation is occurring. Do you understand the reason for this?

    Takahashi