OPA855DSGEVM: High gain peaking on EVM

Hi Parveen,

important for stability is the ratio of "feedback capacitance" to "detector capacitance plus common mode input capacitance and differential mode input capacitance" of OPAmp. All the caps must form a (noise) gain of >=7 to keep the OPA855 stable:

Because of this I think that your layout is suffering from unwanted stray capacitance.

But there's still another issue which is often overlooked: The stray inductance of photodetector! If the photodiode is more than about 10mm away from the OPA855, the gain >= 7 requirement can easily become violated which will make the OPA855 become unstable:

At very low frequencies everything is ok and the gain is factor 7 (16.9dB). But at very high frequencies the stray inductance of copper traces running to and from the photodetector isolates the detector capacitance and the gain >= 7 requirement is violated. -> The OPA855 will become unstable.

parveen_opa855_2.TSC

Kai

Hi Kai

Thank you very much for your detailed explanation. I understand that peaking in my circuit is because of stray inductance of copper traces. Is there any way on the Evaluation module to get rid of the stray inductance (means compensating circuit) at high frequencies. Can you please help me to find the best way to obtain a 3dB bandwidth in my circuit experimentally.

Hello Parveen, 

Where is the photodiode being incorporated into the EVM board? Or is your input a voltage source through J1? Can you clarify on "source impedance not incorporated"? is this 50-ohms added to the board or part of your input source?

Best,

Hasan Babiker

Hi Hasan

At the moment I have not incorporated the photodiode into EVM. I have just placed a capacitance of 1pF instead of the photodiode.

Yes, I have used a sinusoidal input source through J1 whose frequency I have varied from 1MHz to 500MHz to obtain a 3dB bandwidth. I have not added 50-ohms (R12) resistor because it is my source impedance.

Thank you very much for your help.

Hi Parveen,

I don't understand what the many components you added to the -input of OPA855 are for. In my eyes they complicate the things and will not make it easier to understand the OPA855 or help to improve the frequency response.

If you try to make the OPA855 work as TIA then I would experiment with the TIA and not with this complex circuit.

Kai

My simple TIA circuit is attached here. Also can you please suggest how can I calculate the 3dB bandwidth experimentally on EVM.

8463.TIA ON EVM.TSC

Hi Parveen,

this is no TIA but an inverting amplifier.

Kai

Hello Parveen,

Are you still getting significant peaking when using the device in this inverting configuration? (after removal of C1, R1 and C4)

Best,

Hasan Babiker

Hi Kai

Actually I have converted my current source  into voltage source to determine bandwidth that is why I have sent you this file. Now I am attaching my TIA circuit.

Thanks once again.

4846.Transimpedance.TSC

Hi Hasan

When I have removed C1 and C4 in my circuit then I am not getting peaking . My gain is almost constant at higher frequencies . Even upto 900MHz I am not getting 3dB point.

Hello Parveen,

Can you provide more details on your testing method and equipment you are using if possible? What is the power level you are providing into the input of the EVM?

Also, to confirm, when you are measuring this flatband up to 900MHz you are still including the 1pF capacitance at the inverting input of the amplifier?

Best,

Hasan Babiker

Hi Parveen,

can you post a photo of your measuring setup?

Kai

Hi Hasan

Any chance I could send you further details via direct message rather than posting here. I hope you will understand.

Hello Parveen,

Not a problem, feel free to message me through the forums.

Best,

Hasan Babiker