OPA855DSGEVM: OPA855 as high speed TIA, oscillating

Hello David,

   Thank you for the information, I will need to run a stability check and get back to you. Did you remove R10 as well? How are you measuring the output?

Thank you,
Sima

Hi Sima

I did not touch R10 or R1. They are  left at board default.

Measured oscillation with a spectrum analyzer attached to output SMA,

Thanks

David

Hi David,

can you show a schematic?

Kai

Hello Kai,

I am trying to attach schematic, hope it works.

DavidTIA_oscillating.pdf

Yes, did work

Kai

Hi David,

the circuit should be stable:

A phase margin of 80° looks nice.

But if the feedback capacitance is higher than expected -due to the self capacitance of 30k resistor for instance- the phase margin is decreasing and the circuit becomes instable:

david_opa855.TSC

Kai

Another cause of instability is the introduce of copper trace inductance at the input of OPA855 due to a too long wiring of detector:

Remember that -according to the 1nH/mm rule of thumb- a 10mm long copper trace equals 10nH. This would mean a 5mm distanced detector!

Another cause of instability is the lack of bias voltage filtering directly at the bias voltage terminals of detector. Here again an unwanted inductance would appear in series to the detector. And if additional complex impedances are involded an oscillation in the MHz range could be seen.

Kai

Edit: The deleted post was just an unwanted double posting...

That's good news

Good luck!

Kai

hi,

i have a question,can you tell me what package do you use of Rf, 0201 or 0402?

there are also have some oscillation in the tia output of opa855 in my board.

btw,can you share your pcb layout?

thanks ,anyway.

bao

Hello Bao,

   The BOM for the OPA855EVM is located in the User Guide document; the feedback resistors are 0402s. I have shared the OPA855EVM gerber files below.

2335.GerberNCdrills.zip

Thank you,

Sima

Hi Baolin,

look at R2:

And notice the gaps under R2 in the ground plane and power planes:

Kai