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Original question:

OPA855: oscillate in output

OPA855: OPA855

Markus Polster
Prodigy 60 points
Part Number: OPA855

Hello,

we are about to design a TIA with the OPA855. In the end, we intend to use it together with a high-speed photodiode. We have the eval board of the OPA855 which works perfectly well as long as we use it in standard amplifier configuration. But changing it to TIA, we do not get rid of ocsillations around 2GHz. By now, we do not yet use it with the diode, our intention was to first get it stable without it. Instead of the diode, we have tried to short the input, to terminate it with 50R, leave it open.... without changes...We have also increased the feedback resistor, changed the feedback caps. We tried to stick to the recommentations of the data sheet...

For the layout of our pcb, we tried several possiblilties, but all configurations oscillate. I will attach one of these...
Do you have any suggestions?

Regards,
Markus

  • 0
    TI__Genius 10330 points

    Hi Markus,

    Regarding your first question, one potential cause for instability is that the OPA855 is a decompensated part. This means there is a minimum gain required for the amplifier to remain stable, in this case above 7 V/V. By setting it in a buffer or by terminating the IN- with 50-ohms you are likely not in the required minimum gain which is why the amplifier is not in a stable state. There are also a few concerns with using the OPA855EVM in a TIA application as this EVM layout is not optimized for a TIA application. When laying out the circuitry for a TIA application, we strongly recommend putting the PD as close as possible to the IN- node. By doing so you limit trace inductance which can affect the stability of the circuit. At times we also suggest removing the RISO resistor despite being recommended in the datasheet. We have observed that in some circuit designs, the RISO resistor causes more instability by introducing parasitics.

    As for your second question, like mentioned above the RISO sometimes causes these issues and negatively impacts the performance of the circuit. We also recommend limiting the distance between the PD and the input of the amplifier. I would like to ask if there is any information you could share on the capacitance of the PD as well as the expected bandwidth of this circuit. 

    Best Regards,

    Ignacio