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Original question:

OPA818DRGEVM: NOt working as expected

OPA818DRGEVM: noise with photodiode

Romain FRENEHARD
Prodigy 30 points
Part Number: OPA818DRGEVM

Tool/software:

Hello,

I use 2 diodes parts G12180-005A, connected together on the input -. The pin + is connected à the ground.

I have a good offset but I have a big noise at 32Mhz.

Do you know why?

Thank for your helps.

Best Regards

  • 0
    TI__Genius 10670 points

    Hi Romain,

    The first concern would be if you are seeing any sort of stability issues with the signal you are capturing at the output. If you remove the photodiodes, do you see the same noise spikes?

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 30 points

    Hello,

    No the spike is removed.

    So that means photodiode create an unstability. How I can remove spikes with photodiodes ? 

    BR

  • 0 in reply to Romain FRENEHARD
    TI__Genius 10670 points

    Hi Romain,

    Is there a schematic of the values you chose for this design? Layout is also a major contributor to the stability of this type of circuit. Would you be able to share an image of this as well.

    Best Regards,

    Ignacio

  • 0
    Prodigy 30 points

    Hello,

    I keep the board OPA818DRGEVM. I just updated it with R3 removed. Next, I put both diodes like the picture, between VCC and VEE. The signal in the middle of diodes is connected J1 pin 1. I haven't removed J1 and J2. The noise was measured on J5.

    The schematic works because When I put light on diodes, I see the voltage on J5 change with the light intensity.

    BR

  • 0 in reply to Romain FRENEHARD
    TI__Genius 10670 points

    Hi Romain,

    Assuming everything is the same for the EVM, I would tune the circuit by adding a feedback capacitor as this is key to stabilizing a TIA circuit. I would start with around 2pF and adjust up and down from there. The reason this will take some tuning is because the layout is likely not optimized meaning parasitic capacitance is likely affecting the circuit as well. We advise to go with a tight layout when working with TIA circuits as they are very susceptible to additional unwanted capacitance and have seen designs not work because of this.

    Best Regards,

    Ignacio