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LMH32404: Maximum allowed input capacitance

Sariel Hodisan
Prodigy 145 points
Part Number: LMH32404

Tool/software:

Hello,

What is the maximum capacitance the TIA inside the LMH32404 can tolerate before it becomes unstable?
I have a photodiode with a large capacitance (>20pF) that needs to be connected directly to the LMH32404.
I understand that such a large capacitance will increase the TIA noise and reduce its bandwidth, but my primary concern right now is the stability of the TIA.

Thanks,

Sariel



  • 0
    TI__Genius 16775 points

    Hi Sariel,

    I was able to speak with a design engineer on our team and he expects that there could be stability concerns with this high of a PD capacitance. This was not outright captured in the datasheet, but we certainly suspect and caution against such a high value of capacitance with regards to its stability as this device was tuned to optimally perform with a 1pF PD capacitance.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 145 points

    Thank you, Ignacio.

    Could you please estimate or calculate the maximum input capacitance this device can handle before oscillation occurs?
    Additionally, is the supplied SPICE model be accurate enough to detect these oscillations as the input capacitance increases in spice simulations?

    Best regards,
    Sariel

    ---

  • +1 in reply to Sariel Hodisan
    TI__Genius 16775 points

    Hi Sariel,

    Sorry for the delay in response. I was able to get the designer to simulate the device with 20pF of input capacitance and he did not find any signs of instability. He said using a higher value will likely result in the same result where it is not unstable. As for the model, I am not completely certain it can capture those results as accurately as the model is not designed at the transistor level so there is a chance it does not capture every single configuration as accurate as the simulation the designer was able to run.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 145 points

    Thank you so much for your response and for running the simulation. This really helped and solved my issue. I'm relieved to know I can connect such a high capacitance to the device.

    Best regards,
    Sariel