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Hi gain, low noise, photodiode transimpedance design

Mike Price
Other Parts Discussed in Thread: OPA354, LMP7717

Hi I am designing a transimpedance amp that is supposed to be 400M gain, 10kHz bandwidth and 800 micro volt RMS noise on the output.   I am using a single OPA354 opamp  with 100M RF and 30k/10k Tee network to give  a total gain of 400M.  The problem that I have is that the noise level is too high (3mV RMS) once the photodiode is in circuit - presumably from the capacitance giving noise gain.  The capacitance is high as I currently have no bias applied to the photodiode as this will cause leakage and shot noise, particularly at high temperatures.

Does anyone have any advice on getting the noise level down? Is the noise that I am seeing simply an amplification of the opamp input voltage noise caused by the photodiode capacitance?

Thanks

Mike Price

  • 0
    Intellectual 860 points

    Start by getting rid of the T network and replacing it with a 400M resistor.  Also, watch your stray capacitance.  You don't give the capacitance of your photodiode but in your PCB layout, remove any ground plane under the inverting pin, feedback network, and connection to the photodiode.  This might reduce the peaking in the noise gain.

    Tim

  • 0 in reply to Tim Sobering

    Hi Tim,

    Greatly appreciate your input.  I couldn't quite get the bandwidth with 100M resistors as they have an inherent low pass roll off at 6kHz.  That's ok, I can put lots of 30M to make up for it.  I also found that most of the output noise was due to the current noise density of the LMP7717 - only 10fa but enough to cause a problem with such high gain. 

    Thanks again

    Mike

  • 0 in reply to Mike Price
    Intellectual 860 points

    Body capacitance will always be an issue with large value resistors.  A few tenth of a pF is all it takes so board layout is critical.  Be sure to cut out the ground plane under the inverting node to reduce (and possibly the output node as well) the parasitic capacitance.  I have not looked, but try to find an amp with sub fA current noise.

    Glad to help.  Let me know if you have more questions...I'll do my best. 

    Tim