Bandwidth mismatch between photodiode wizard and LTSpice

Hello,

Attached is a design from the photodiode wizard tool. It is a two stage design. When second stage is non-inverting, -3dB bandwidth from the tool (fN) roughly matches with that from LTspice. However, when the second state is inverting, the tool reported bandwidth is way off (tool is at least 3 dB more optimistic than LTspice). I tried with two different second stage opamps LTC6228 as well as LTC6252 and both have problems. It almost looks like the tool is not comprehending bandwidth degradation on the first stage as a result of low load impedance Rg gain resistor of the second stage.

Can some one please look into it? 

It looks like this site does not like to upload the .json file generated by the tool. In any case, here are the relevant inputs to tool to replicate the issue:

Photodiode bias (change this from positive to negative to see the issue), 200 nA peak current, 4G shunt, 900fF capacitance, 200mV peak voltage, Bandwidth 72MHz, Q =0.66, 2-stage design, 1st stage LTC6268-10, 2nd stage LTC6228 or LTC6252, expected fN = ~38.5MHz. Ideally, fN should match with -3dB from LTspice.

(On a separate minor issue, fN reported from Circuit Design -> Frequency Response Tab of the tool does not match with fN reported under Next Steps -> Frequency Response Tab. I think the latter reporting is incorrect.)

Thanks and regards,

Satish

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  • +1
    •  Analog Employees 
    on May 19, 2020 7:33 PM 9 months ago

    Hey Satish,

    Thanks for including the JSON file in the zip.  It was very helpful to easily reproduce your exact design.

    It almost looks like the tool is not comprehending bandwidth degradation on the first stage as a result of low load impedance Rg gain resistor of the second stage.

    You hit the nail on the head.  The photodiode tool does not simulate with the output impedance of the amplifier and therefore does not take into the account the loading.  From playing around with the spice file, it does indeed look like adding a G=1 voltage dependent voltage source between the stages in spice gets a much closer answer to the photodiode tool.  We are looking into some options in the future to address this, but right now this is a limitation of the tool.  

    Matt

Reply
  • +1
    •  Analog Employees 
    on May 19, 2020 7:33 PM 9 months ago

    Hey Satish,

    Thanks for including the JSON file in the zip.  It was very helpful to easily reproduce your exact design.

    It almost looks like the tool is not comprehending bandwidth degradation on the first stage as a result of low load impedance Rg gain resistor of the second stage.

    You hit the nail on the head.  The photodiode tool does not simulate with the output impedance of the amplifier and therefore does not take into the account the loading.  From playing around with the spice file, it does indeed look like adding a G=1 voltage dependent voltage source between the stages in spice gets a much closer answer to the photodiode tool.  We are looking into some options in the future to address this, but right now this is a limitation of the tool.  

    Matt

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