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
