Hey everyone! I work in a physics lab and we need to measure an optical signal via the difference of two photodiodes. We don't like the noise spec on current commercial options, so we've decided to make one in house.
I'd like a quick design review on the output stage!
We have a 500 MHz TIA (tuned via the feedback network for bandwidth to be great up to 10 MHz) that goes into R23:
We then read the signal via an SMA on a 50 Ohm terminated instrument. Here is what the layout looks like on board:
The 50 Ohm -> 270 nH L -> (220 pF C->Ground) -> 270 nH L Pi filter is with a corner at ~20 MHz, designed to pass the 10 kHz – 10 MHz signal with a more or less flat response.
The 50 Ohm -> 100 pF C -> 50 Ohm -> GND is desinged as a Zobel to keep the load resistive with a cutoff of ~30 MHz.
Finally, the two ouput caps in parallel were chosen to AC couple the circuit with a ~1.5 kHz corner frequency and meant to keep the response stiff up to 10 MHz.
Things I maybe don't like about this circuit but don't know if I should care:
I have to break the 50 line to go into the output coupling caps (big pour for parallel)
the LC Pi filter is apparently at 35 ohms?
Should I fix 1 and 2? And are there any other obvious red flags in this design or layout? Thank you all so much?