Re: Photodiode case

Wow. So many other people withh nothing better to do on a Saturday ;-)
Thanks for the comments, I will head into the lab later this weekend.

I have always grounded the can directly to chassis resp. the ground
plane on the circuit boards. Reverse bias it, to gain a wee bit more BW
and avoid rectification of RF noise.

Reverse bias helps a lot. It not only reduces capacitance, mostly in
the first few volts, but higher voltages sweep charges out of the
junction faster. So use all the volts you can.

I had tried just the cascode and reverse biasing, but I only had
available -5V reverse bias.
That did not get me the BW I needed. The Cd on the part is about 2.5pF
at that bias. I could
not resolve this, as I should have had an r_e on the cascode of about
725 ohms (should have been
a BW of about 87MHz).

RC filter it close to the pd.

Yep, I would do this, plus use a ferrite. Or maybe a capacitance
multiplier.

bias of about 80% of spec *** limit. If BW is still an issue you can
cascode to reduce the capacitive load by the TIA. Phil Hobbs explains
this very nicely:

http://users.bestweb.net/~hobbs/frontends/frontends.pdf

His book is worth having if you're doing electro-optical stuff. It's
fun to read anyhow.

I have this, it is quite a good book. The circuit is actually the
bootstrap plus cascode he covers in the book and the paper, so there's
the schematics (different op-amp in the transimpedance stage). I've
also corresponded with him several times for clarification (not that
he was unclear, I was just being dense). Quite a nice fellow.


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