Mostly this is for Phil Hobbs who was lamenting the demise of some low noise, low capacitance, fast opamp in a previous post. I received some email today that led me to order samples of the following,
Dang! Check out the simple photodiode preamp in the application notes.
George Herold
Nice part, thanks--added to the data file. It's sort of like an OPA627 on steroids. Unfortunately it's running at a noise gain of about 30 dB at 5 MHz, so its output noise voltage there is more like 100 nV/sqrt(Hz). Even without that nasty gain peak, a 50k feedback resistor needs about 4 uA of photocurrent to get to within 1 dB of the shot noise, which is my rule-of-thumb starting point. A nice cascode transistor would get you at least 25 dB SNR improvement in that circuit. The input capacitance is a bit better than an LF357's, even, and of course it's 60 times faster. It has the usual CMOS problems of +-5V max supplies (a 10 dB dynamic range penalty vs +-15V) and horrible 1/f noise below 10 kHz.
OTOH, it's half the price of an OPA656 and has lower C_in. (And _much_ lower than the OPA627's.) On balance, with split feedback, a cascode transistor, and a decent booster stage, that could be a very nice TIA indeed. The OPA657's C_in is horrible, so it isn't as good for TIA service as TI would have one believe.
Cheers,
Phil Hobbs
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