Re: Anything better than OPA656 these days, Bill?

Winfield Hill wrote:
Chris Carlen wrote...
I'll be building a photodiode amp. I was planning to use OPA655 which was "the best" for this purpose a few years ago. It is now obsolete, replaced by OPA656. These are not stocked by my usualy distributors. I will likely give them a call and see if they can get me a handful.

The opa657 is a better choice, and is stocked at DigiKey. It's
not a unity-gain opamp, but as you should know, properly-designed
transimpedance amplifiers don't need unity-gain opamps. Still,
I'll miss the opa655, with its miniDIP package and cool isolated
input-stage power connections.

My specs/requirements are highly vague.

I'll say. Not a good idea. The best amplifiers are optimized for
the gain, bandwidth and input-capacitance region in which they'll
be used. You'll end up doing a lot of work, but get an inferior
instrument. If your goal is flexibility, I suggest an opamp with
lower voltage-noise and more input capacitance, like the LT1792,
with 4nV/rt-Hz at 1kHz compared to 6nV for the opa657. Then you
can make another customized high-speed amplifier using the opa657.



Actually perhaps OPA627 might give a little more BW than LT1792 with about the same noise.

I used LT1792 in a 9Gohm total transimpedance amp with about 3 or 10kHz BW (can't recall) once. It sure was sensitive. Almost like a PMT. It was to look at a small sample volume imaged from a faint blue turbulent methane in air flame.

Win, the distressing fact is that I would love to learn enough to highly optimize just about anything, but functional non-optimal results seem always to have greater value than optimized designs in 90% of real-world problems. If I needed something optimized, I'd just spend the $800 or so for a detector from a company like New Focus and be done with it. But non-optimum is very Ok in a case where I'd simply like to improve upon the performance of a PD with a battery and 1k resistor in a box.

I was down to zero hours a week for studying and hobby projects for the past year since having the baby. At work it's not much better, with broken lasers needing fixing all the time coupled with having committed to several very big electronics projects (mostly digital and uC/DSP oriented, something on which one can wing-it without a lot of analysis). They'll be looking for a new Laser Tech soon to let me concentrate on electronics. That ought to improve things. Also, I've managed to commit myself to a 30 minute per day project session at home. It seems that the right strategy was to go for a little bit of time every day, rather than a big multi-hour session once or twice a week.

I started a thorough analytical study of photodiode amps a few months ago, but shelved it due to time pressures. I'm forgetting stuff faster than I'm learning anymore.

Oh well...


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Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
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