Photodiode amplifier noise

noise

with a

certainly

on

physical

website

This weekend I'm going to buy a tin of Danish sugar cookies, eat all the cookies, and build a pA range diode-curve measuring setup into it. I have a bunch of LMC6001s (Ib is 10 fA typ) to use. The datasheet hints somewhere in one of the app circuits that Cin is big. But it's not specified anywhere.

The rule seems to be that if Cin is large, don't mention it on the datasheet.

John

Sometimes you can pull it out of the macromodel, but sometimes they don't even put it in there. Pathetic. Plus they never specify how they measure the input capacitance.

Cheers

Phil Hobbs

It's "Danish Butter cookies" :-)

Make sure to _throughly_ clean it before you build anything into it, until wiping shows no grease residue. Else you might experience an impressive meeting of ants in there. BTDT.

Cin is probably largish and usually not used directly:

Interesting; that's vaguely like what I'm tying to do. But I can't find a datasheet on the JU421, even on the Vishay site, so I assume it's another FormerFet.

If I do find something that works, I'd better nab a few reels. Jfets are dying all over the place.

And what's with the 51K resistors driving an OPA27? I just love the way scientists design electronics, wiring in enormous amounts of noise so we can come along later and get rich and famous by merely doing it right.

John

Yep, long gone :-(

Your best bet might be Linear Systems. But they don't exactly make it easy to try their stuff. Since they are in Fremont maybe you can cajole one of them into giving you some samples, over a few brewskys at Zeitgeist.

Shhht. Don't spill the beans here :-)

My favorite re-design job was when a respected Californian professor designed in resistors around 50M and at the slightest hint of humidity in the weather forecast the whole chebang would quit working. Heck, you couldn't even breathe out anywhere close to it. Plus those resistors happened to be super-expensive.

Cheers, James Arthur

Cool. Vishay didn't recognize "JU421".

Nice part for leakage, but with En=30 nv/rthz and Gm=200 uS, it's sort of the Marching Band of Noise. This guy adds a second one in differential mode to add bonus noise and lower the gain to boot.

Why do these scientific types so love differential jfet front-ends?

Maybe the epoxy has set by now.

John

Well, good luck finding one :-)

If you do it'll probably have "DoD pricing".

In electronics they tend to hang on to stuff much longer than we do.

I hadn't paid much attention to discrete jfets in, well, some decades. They have low transconductance, high capacitances, miserable matching, and astounding datasheet parameter spreads, like 10:1 Idss limits.

But for really low noise (like, under 1 nV/rthz) amplification of high-impedance signals, nothing can touch them. The best fet opamps are, like, 5x as noisy as the best jfet. Pity that most people, at least the scientists, trend to use them wrong.

John

Really interesting are very small geometry device such as the 2SK3372. But with JFETs one must be on the lookout regarding discontinuance. Now even the 2SK3372 was set to 10,000 MOQ, not a good sign IMHO. I bought a few similar JFETs at Digikey in December and promptly received an obsolescence notice from them a few months later. At least they do send those out which is nice.

Yes, for very low noise apps I never left the discrete path. Even if there was an integrated solution I'd have a hard time trusting that it'll still be there five years down the road. Having a client call me because of an obsolescence issue in one of my design would be quite embarrassing.

True.

What wrongness have you seen? The commercial stuff for research applications looks pretty decent to me.

Best regards, Spehro Pefhany

Nothing can touch them on noise performance, although differential input costs noise and power consumption.

Best regards, Spehro Pefhany

I've seen a lot of diffamps where it wasn't necessary, really rotten second stages (the fets have low gain, so 2nd stage noise matters) and circuits where the Johnson noise of gate bias resistors (and occasionally drain load resistors!) overwhelmed the fet's noise. One paper got around that by testing the amp with a grounded input!

Could it be that there just aren't a lot of good disctete circuit designers around? I would hate to think that.

John

I may have a wacky idea. What's your signal bandwidth (if you can say)?

Cheers, James Arthur

Pity, but I can't get too close to the actual application in public. We can go private if you're interested. It is a fascinating and important problem where good circuit design could affect the world, at least some.

John

ps: tell S thanks for the PC! Everybody will be pleased except the potatoes.

I see mostly botched circuit protection. Last one: A freaking expensive laser diode driver with TEC in there was running. Janitor plugged vacuum into same circuit, turned it on ... tzzzt ... phut ... *POP*

That isn't supposed to happen.

That is most definitely the case :-(

OTOH, this puts bread on the table around here ...

Most people can be adequate carpenters or painters or cooks. Not many people, even with engineering or physics degrees, can be decent circuit designers.

That's sort of weird. I guess circuit design is really more art and instinct than science.

John

Absolutely! I was a circuit designer _before_ I trotted off to MIT. The education only honed my capabilities.

...Jim Thompson

Big JFETs are great, but only for slow measurements...once you get past BF862 territory you get to choose between horrible 1/f noise (in the RF parts) or horrible input capacitance (in the AF parts). The AC input resistance drops quadratically with frequency once you get above the corner frequency.

Or are there sterling counterexamples that I'm missing?

Cheers

Phil Hobbs