Re: Cell phone noise into opamps

It's not completely possible in this app. Maybe we can find a JFET or MOSFET that's low noise instead.

On a sunny day (Tue, 13 Nov 2007 16:40:37 GMT) it happened Joerg wrote in :

Joerg, I find this difficult to answer. It seems to me however, if I make something with say -80dB noise level, that that should stay -80dB, even when somebody puts a cellphone on top of it. Else what good is a noise spec? We will only see more and more use of the spectrum, Wimax is coming, the higher bands will be used too, EU is working on an "EU wide' spec that would leave much more space for unlicensed applications, and give complete freedom for the modulation type... Expect at lot of RF.

complete freedom

I know. That's just the problem, here we have to deal with cell phones that are just a foot or two away from the amp and shielding is a major challenge on this one. So a JFET that could reliably deliver around

complete freedom

P.S.: Somehow, whenever I am looking up the old faithfuls that could have done it such as the 2SK662 they are discontinued. Man, that thing rocked when it came to audio noise.

I would try the OPA627 of Burr-Brown (ok, TI). They don't get any better than that - the speed should be fine for you, noise too. But it is not dual... A similar - but somewhat noisier and a bit more than somewhat slower, but dual is the OPA2107, perhaps it would do. However, the way this sounds I would be pretty sceptic that much can be achieved without shielding. The cellphone here drives the power supply of the TV-set mad from 1 ft... what 80+ dB.

Dimiter

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it.

complete freedom

Says 40nV/rtHz max at 10Hz. That's almost like the noise of the toilet that I installed a couple years ago (where one of our dogs runs away the instant I press the flush button).

Also, right now I am trying to stay clear of BB parts because they appear to have some major issues in getting product out the door. Digikey has a few old TO cans left for the OPA627 but I assume when they are gone that's it. Not good for something that has to go into production soon.

Similar noise but Digikey has absolutely no stock on this one.

It can be done but not with anything bipolar. Has to be something where all the junctions are well biased, like JFETs. We'll go discrete but unfortunately JFET datasheets are rather skimpy. Often they don't even contain a noise versus frequency graph.

The graph says something like 15 at 10 Hz. The graph underneath indicates at Rs > apr. 5k the noise is all R... If the 627 is too noisy for this application, don't waste your time looking for anything better - there is nothing even very close to it (meaning fast FET opamp).

Now that would be surprising. I have been using them since 1991 or so and they are still in production...

Dimiter

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It doesn't have to be fast. Looks like this board has to be migrated over to a discrete design. Oh well, it ain't the first time.

Have you tried to buy any lately? I mean actually ordering a few thousand? I don't know what the issues are but I have met a lot of people who were in a bit of a frantic mode because their purchasing guys cannot get parts.

Now from surprising that gets depressing. I wish I had orders to need the few thousand 627s, though... :-).

I am not familiar with the slow parts (not many FET opamps too slow anyway, IIRC), but I would still be surprised if there were any FET opamp to match the noise figure of the OPA627.

Dimiter

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How low do you want? Have a look at AD745 : 3nV/rtHz@10kHz and 5.5nV/rtHz@10Hz typ. (4 and 10 max) It's the only really low noise one I know of. But be warned when seeing the package size. And check the footprint, I got burned on this one: they just say soic16(R) but it's a _wide_ package. Ouch...

Thanks. Yes, that's one large amp. We now have 8-TSSOP in there and it might not fit.

The story is that B-B had a lot of old production test sets that have started to fail, and they can't get parts to fix them, and it will take a while to design new ones.

TI tech support has been atrocious lately. I suppose it's all backed up.

John

I've heard such rumors as well. What's so difficult about a test set? And why do they suddenly fail? Somehow it sounds like a management problem. I mean, it's been months now if not a year.

Lately I have become a bit more bold there. If a part is hard to get or support isn't forthcoming it's outta here. Usually for good or at least for a long, long time. Even if it has to be done discrete, so be it.

Joerg snipped-for-privacy@removethispacbell.net posted to sci.electronics.design:

Given the recent purchase it sounds like an inherited management oops. Just the same, test sets should not be failing.

What's a test set?

robert

Production test rigs :-)