Cell phone noise into opamps

In a really sensitive sub-audio application you use LM833 despite of all your complaints :-)

It could be some resonance affected by the parasitics of the opamp.

VLV

Yup. The b-e junctions think they're crystal sets. I've sold about

4000 thermocouple temperature controllers because the Ox**** units rectified RF so well you could shut them down with an old GenRad generator from across the room. Well, that and the fact that they lost the source code.

Ferrite beads and small caps to ground help enormously, and won't affect baseband performance. I've also observed that such rf sensitivity is often associated with sharp resonances in the wiring and packaging. A well-placed bead or figure-8 ferrite, in a cable, can kill a resonance nicely. We picked up another 20 dB by running our thermocouple wiring, from a connector to the pcb, through a fig-8 (dual-hole) bead.

One quick fix would be to dump in a jfet opamp. Some of them are very good.

I saw an appnote somewhere that graded various opamps on RF sensitivity, but I can't remember where.

John

On a sunny day (Mon, 12 Nov 2007 18:55:56 -0800) it happened Joerg wrote in :

But you will have to. Metal case All inputs and outputs including power. Do not blame it on the chips. (Well there may be some chips.....) :-)

John Larkin schrieb:

Ack. However, the JFET types often tend to be more noisy.

I made the experience that it's often good to place a small capacitor between the two inputs of the OP. That doesn't affect baseband behaviour much, but effectively short circuits the RF before it can be rectified by the junctions at the OPs input.

Tilmann

--
http://www.autometer.de - Elektronik nach Maß.

Basically, if you hit the NPN inputs of a bipolar op amp with more than about 26mV of ptp noise at a frequency too high for the output put to follow, the inputs follow the upper edge of the envelope of the noise, because you've got out of the region of linear response.

For FET input op amps, the corresponding level is about a volt, which is what John is talking about. Low noise FET-input op amps are not cheap.

-- Bill Sloman, Nijmegen

Which complaints? Yes, we might go discrete at some point by I think the LM833 is a nice amp. Just somewhat fussy when RF is around.

--
Regards, Joerg

http://www.analogconsultants.com/

Hmm, maybe I could brain-wash the b-e junctions ...

We are pretty much capped and beaded out already on this one. It helps, but it ain't enough.

As Tilmann pointed out those are too noisy at low frequencies.

That would be great to have. If I find it via Google I'll post. However, in our case we are pretty much stuck because only a handful of opamps fulfill the noise criteria. Now I wish there was more LF noise data for JFETs because they don't have an engaged b-e junction but are a few volts away. Or MOSFETs but there are definitely no LF noise specs for those. Maybe I'll start a separate thread, see which FETs people think are the best contenders, then buy all those and try one after the other.

--
Regards, Joerg

http://www.analogconsultants.com/

Not cheap would be ok right now, for this application. We can always worry about going discrete later. Got one in mind that is in the

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

--
Regards, Joerg

http://www.analogconsultants.com/

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.

In the signal path?

John

Yes, and also in the supply path.

--
Regards, Joerg

http://www.analogconsultants.com/

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

5nv/rtHz at 20Hz would be great :-)

--
Regards, Joerg

http://www.analogconsultants.com/

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------

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.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

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

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

------------------------------------------------------