Cell phone noise into opamps

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

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

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

Digikey: Only 31 OPA627 left, in the classy TO can. Nice for a retro project but don't count on building more than 31 units. Sometimes I have the impression the time you were talking about has come. Lots of BB parts shortages.

The OPA656 is around 80nV/rtHz at 10Hz or as Archie Bunker would have put it, lousay.

--
Regards, Joerg

http://www.analogconsultants.com/

Joerg a écrit :

Oh, you can find consolation by looking at some CMOS opamp datasheet.

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Thanks,
Fred.

Hi Joerg

Does the circuit run on a single supply ? Try dual ! and separate 0V from shielding ground by a few Ohms.

Robert

It has dual supplies. Uncoupling anyb of the grounds makes things a lot worse, pretty much what I've always seen. That's usually because the source cannot be truly differential.

--
Regards, Joerg

http://www.analogconsultants.com/

datasheets:

Just wondering, are the cell phones in question GSM T-Mobile or Cingular/AT&T phones?

Michael

datasheets:

We tried a lot of them and it was all over the map. Mostly CDMA I guess, at least none was from T-Mobile. Mine (Nokia 2115i on VirginMobile which uses the Sprint Net) was actually the most docile. A Verizon one really hit hard, so did a BlackBerry. But it'll depend on how far the next tower is and thus the ouput power it sets itself to.

--
Regards, Joerg

http://www.analogconsultants.com/

datasheets:

Ah, ok. I thought the interference was predominantly from GSM phones.

"It is a common occurrence for a nearby GSM handset to induce a "dit, dit di-dit, dit di-dit, dit di-dit" output on PA's, wireless microphones, home stereo systems, televisions, computers, cordless phones, and personal music devices."

I had thought this was our car stereo going bad; knowing this now, it was most likely my wife's GSM phone.

Regards,

Michael

datasheets:

Yep, just had a thread in a German NG. Dits and grumbles in the stereo ever hour or so. One of the guys asked whether they had plopped the cell phone down anywhere close. Answer: Uhm, ah, well, yes! Problem solved.

--
Regards, Joerg

http://www.analogconsultants.com/

I'd have thought *everyone* over there knew about that. They've been exclusively GSM for years in Europe.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Ahem, this happened in Germany :-)

--
Regards, Joerg

http://www.analogconsultants.com/