Cell phone noise into opamps

[Fred Bartoli]

Joerg a écrit :

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.

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...

 

[Joerg]

Joerg a écrit :

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.

 

[John Larkin]

Yes, and also in the supply path.

Mail me the schematic and layout, and I'll do some hand-waving.

John

 

[John Larkin]

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).

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.

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

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.

[...]

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

 

[Joerg]

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).
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).

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.

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

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.

[...]

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.

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.

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

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]

Mail me the schematic and layout, and I'll do some hand-waving.

Then I'll get shot

I already did the convincing session (thou shalt not split grounds
etc.). The layout is going to be pretty good and with some extra shield
cans and stuff I am sure we can make it work. But no matter how much
filtering there comes a point where something without a b-e junction
would be kind of nice. This is an app where a cell phone right next to
the unit is something to be reckoned with. Almost like good ole mil stuff.

 

[Adrian Jansen]

Then I'll get shot

I already did the convincing session (thou shalt not split grounds
etc.). The layout is going to be pretty good and with some extra shield
cans and stuff I am sure we can make it work. But no matter how much
filtering there comes a point where something without a b-e junction
would be kind of nice. This is an app where a cell phone right next to
the unit is something to be reckoned with. Almost like good ole mil stuff.

Just an observation:

These tiny Bluetooth headsets seem to get away with this sort of stuff.
I have one which happily works without any audible interference with
the cell phone sending Bluetooth data , and of course its own radiation
at 8-900 MHz, with the phone effectively right on top of the earpiece.
And its all in a plastic moulding.

--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

 

[Joerg]

Just an observation:

These tiny Bluetooth headsets seem to get away with this sort of stuff.
I have one which happily works without any audible interference with
the cell phone sending Bluetooth data , and of course its own radiation
at 8-900 MHz, with the phone effectively right on top of the earpiece.
And its all in a plastic moulding.

It's ok for simple audio stuff. But we have to extricate signals in the
uV region.

 

[Phil Allison]

"Joerg"

It's ok for simple audio stuff. But we have to extricate signals in the uV
region.

** What the heck do you imagine all microphone, tape head and phono
pre-amps have been doing for the last 70 years - PAL ????


Grrrrrrrrrrr..........



........ Phil

 

[Joerg]

"Joerg"



** What the heck do you imagine all microphone, tape head and phono
pre-amps have been doing for the last 70 years - PAL ????


Grrrrrrrrrrr..........

Yes, electret microphones. I only know the cheaper ones and they had
rather mundane JFETs in there, not good enough here.

70 years, hmm, maybe I should check out nuvistors as well

 

[Phil Allison]

"Joerg"

Phil Allison

Yes, electret microphones.

** NO !! All microphones.

Originating audio signal sources have sub uV noise levels at their
utputs - so their associated pre-amps all have to " extricate signals in
the uV region " .

70 years, hmm, maybe I should check out nuvistors as well

** Don't try be smart about stuff you have no comprehension of - or you
will fall flat every time.




....... Phil

 

[Joerg]

"Joerg"


** NO !! All microphones.

Originating audio signal sources have sub uV noise levels at their
utputs - so their associated pre-amps all have to " extricate signals in
the uV region " .

Yeah, I guess that's why you guys use all those low noise opamps like
the LM833 or the NE series.

** Don't try be smart about stuff you have no comprehension of - or you
will fall flat every time.

I did build audio gear with tubes, back when even Ge transistors were
expensive. Pre-amps, compressors, bigger amps, and one really big amp.
That one had six color-TV H-tubes in it. You could make the plaster rain
down from the ceiling with a few cords on the E-guitar. Sure enough,
shortly after we fired it up the laws came. Luckily they did not check
out how I had done the power transformer. There was none ...

 

[Jan Panteltje]

"Joerg"


** NO !! All microphones.

Bullshit, carbon mikes, throat mikes (or whatever you English care to call these),
dynamic mikes, and a ton of other ones, have much higher output.

 

[Phil Allison]

"Joerg"

Yeah, I guess that's why you guys use all those low noise opamps like the
LM833 or the NE series.

** They will all "extricate signals in the uV region".

I did build audio gear with tubes, back when even Ge transistors were
expensive. Pre-amps, compressors, bigger amps, and one really big amp.
That one had six color-TV H-tubes in it. You could make the plaster rain
down from the ceiling with a few cords on the E-guitar. Sure enough,
shortly after we fired it up the laws came. Luckily they did not check out
how I had done the power transformer. There was none ...

** Yawn....

What an appalling web site.

Cleary, Joerg = another rabid nut case.



....... Phil

 

[Phil Allison]

"Jan Panteltje"



** **** OFF -

you ASININE WOG MORON





........ Phil

 

[[email protected]]

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
<10nV/rtHz class and has a farily low 1/f knee?

OPA627 - about 5nV per root Herz and 100Hz knee

http://www.ortodoxism.ro/datasheets/texasinstruments/opa627.pdf

The OPA656 is cheaper, but 7nV and 1kHz. Farnell stocks both. Burr-
Brown were always good for low-noise FET-input op amps, and TI hasn't
yet managed to wreck them, though I imagine that it is just a matter
of time.

 

[Joerg]

OPA627 - about 5nV per root Herz and 100Hz knee

http://www.ortodoxism.ro/datasheets/texasinstruments/opa627.pdf

The OPA656 is cheaper, but 7nV and 1kHz. Farnell stocks both. Burr-
Brown were always good for low-noise FET-input op amps, and TI hasn't
yet managed to wreck them, though I imagine that it is just a matter
of time.

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.

 

[JosephKK]

Adrian Jansen [email protected] posted to sci.electronics.design:

Just an observation:

These tiny Bluetooth headsets seem to get away with this sort of
stuff.
I have one which happily works without any audible interference
with
the cell phone sending Bluetooth data , and of course its own
radiation at 8-900 MHz, with the phone effectively right on top of
the earpiece. And its all in a plastic moulding.

I saw your frequencies and thought "that's not right". So i googled
and the correct frequencies are 2400 to 2500 MHz pretty much
worldwide. Right there with 802.11b/g network equipment.

 

[JosephKK]

Jan Panteltje [email protected] posted to
sci.electronics.design:

Bullshit, carbon mikes, throat mikes (or whatever you English care
to call these), dynamic mikes, and a ton of other ones, have much
higher output.

Phil aside, low Z dynamic microphones have maximum outputs on the
order of 10s of mV, magnetic phonograph pickups have full amplitude
ratings of a few mV. This means if there is a quiet passage the
output is in the 10's of uV range, and the noise floor is at least 40
dB below that (fractional uV). This is one of the reasons that high
futility (Hi-Fi) used to be very hard.

 

[JosephKK]

Joerg [email protected] posted to
sci.electronics.design:

Didi wrote:
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).
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).

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.


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

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.

[...]

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.

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.

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