Noise in tantalum SMT caps?

[Spehro Pefhany]

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?


Best regards,
Spehro Pefhany

 

[Joerg]

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?

A very loud bang?

 

[Phil Allison]

"Spehro Pefhany"

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?

** This a trick question ??

Like how many ohms in a coulomb ?



........ Phil

 

[John Devereux]

"Spehro Pefhany"

** This a trick question ??

Like how many ohms in a coulomb ?

Yes I am puzzled too, although he must have some reason for asking...

I did find that tantalums have very low microphony (I could not detect
it) compared to high K chip ceramics which have lots. It makes them a
good choice for filtering any kind of reference voltage.

 

[[email protected]]

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?

Celine Dion?

 

[Boris Mohar]

Polarity marking is backwards so it is starting to break down?

 

[John Larkin]

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?


Best regards,
Spehro Pefhany

I'd guess just the thermal noise of the esr, which is only a couple
tenths of an ohm maybe.

John

 

[Spehro Pefhany]

I'd guess just the thermal noise of the esr, which is only a couple
tenths of an ohm maybe.

John

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

I'm looking at a design (not my own) where there's an RC on the output
of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

Is the reference input current constant with input voltage?
Temperature? I doubt it. 8-(

 

[MooseFET]

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?

Look at the leakage current. The electrons rattle as they go
through. The more leakage, the more noise.

 

[John Larkin]

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

Dunno. I recently tested some standard alums and some tants, posted
elsewhere, and the alums had serious chemical things going on inside;
the tants and polymer aluminums much, much less.

I'm looking at a design (not my own) where there's an RC on the output
of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

Is the reference input current constant with input voltage?
Temperature? I doubt it. 8-(

It certainly varies as a function of adc activity, which can vary in
some systems, even for delta-sigmas.


Well, the cap-versus-temp bahavior of a tantalum isn't great. So if
there were standing DC on it, as there is here, minute temperature
fluctuations will modulate C and, since CV is conserved, V would
wobble around. The same thing would happen on hi-K ceramics.

The R is maybe a bad idea, if unbuffered by an opamp.

John

 

[Fred Bloggs]

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

There most certainly is and it has been characterized as 1/f^2 down to
the millihertz region. From what I gather from the research papers
available on the web, most of this noise is due to thermal instability
around dielectric microleakage areas which self-heal via a chemical
transformation of MnO2->Mn2O3. These are characterized by a gradual
buildup of noise to a peak and then a substantial drop once the healing
has occurred. Also the very construction of the things makes them a
hotbed of tunneling and current trapping mechanisms as well as
possessing a nearly shot noise effect due to a
metal-insulator-semiconductor conduction mechanism from anode to
cathode. And all of these things are not even close to stationary,
varying all the time with a very slow trend into deterioration with
aging. This is all transparent to the majority of applications requiring
bulk capacitance at low ESR in small packages, but entirely visible as a
reference for a 24-bit A/D conversion application.

 

[Joerg]

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

I'm looking at a design (not my own) where there's an RC on the output
of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

Ouch! Did they split the grounds? That can cause part of it. I've never
had a case where that brought the ENOB down this much, but it put a
measurable crimp into it.

Is the reference input current constant with input voltage?
Temperature? I doubt it. 8-(

There's probably a lot more that's not constant.

 

[John Larkin]

There most certainly is and it has been characterized as 1/f^2 down to
the millihertz region. From what I gather from the research papers
available on the web, most of this noise is due to thermal instability
around dielectric microleakage areas which self-heal via a chemical
transformation of MnO2->Mn2O3. These are characterized by a gradual
buildup of noise to a peak and then a substantial drop once the healing
has occurred. Also the very construction of the things makes them a
hotbed of tunneling and current trapping mechanisms as well as
possessing a nearly shot noise effect due to a
metal-insulator-semiconductor conduction mechanism from anode to
cathode. And all of these things are not even close to stationary,
varying all the time with a very slow trend into deterioration with
aging. This is all transparent to the majority of applications requiring
bulk capacitance at low ESR in small packages, but entirely visible as a
reference for a 24-bit A/D conversion application.

What's the magnitude of the currents? The source impedance of the
reference (or of the series resistor) then determines the noise
voltage. I've done analog systems that were good to a few ppm
stability, with tantalums bypassing refs and excitation amps.

The resistor is a bad idea. What's its value, Spehro?


Numbers, gentlemen!


John

 

[Tom Bruhns]

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

I'm looking at a design (not my own) where there's an RC on the output
of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

Is the reference input current constant with input voltage?
Temperature? I doubt it. 8-(

I've used tantalums at a point in a noise-filtering circuit where I'd
for sure have noticed noise much above 10 nanovolts/rtHz, and not seen
any. If you'd like, I could make a measurement with an AD797
amplifier connected to a couple biased-up caps, driving an HP89410
analyzer, and get a noise spectral density plot. I'm curious enough
about it now I may just do that anyway...

Cheers,
Tom

 

[Joerg]

I've used tantalums at a point in a noise-filtering circuit where I'd
for sure have noticed noise much above 10 nanovolts/rtHz, and not seen
any. If you'd like, I could make a measurement with an AD797
amplifier connected to a couple biased-up caps, driving an HP89410
analyzer, and get a noise spectral density plot. I'm curious enough
about it now I may just do that anyway...

Same experience here. Tantals are either very quiet or very loud (as in
*BANG*). Quite nicely suited for filters if the tolerance were smaller.

 

[John Devereux]

I've used tantalums at a point in a noise-filtering circuit where I'd
for sure have noticed noise much above 10 nanovolts/rtHz, and not seen
any. If you'd like, I could make a measurement with an AD797
amplifier connected to a couple biased-up caps, driving an HP89410
analyzer, and get a noise spectral density plot. I'm curious enough
about it now I may just do that anyway...

FWIW I would love to see this. I have been using e.g. 2k2 feeding a
22uF tant as a reference noise filter. (Feeding a high impedance opamp
input). As I mentioned chip ceramics are a disaster here because of
microphony and perhaps temperature fluctuations.

 

[Joerg]

FWIW I would love to see this. I have been using e.g. 2k2 feeding a
22uF tant as a reference noise filter. (Feeding a high impedance opamp
input). As I mentioned chip ceramics are a disaster here because of
microphony and perhaps temperature fluctuations.

Can't say that. I use ceramics all the time, never had such issues in a
Ref distribution and some of that is rather sensitive.

 

[John Devereux]

[...]

FWIW I would love to see this. I have been using e.g. 2k2 feeding a
22uF tant as a reference noise filter. (Feeding a high impedance opamp
input). As I mentioned chip ceramics are a disaster here because of
microphony and perhaps temperature fluctuations.

Can't say that. I use ceramics all the time, never had such issues in
a Ref distribution and some of that is rather sensitive.

IIRC I measured about a mV disturbance across the cap, when tapping
the PCB. This was when using some 0805 1uF X7R. I had to go through
the circuit and replace them with tantalums, in the critical areas,
which fixed the problem.

ref-[R]-.----- opamp> --.
[C] LED ----> Photodiode -> AC amplfier -> output

Basically I was looking for a small fluctuation on a large background
signal.


Hook up a 1M resistor and a 10u chip ceramic, with wires, to a voltage
source. Put a scope on AC across the cap. If you move the wires you
strain the body of the cap - this results in a visible output voltage
change.

I assume the same thing is going on when the PCB is flexed.

 

[Joerg]

[...]

I've used tantalums at a point in a noise-filtering circuit where I'd
for sure have noticed noise much above 10 nanovolts/rtHz, and not seen
any. If you'd like, I could make a measurement with an AD797
amplifier connected to a couple biased-up caps, driving an HP89410
analyzer, and get a noise spectral density plot. I'm curious enough
about it now I may just do that anyway...
FWIW I would love to see this. I have been using e.g. 2k2 feeding a
22uF tant as a reference noise filter. (Feeding a high impedance opamp
input). As I mentioned chip ceramics are a disaster here because of
microphony and perhaps temperature fluctuations.

Can't say that. I use ceramics all the time, never had such issues in
a Ref distribution and some of that is rather sensitive.

IIRC I measured about a mV disturbance across the cap, when tapping
the PCB. This was when using some 0805 1uF X7R. I had to go through
the circuit and replace them with tantalums, in the critical areas,
which fixed the problem.

ref-[R]-.----- opamp> --.
[C] LED ----> Photodiode -> AC amplfier -> output

Basically I was looking for a small fluctuation on a large background
signal.


Hook up a 1M resistor and a 10u chip ceramic, with wires, to a voltage
source. Put a scope on AC across the cap. If you move the wires you
strain the body of the cap - this results in a visible output voltage
change.

I assume the same thing is going on when the PCB is flexed.

I'll try that. Interesting, I always give my circuit boards a good
banging and never saw issues like that. Of course we don't have to use
RoHS solder out here

 

[Tom Bruhns]

FWIW I would love to see this. I have been using e.g. 2k2 feeding a
22uF tant as a reference noise filter. (Feeding a high impedance opamp
input). As I mentioned chip ceramics are a disaster here because of
microphony and perhaps temperature fluctuations.

Have you tried C0G ceramics? You can get 0.1uF in 1206 size these
days, though a bit pricey.

When I get a chance I'll set up the (rather simple) noise spectral
density thing on the tants. Doubt I'll do any microphonics testing on
them though; it'll just be to see if they are stable under bias. Per
John Larkin's comments, I'll try it in "free air" and with some sort
of thermal insulation to at least slow down the changes. That's one
area where C0G should shine. Though they are rated +/-30ppm/C, you
can get batches that are much lower. I accidentally mixed up some X5R
and some C0G 1000pF caps the other day, and sorted them by putting a
soldering iron on them while measuring (I know, I know, they weren't
worth the effort, but it was an interesting exercise). Some of the
C0Gs changed by about .01%, 100ppm, between room temp and way too hot
to touch -- that's a lot less than 30ppm/C.

Cheers,
Tom