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Low absorption SMD capacitors

Does anyone make SMD polypropylene capacitors? I need a type with low
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.

Cost isn't much of an issue; it's a low-volume, high-value product.

I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html

There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.

The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.

Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something? :)

TIA
 
R

Roger Hamlett

Jan 1, 1970
0
Does anyone make SMD polypropylene capacitors? I need a type with low
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.

Cost isn't much of an issue; it's a low-volume, high-value product.

I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html

There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.

The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.

Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something? :)
Look for polyphenylene sulphide capacitors. It has worse drift than the
polypropylene, but otherwise very good characteristics, and a few types
are available in SMD. Also try a company called 'Electronic Concepts
Inc.', who specialise in slightly 'upmarket' capacitors, and were helpful
to me in the past when looking for something with better characteristics.
I think they do a polypropylene type.
The reason for the 'lack', is that the film ceased production a little
while ago. They actually arranged a run of film specially for these
capacitors. They also do PTFE capacitors. :)

Best Wishes
 
S

Steve Goldstein

Jan 1, 1970
0
Does anyone make SMD polypropylene capacitors? I need a type with low
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.

Cost isn't much of an issue; it's a low-volume, high-value product.

I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html

There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.

The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.

Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something? :)

TIA

Some years ago I found that NPO ceramic microwave capacitors worked very
well. The ones we used were made by American Technical Ceramics. This
was a true 16-bit hybrid track/hold amplifier (AD386 if you still have an old
Analog Devices catalog on your shelf). They provided about 20-40ppm of
DA error alone IIRC, we were able to get nearly an order of magnitude better
with some laser trimming (not the cap, but some other compensating stuff).

We learned some other interesting things during this project as well. They
may not apply to your design. When we were breadboarding the circuit
prior to designing the substrae for this chip-and-wire hybrid, we found that
sometimes the DA was awful, and sometimes not. Ultimately it came down
to the op-amp packaging! Metal cans were great, plastic nearly as good,
ceramic (a.k.a. CERDIP) was right out. It definitely wasn't the op-amp,
(AD744, BTW) we had part from the same wafer lot in all three packages.

Steve - still at ADI after all these years

For direct email replies remove the 3-letter food item from the address
 
Steve said:
Some years ago I found that NPO ceramic microwave capacitors worked very
well. The ones we used were made by American Technical Ceramics. This
was a true 16-bit hybrid track/hold amplifier (AD386 if you still have an old
Analog Devices catalog on your shelf). They provided about 20-40ppm of
DA error alone IIRC, we were able to get nearly an order of magnitude better
with some laser trimming (not the cap, but some other compensating stuff).

Thanks, I'll look them up.
We learned some other interesting things during this project as well. They
may not apply to your design. When we were breadboarding the circuit
prior to designing the substrae for this chip-and-wire hybrid, we found that
sometimes the DA was awful, and sometimes not. Ultimately it came down
to the op-amp packaging! Metal cans were great, plastic nearly as good,
ceramic (a.k.a. CERDIP) was right out. It definitely wasn't the op-amp,
(AD744, BTW) we had part from the same wafer lot in all three packages.

One thing which has bitten me already is surface leakage on the
prototype board (soldering residues etc). I must have got lucky with
the first one as it worked perfectly. #2 and #3 were totally unusable
at first, I couldn't scrub the residue off as it would rip all the fine
wires off the SOT23 transistors I had bodged onto the protoboard. A
gentle wash with isopropanol improved things a lot, with a proper PCB I
can give it a proper scrub and then conformally coat it.
We are trying to measure photocurrents of about 200 pA upwards, so I
will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
with conformal coating adequate for this job or will I need to look at
ceramic hybrids etc?
 
Roger said:
Look for polyphenylene sulphide capacitors. It has worse drift than the
polypropylene, but otherwise very good characteristics, and a few types
are available in SMD. Also try a company called 'Electronic Concepts
Inc.', who specialise in slightly 'upmarket' capacitors, and were helpful
to me in the past when looking for something with better characteristics.
I think they do a polypropylene type.

OK, thanks.
The reason for the 'lack', is that the film ceased production a little
while ago. They actually arranged a run of film specially for these
capacitors. They also do PTFE capacitors. :)

Oh dear. This design will be in production for 5+ years so I may end
up having to do a lifetime buy. At least we aren't talking massive
volumes here...
 
Roger said:
Look for polyphenylene sulphide capacitors. It has worse drift than the
polypropylene, but otherwise very good characteristics, and a few types
are available in SMD. Also try a company called 'Electronic Concepts
Inc.', who specialise in slightly 'upmarket' capacitors, and were helpful
to me in the past when looking for something with better characteristics.
I think they do a polypropylene type.

OK, thanks.
The reason for the 'lack', is that the film ceased production a little
while ago. They actually arranged a run of film specially for these
capacitors. They also do PTFE capacitors. :)

Oh dear. This design will be in production for 5+ years so I may end
up having to do a lifetime buy. At least we aren't talking massive
volumes here...
 
W

Winfield Hill

Jan 1, 1970
0
[email protected] wrote...
Thanks, I'll look them up.

Steve is right. I have taken measurements on three brands of NP0 C0G
0805-size capacitors and found them to be either perfect, or below
0.1% D.A. over a 100Hz to 250kHz range (this is close to my hp 4192's
supposed 0.2% measurement accuracy), which is as good as my "pefect"
polypropylene capacitor measurements. This compares to 5 to 15% D.A.
that I measured for other ceramic-material 0805 capacitors.

Steve, is your concern the D.A. of a ceramic package's capacitance?
One thing which has bitten me already is surface leakage on the
prototype board (soldering residues etc). I must have got lucky with
the first one as it worked perfectly. #2 and #3 were totally unusable
at first, I couldn't scrub the residue off as it would rip all the
fine wires off the SOT23 transistors I had bodged onto the protoboard.
A gentle wash with isopropanol improved things a lot, with a proper
PCB I can give it a proper scrub and then conformally coat it.
We are trying to measure photocurrents of about 200 pA upwards, so I
will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
with conformal coating adequate for this job or will I need to look
at ceramic hybrids etc?

You have to clean the board shortly after soldering, then add a
conformal layer to a dry board after test. FR4 should be OK in the
region you're using, provided you have appropriate guards. In some
instances you'll want to add rows of closely-spaced guarded vias to
intercept the currents that could flow along fibres inside the PCB.
 
R

Roger Hamlett

Jan 1, 1970
0
Thanks, I'll look them up.


One thing which has bitten me already is surface leakage on the
prototype board (soldering residues etc). I must have got lucky with
the first one as it worked perfectly. #2 and #3 were totally unusable
at first, I couldn't scrub the residue off as it would rip all the fine
wires off the SOT23 transistors I had bodged onto the protoboard. A
gentle wash with isopropanol improved things a lot, with a proper PCB I
can give it a proper scrub and then conformally coat it.
We are trying to measure photocurrents of about 200 pA upwards, so I
will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
with conformal coating adequate for this job or will I need to look at
ceramic hybrids etc?
You will find that companies who are used to working with low leakage
boards, will offer low residue cleaning, as a special service. I have some
boards used for reading a couple of sensors, where virtual grounds around
the inputs are used to make the input 'see' an extremely high
characteristic impedance, and the input feeds in through Teflon standoffs
and connectors, and with properly specified cleaning proceedures it is
suprising how good FR4 can be.

Best Wishes
 
T

Tom Bruhns

Jan 1, 1970
0
Does anyone make SMD polypropylene capacitors? I need a type with low
dielectric absorption for use in an integrating photodiode amplifer,
value 100pF - 10nF. 5% tolerance is OK as the product will be
calibrated before shipping; tempco also isn't much of an issue as we
can measure the ambient temp and compensate.
I've built a prototype with a through-hole polypropylene cap which
works well; now I need to build the production version with all SMD
parts, as there isn't much space to work with.

Cost isn't much of an issue; it's a low-volume, high-value product.

I'm basing my choice of dielectric on this app note:
http://www.national.com/rap/Application/0,1570,28,00.html

There is some long-time-constant effect visible with the circuit I have
built but it's negligible compared with the signal. Just for an
experiment I tried an X7R SMD ceramic, the dielectric absorption made
it totally unusable.

The nearest SMD type I can find is polypropylene sulphide from several
different manufacturers; they all *claim* low dielectric absorption but
don't provide any figures. I can't find any comparisons between PPS
and polypropylene along the lines of the graph at the end of the
National app note.

Teflon would obviously be better still but I can't find *any* of these,
leaded or SMD - have they been banned under RoHS or something? :)

TIA

Others have mentioned NPO caps--AKA C0G. You can get these over the
whole range you're looking for, and a decade on either side. It's
possible to get them in 1% tolerance, if you ever do need closer than
5%. One thing to be careful about, though, is that large physical size
multilayer ceramics, in any of the various dielectrics, are prone to
cracking. Recommend you stay at 1210 maximum size.

Interesting to hear from another poster about the polyphenylene
sulphide film production. I need to pass that info along to a team
member who is planning on using them (in part because of the low DA).

Cheers,
Tom
 
E

Eeyore

Jan 1, 1970
0
Roger said:
The reason for the 'lack', is that the film ceased production a little
while ago. They actually arranged a run of film specially for these
capacitors.

Polypropylene ?

I heard that polystyrene film was no longer available but polypropylene's widely
used.

Graham
 
Winfield said:
[email protected] wrote...[snip]

Steve is right. I have taken measurements on three brands of NP0 C0G
0805-size capacitors and found them to be either perfect, or below
0.1% D.A. over a 100Hz to 250kHz range (this is close to my hp 4192's
supposed 0.2% measurement accuracy), which is as good as my "pefect"
polypropylene capacitor measurements. This compares to 5 to 15% D.A.
that I measured for other ceramic-material 0805 capacitors.

OK, great. These seem to be a lot more readily available in SMD.

[more snippage]
You have to clean the board shortly after soldering, then add a
conformal layer to a dry board after test. FR4 should be OK in the
region you're using, provided you have appropriate guards. In some
instances you'll want to add rows of closely-spaced guarded vias to
intercept the currents that could flow along fibres inside the PCB.

By 'guards' I assume you mean a ring around the sensitive nodes
connected to GND?

All our boards are made by a subcontractor so I don't have much control
over the cleaning - some have been fine, some have come back covered in
white detergent residue and other cr@p which has played havoc with even
fairly low-impedance circuits. If I scrub them myself with a
toothbrush and isopropanol after they've been sitting on a shelf for a
week will this be OK or does the contamination leach into the FR4
itself?

I have used the LT1112 (plastic DIP package) for the prototype, it
seems to work fine but I note from the data sheet that the worst-case
input bias current and offset voltage could be a problem. In one sense
it doesn't matter because the photodiode dark current will usually
dominate, and I subtract this out by taking a dark measurement at the
same integration time immediately afterwards, but if there is a large
enough negative bias current the ADC will clip at zero and this method
won't work.
Another 'oddity' is that the SO8 package of this device (which I'm
planning on using for production) has significantly worse specs than
the PDIP - any ideas why? I always assumed the die was identical
between different packages.
 
S

Steve Goldstein

Jan 1, 1970
0
[email protected] wrote...

Steve is right. I have taken measurements on three brands of NP0 C0G
0805-size capacitors and found them to be either perfect, or below
0.1% D.A. over a 100Hz to 250kHz range (this is close to my hp 4192's
supposed 0.2% measurement accuracy), which is as good as my "pefect"
polypropylene capacitor measurements. This compares to 5 to 15% D.A.
that I measured for other ceramic-material 0805 capacitors.


Steve, is your concern the D.A. of a ceramic package's capacitance?

Yes, that was it precisely. Parts from the same wafer lot in different
package types showed different amounts of dielectric absorption. We
never bothered to figure out if it was the ceramic in the package, or the
glass frit seal material, but parts in the CERDIP package showed much
worse DA than in plastic or metal cans. Ultimately we didn't really care
as the final product was a chip-and-wire hybrid on a single-layer thinfilm
substrate with laser-trimmed Nichrome resistors.

We did also see a difference in cap materials. NP0 were generally good,
but the best ones were the ATC microwave caps designed for really low
ESR at superhigh frequencies. I can't remember if they actually specified
DA, but those caps were quite good. At $1+ each they should have been!

steve
 
T

Tom Bruhns

Jan 1, 1970
0
By 'guards' I assume you mean a ring around the sensitive nodes
connected to GND?

Guards are not necessarily connected to ground (optimally), but to a
potential which is driven from a low impedance and follows the node
(trace; vias) that you want to protect. If the node you want to
protect is a virtual ground, then a grounded guard is perfectly
appropriate. But if you want to guard the + input of an op amp that's
used in the non-inverting configuration, you can guard that node with
traces and vias that are connected to the - op amp input. You have to
beware of capacitance to ground (or to other nodes) from that guard
trace, though, because that can affect the response. You can also use
an op amp buffer to drive the guard. Using a guard reduces both DC
leakage currents and effective capacitance.
All our boards are made by a subcontractor so I don't have much control
over the cleaning - some have been fine, some have come back covered in
white detergent residue and other cr@p which has played havoc with even
fairly low-impedance circuits. If I scrub them myself with a
toothbrush and isopropanol after they've been sitting on a shelf for a
week will this be OK or does the contamination leach into the FR4
itself?

Don't have much control over your subcontractor? Ouch! Complain, and
if they don't solve the problem, fire them and find one that will
listen to you. Your situation may prevent doing something that
drastic, but at least give them a good piece of your mind if they
deliver cr@p. We've found that our contract manufacturers don't always
get it right the first time, but they do want to deliver a product that
meets our needs, and they do listen to us.
....
Cheers,
Tom
 
V

vasile

Jan 1, 1970
0
Tom said:
My recollection is that glass has poor dielectric absorption
characteristics. Do you have other info on it? The link seemed silent
on the subject.

I'm asking info directly from the AVX when the parts meet my
requirements. They answer quite fast. However AVX is expensive (twice
than Panasonic for example).
On the application posted, the capacitor's dielectric loss is not the
only problem as long the photodiode current is *very* small. Keeping
the board clean around the low leackage points is much difficult than
founding a good capacitor.

greetings,
Vasile
 
T

Tom Bruhns

Jan 1, 1970
0
requirements. They answer quite fast. However AVX is expensive (twice
than Panasonic for example).
On the application posted, the capacitor's dielectric loss is not the
only problem as long the photodiode current is *very* small. Keeping
the board clean around the low leackage points is much difficult than
founding a good capacitor.

greetings,
Vasile

Hi Vasile,

But the question is not about dielectric loss or leakage, it's about
dielectric absorption. See, for example, Bob Pease's article at
http://www.national.com/rap/Application/0,1570,28,00.html (also posted
in the basenote to this thread). Bob says he has never seen any other
articles about the effect, but my uncle was issued a patent on a method
to get around the effect, long before Bob's article came out.

I have an ongoing experiment involving the room-temperature
self-discharge time constant of polyester (mylar) and polypropylene
caps. Last time I checked, the polyesters managed between 5 and 10
years, and the polyprops came in at about 50 to 100 years. Indeed, it
becomes diffidult to guard traces with a voltage that's close enough to
the voltage on the guarded trace to get leakage currents that low.
Even a 100uV offset, with 10^10 ohms, comes out 10fA I believe, and
it's easy to end up with less than 10^10 ohms, especially if the
humidity is high.

Cheers,
Tom
 
J

joseph2k

Jan 1, 1970
0
Thanks, I'll look them up.


One thing which has bitten me already is surface leakage on the
prototype board (soldering residues etc). I must have got lucky with
the first one as it worked perfectly. #2 and #3 were totally unusable
at first, I couldn't scrub the residue off as it would rip all the fine
wires off the SOT23 transistors I had bodged onto the protoboard. A
gentle wash with isopropanol improved things a lot, with a proper PCB I
can give it a proper scrub and then conformally coat it.
We are trying to measure photocurrents of about 200 pA upwards, so I
will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
with conformal coating adequate for this job or will I need to look at
ceramic hybrids etc?

Wow, that triggered an old memory. Back in the day ~20+ years ago as a test
engineer i had to create fixtures for repeatable mass testing of SMD
capacitors from very large (0.250 inch by 0.350 inch) down to about 0603
size. It turned out to be precision construction (less that 0.001 inch
tolerances) and air insulated.
 
G

Gerhard Hoffmann

Jan 1, 1970
0
never bothered to figure out if it was the ceramic in the package, or the
glass frit seal material, but parts in the CERDIP package showed much
worse DA than in plastic or metal cans. Ultimately we didn't really care
as the final product was a chip-and-wire hybrid on a single-layer thinfilm
substrate with laser-trimmed Nichrome resistors.

I heard there used to be problems long ago when they changed EPROMS
from sidebrazed to CERDIP. The sealing glass released quite a lot of water
during fritting and the chip passivation did not take that into account.

regards, Gerhard
 
G

Gerhard Hoffmann

Jan 1, 1970
0
But the question is not about dielectric loss or leakage, it's about
dielectric absorption. See, for example, Bob Pease's article at
http://www.national.com/rap/Application/0,1570,28,00.html (also posted
in the basenote to this thread). Bob says he has never seen any other
articles about the effect, but my uncle was issued a patent on a method
to get around the effect, long before Bob's article came out.

Some articles on DA are on

<http://waltjung.org/Classic_Articles.html>

regards, Gerhard
 
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