What's the best bypass caps?

[Chris Carlen]

Greetings:

Before I go reading datasheets for hours and getting nothing done today,
do you folks have any "favorite" bypass capacitor makes/models for
medium speed digital stuff like HC and AC logic devices, CPLDs and FPGAs
running no more than 200MHz, microprocessors of the less than 100MHz
variety, etc.?

I am looking to stock up on some 0805 and 1206 chips, as well as
through-hole 0.1uF types, and perhaps some 100pF ones for higher
frequency stuff, to put in parallel with the 0.1uF ones.

Would you go for ceramic disks as opposed to "mono" caps for the
through-hole types?

Are there any quick ways to know that a cap is going to be an effective
bypass without reading datasheets for hours? (Like getting someone to
tell you the one they use?) ;-)

Thanks for suggestions.


Good day!




--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected] -- NOTE: Remove "BOGUS" from email address to reply.

 

[Frank Bemelman]

Greetings:

Before I go reading datasheets for hours and getting nothing done today,
do you folks have any "favorite" bypass capacitor makes/models for
medium speed digital stuff like HC and AC logic devices, CPLDs and FPGAs
running no more than 200MHz, microprocessors of the less than 100MHz
variety, etc.?

I am looking to stock up on some 0805 and 1206 chips, as well as
through-hole 0.1uF types, and perhaps some 100pF ones for higher
frequency stuff, to put in parallel with the 0.1uF ones.

Would you go for ceramic disks as opposed to "mono" caps for the
through-hole types?

Are there any quick ways to know that a cap is going to be an effective
bypass without reading datasheets for hours? (Like getting someone to
tell you the one they use?) ;-)

Ceramic multilayers.

John Larkin made some comments about decoupling caps lately. Something
about appnotes and manufaturers thinks their little 25 cents chip is
the centre of the universe. I have another thing to add, take a nice
board loaded with chips, make sure that it works, and then start cutting
away decoupling caps until it stops functioning. Just do it. It's
a nice experiment. Then *forget* the outcome of the experiment, and use
plenty decoupling caps in your design

 

[Charles Schuler]

Greetings:

Before I go reading datasheets for hours and getting nothing done today,
do you folks have any "favorite" bypass capacitor makes/models for
medium speed digital stuff like HC and AC logic devices, CPLDs and FPGAs
running no more than 200MHz, microprocessors of the less than 100MHz
variety, etc.?

I am looking to stock up on some 0805 and 1206 chips, as well as
through-hole 0.1uF types, and perhaps some 100pF ones for higher
frequency stuff, to put in parallel with the 0.1uF ones.

Would you go for ceramic disks as opposed to "mono" caps for the
through-hole types?

Are there any quick ways to know that a cap is going to be an effective
bypass without reading datasheets for hours? (Like getting someone to
tell you the one they use?) ;-)

If you are in control of the PCB design, then you probably realize that the
ground traces/ground planes can be just as important as the choice of bypass
capacitors, if not more important.

 

[John Larkin]

Greetings:

Before I go reading datasheets for hours and getting nothing done today,
do you folks have any "favorite" bypass capacitor makes/models for
medium speed digital stuff like HC and AC logic devices, CPLDs and FPGAs
running no more than 200MHz, microprocessors of the less than 100MHz
variety, etc.?

I am looking to stock up on some 0805 and 1206 chips, as well as
through-hole 0.1uF types, and perhaps some 100pF ones for higher
frequency stuff, to put in parallel with the 0.1uF ones.

Would you go for ceramic disks as opposed to "mono" caps for the
through-hole types?

Are there any quick ways to know that a cap is going to be an effective
bypass without reading datasheets for hours? (Like getting someone to
tell you the one they use?) ;-)

Thanks for suggestions.


Good day!

I'd go for 0.1 uF 0805 as the universal cap. There's no reason to use
a lower value for "faster" situations, nor any reason to put a smaller
value in parallel with the .1s... they all have about the same ESL, so
all look about the same at high frequencies.

Leaded caps will have a lot more lead inductance, which will matter
when dealing with fast digital stuff and extreme analog bits. The only
real advantage of the mono parts is that they're smaller and available
in square shapes, so you can keep the leads shorter to the board.

Thru-hole is dead: time to move on. Even 1206s are becoming dinosaurs.

For clean, fast stuff, use a multilayer board with a thin (say, 0.005"
or less) dielectric between the Vcc and ground planes, then scatter a
moderate number of the 0.1 uF 0805s around. The planes themselves are
the best bypass cap.

John

 

[Buck Buchanan]

Greetings:

Before I go reading datasheets for hours and getting nothing done today,
do you folks have any "favorite" bypass capacitor makes/models for
medium speed digital stuff like HC and AC logic devices, CPLDs and FPGAs (snip)
(Like getting someone to
tell you the one they use?) ;-)

I'm a big fan of Panasonic multi layer ceramics in your favorite
flavor of SMT (so you can mount right at a pin and via straight to a
ground plane). .1uF in 0402 - 0805 for me. Digi-Key has 'em.

I would think that if you want to get super duper tweaky about
individual ceramic cap specs, then focus might be more appropriately
turned towards PCB layout - lead inductances, ground plane/paths, etc.

Cheers,

Buck Buchanan

 

[Michael Black]

Ceramic multilayers.

John Larkin made some comments about decoupling caps lately. Something
about appnotes and manufaturers thinks their little 25 cents chip is
the centre of the universe. I have another thing to add, take a nice
board loaded with chips, make sure that it works, and then start cutting
away decoupling caps until it stops functioning. Just do it. It's
a nice experiment. Then *forget* the outcome of the experiment, and use
plenty decoupling caps in your design

Isn't that how "old man Muntz" designed his tv sets? Start taking out
parts until it stops working, and then put the last piece back in?

Somehow this bit about datasheets seems a tad overkill. In the old
days when we went to the wooden floored parts place, nobody asked
to see the datasheets on the capacitors. I doubt they existed.

What seems to be lost is common sense. "Oh, it's this frequency"
we'd say, "then use a .01uF". "It's a bit higher", then it would
be a .001". Only when you get into UHF would you switch to 470pF bypass
capacitors, and then you'd try to get the button type (suddenly I can't
remember the exact term, sort of looked like feedthroughs, but not actual
feed through) that you screwed into the chassis. Or if the signal is
going through a panel, use a feedthrough. You knew that as you went up in
frequency, short leads became fairly important, and those other types made
it easier to get a low inductance connection to ground.

In tube days, getting a good ground return was often a bigger issue
than the specific capacitor, at least in the shortwave and low
VHF range. If you had to drill another hole, and expend a ground
lug, one might try to use a longer lead on that bypass than necessary,
or have problems with ground rings around tube sockets.

Circuit board makes it easier to get a good ground, so long as
layout is good. That's one of the points of a ground plane layer,
so there is a really good ground layer, wherever you need it. Because
it's a sheet of copper, the inductance between points is much lower
than if narrow traces were used to wire up all the ground returns.

I threw out all the paper capacitors a couple of decades ago.
The ceramic capacitors I pull of scrap boards are fine for most
bypass uses. Getting too fancy may simply complicate things.

Michael

 

[qrk]

I like ceramic MLC's, surface mount in 0603. 0402 is too small for my
tastes. Ground/power plane is a very important capacitor. Has
wonderful high freq characteristics. Don't forget bulk capacitance,
like 10 to 1000 uF - depending on what your doing. This goes for high
speed digital junk.

Last 957-pin FPGA we used only had about 20 local 0.1 uF MLC
capacitors for all the power supplies. Bypass caps sat opposite the
part.

AVX has some very nice info on their MLC capacitors.
http://www.avxcorp.com/SpiApps/spicap/

Mark

 

[Active8]

[snip]

Isn't that how "old man Muntz" designed his tv sets? Start taking out
parts until it stops working, and then put the last piece back in?

that's the way i remember the story.

Somehow this bit about datasheets seems a tad overkill. In the old
days when we went to the wooden floored parts place, nobody asked
to see the datasheets on the capacitors. I doubt they existed.

i think it only becomes important at UHF and above, or when you really
need low ESR or low inductance. even then, a whole sheet is overkill.

What seems to be lost is common sense. "Oh, it's this frequency"
we'd say, "then use a .01uF". "It's a bit higher", then it would
be a .001". Only when you get into UHF would you switch to 470pF bypass
capacitors, and then you'd try to get the button type (suddenly I can't
remember the exact term, sort of looked like feedthroughs, but not actual
feed through) that you screwed into the chassis. Or if the signal is
going through a panel, use a feedthrough. You knew that as you went up in
frequency, short leads became fairly important, and those other types made
it easier to get a low inductance connection to ground.

if you can resonate the cap with it's own L, it's an rf short to ground
and you can forget about worrying. but that's a UHF thing. i've never
seen this technique used at 40MHz.

br,
mike

 

[Bill Sloman]

I'd go for 0.1 uF 0805 as the universal cap. There's no reason to use
a lower value for "faster" situations, nor any reason to put a smaller
value in parallel with the .1s... they all have about the same ESL, so
all look about the same at high frequencies.

Leaded caps will have a lot more lead inductance, which will matter
when dealing with fast digital stuff and extreme analog bits. The only
real advantage of the mono parts is that they're smaller and available
in square shapes, so you can keep the leads shorter to the board.

Thru-hole is dead: time to move on. Even 1206s are becoming dinosaurs.

For clean, fast stuff, use a multilayer board with a thin (say, 0.005"
or less) dielectric between the Vcc and ground planes, then scatter a
moderate number of the 0.1 uF 0805s around. The planes themselves are
the best bypass cap.

The multilayer 0.1uF 0805 parts witn X7R dielectric have an impedance
minimum at of the order of 100MHz, and - back in 1989 - we had an
object lesson in what this means at 800MHz. One of my guys didn't
really believe the application notes for the Gigabit Logic GaAs parts,
and skipped the (1810) 1nF porcelain dielectric microwave capacitor
(good to about a GHz) that we normally used to parallel the regular
100nF 0805 decoupling cap.

We had as much 800MHz on the supply rail at the part with the 100nF
multilayer in place as we had with it removed. I think we replaced it
on the layout with a 100pF single layer part with a COG dielectric,
which at least had a visible effect. The next version of the layout
had the full complement of capacitors.

 

[Spehro Pefhany]

What do you mean? Are 1206's getting hard to get? Are manufacturers going
to quit making them? I have a lot of designs out there that use only 1206
caps.

As all the volume shifts to 0603 and 0402 eventually, the 1206 parts
of popular value will become more expensive (or you might have to buy
higher voltage, higher value or better dielectric than you'd otherwise
require).

Example: 0.1uF/16V or 25V or 50V cap in high volume from my supplier,
cheapest dielectric in stock:

Mfr. Relative price Relative stock
0402 TY 1.0 1.0
0603 AT 0.85 94
0805 AT 0.97 5.8
1206 VI 4.4 1.1

Another comparison (Digikey)

Mfr. Relative price
0402 MA 1.0
0603 KE 1.08
0805 MA 1.5
1206 KE 2.5

Not shown is the number of different suppliers, which is large for the
0603 and 0805 and less at either end. You can see the "sweet spot"
right now is 0603, with 0402 coming in the future.

Best regards,
Spehro Pefhany

 

[Jeroen Belleman]

The multilayer 0.1uF 0805 parts witn X7R dielectric have an impedance
minimum at of the order of 100MHz, and - back in 1989 - we had an
object lesson in what this means at 800MHz. One of my guys didn't
really believe the application notes for the Gigabit Logic GaAs parts,
and skipped the (1810) 1nF porcelain dielectric microwave capacitor
(good to about a GHz) [...]

Both of these caps then seem to have about 25pH of ESL, which
doesn't sound credible. The way you mount them is more important
than the properties of the caps themselves. (How *did* you
mount them then? 25pH of ESL would be quite a trick! A simple via
between opposite sides of a standard 1.6mm PCB is ten times that!)

I measured the ESL of a bunch of capacitors some time back.
A 1206 ceramic standing upright on a ground plane measures
about 900pH. I'd expect about the same for an 1810 component,
since it's both 50% longer and wider. ESL does not depend
on capacitance, nor on dielectric. It's all in the geometry
of the part and its surroundings.

Once you have a 100nF ceramic chip capacitor as bypass, there
is no advantage in adding smaller-valued similarly shaped caps
for HF. On the other hand, some manufacturers make, e.g., *0612*
parts, that is, they're short and wide. Syfer comes to mind.
That *does* help. (Their web site sucks, though :-( )

Jeroen Belleman

 

[Blake]

As all the volume shifts to 0603 and 0402 eventually, the 1206 parts
of popular value will become more expensive (or you might have to buy
higher voltage, higher value or better dielectric than you'd otherwise
require).

Wow thanks for the thorough analysis. I did look at the Digikey catalog
myself, and you are right, the 1206's cost a few cents more per cap. Also,
I am amazed to see that ceramic caps of > 1uF can be found in a 1206
package.

Blake

 

[Jim Weir]

It went further than that. Muntz engineers were amongst the most talented
folks I've ever come across...

So you want an RF amplifier? No problem, but since RF and audio are so far
apart, do a bit of clever decoupling and use the same tube section for RF and
audio. Hm. Audio only goes up to 3 KHz. if you don't care for hifi, so do a
little more work and use it to split the sync at 15 kHz...and while you are at
it, nothing in that tube at 4.5 MHz., so use it as the sound intercarrier
amplifier. ONE TUBE section for several functions.

Those people were CLEVER.

Jim


[email protected] (Michael Black)
shared these priceless pearls of wisdom:

->
->Isn't that how "old man Muntz" designed his tv sets? Start taking out
->parts until it stops working, and then put the last piece back in?

 

[Spehro Pefhany]

It went further than that. Muntz engineers were amongst the most talented
folks I've ever come across...

So you want an RF amplifier? No problem, but since RF and audio are so far
apart, do a bit of clever decoupling and use the same tube section for RF and
audio. Hm. Audio only goes up to 3 KHz. if you don't care for hifi, so do a
little more work and use it to split the sync at 15 kHz...and while you are at
it, nothing in that tube at 4.5 MHz., so use it as the sound intercarrier
amplifier. ONE TUBE section for several functions.

Those people were CLEVER.

I guess Asian sourcing and solid engineering trounces cleverness, as
they're not around anymore. Or are they?

Best regards,
Spehro Pefhany

 

[Paul Burridge]

if you can resonate the cap with it's own L, it's an rf short to ground
and you can forget about worrying. but that's a UHF thing. i've never
seen this technique used at 40MHz.

Me neither. I don't think the OP was talking about this frequency
though, was he? Anyway, whilst getting a short at RF is very nice,
it's hardly necessary. I'm just left wondering what sort of maximum
reactance is acceptable for bypassing purposes. Anyone know if there's
a specific 'figure' to aim for?

 

[Paul Burridge]

Example: 0.1uF/16V or 25V or 50V cap in high volume from my supplier,
cheapest dielectric in stock:

Mfr. Relative price Relative stock
0402 TY 1.0 1.0
0603 AT 0.85 94
0805 AT 0.97 5.8
1206 VI 4.4 1.1

Sorry to engage in another shameless public display of ignorance, but
what the hell do these four digit numbers refer to?

 

[Bill Sloman]

The multilayer 0.1uF 0805 parts witn X7R dielectric have an impedance
minimum at of the order of 100MHz, and - back in 1989 - we had an
object lesson in what this means at 800MHz. One of my guys didn't
really believe the application notes for the Gigabit Logic GaAs parts,
and skipped the (1810) 1nF porcelain dielectric microwave capacitor
(good to about a GHz) [...]

Both of these caps then seem to have about 25pH of ESL, which
doesn't sound credible.

We didn't have the gear to measure the high frequency impedances, but
whatever the loss mechanism, the 100nF multilayer X7R clearly had a
much higher ESR at 800MHz. I always assumed that the dielectric was
the problem rather than the inductance of the connections. American
Technical Ceramics made a great deal of fuss about the porcelain
dielectric in thier microwave capacitors.

The way you mount them is more important
than the properties of the caps themselves.

We mostly mounted them on the under-side of the board, on the opposite
side to the surface-mount parts we were decoupling. At least four of
the inner layers of the board were devoted to various power planes -
it went component(IC) side, -2V, -5.2V, +5V/-3.4V, 0V and track
(decoupling caps) side.

The -3.4V was for the GaAs logic which was not mixed in with the TTL
parts, so that ground plane split pretty naturally.

The via's went straight to the relevant power planes, and were offset
from the lands for the IC leads and the capacitor contacts - IIRR the
boards came with the vias filled with solder and covered with
solder-mask.

(How *did* you
mount them then? 25pH of ESL would be quite a trick! A simple via
between opposite sides of a standard 1.6mm PCB is ten times that!)

I measured the ESL of a bunch of capacitors some time back.
A 1206 ceramic standing upright on a ground plane measures
about 900pH. I'd expect about the same for an 1810 component,
since it's both 50% longer and wider. ESL does not depend
on capacitance, nor on dielectric. It's all in the geometry
of the part and its surroundings.

Once you have a 100nF ceramic chip capacitor as bypass, there
is no advantage in adding smaller-valued similarly shaped caps
for HF.

That wasn't our experience - my colleague (technically my project
leader at the time, though he didn't take his status all that
seriously) shared your opinion, encouraged by a rather cramped layout,
and had to recant.

On the other hand, some manufacturers make, e.g., *0612*
parts, that is, they're short and wide. Syfer comes to mind.
That *does* help. (Their web site sucks, though :-( )

Sounds like a good idea. Back in 1989, we got what we could from the
microwave suppliers.

 

[Jim Weir]

So did I, sir. I started by sweeping out the TV shop in 1958 in the 9th grade.
Beat the hell out of delivering papers in the rain. Started fixin' them later
that year, and made a high school job of it. Got pretty darned good at it after
my junior year and went on to other electroniker pursuits, but never forgot the
smell of burning selenium (peanut butter) or the sight of horizontal output
tubes that got so hot the glass sucked in to the plate.

Nor how far into the plaster wall one's elbow will penetrate when getting bit
off the yoke.

As to the drifting parameters and sync buzz, yup. But you could replace every
double/triple...duty tube in the damned thing with brand new for under $5 and
bring it back to like-new.

Jim


Boris Mohar <[email protected]>
shared these priceless pearls of wisdom:

-> You can stop a car with only one brake caliper engaging. Not very well.
->Why am I so cantankerous? I used to service those beasts. I can still
->smell the burned selenium stack.

 

[[email protected]]

Wow! That's gross overkill. I bet you could depopulate 90% of them and
never see the difference. I'm about to do a Xilinx FG456 board, and
I'll use maybe 4 bypasses for the core and 4 more for VccIO. I'll let
you know how it works.

John

Hi John,

Just so you know, the core can (and does) suck 2.5A for a very short
period of time. Just after the download is complete, as it sets all
of the flops into their initial state.

This <http://www.xilinx.com/xapp/xapp189.pdf> may seem like overkill,
but if that 2.5A spike causes the voltage to droop enough, the chip
will reset back to its uninialized state.

Gary
g w helbig -at- yahoo -dot- com

 

[R.Legg]

"Case" size for SMT parts. The physical dimensions (in mm) are
approximately 1/4 of the value of the first two digits x 1/4 of the
value of the second two digits, so an 0805, for example, is 2mm x
1.25 mm.

You might want to try the 0201 size as it's not near as bulky as the
above old-fashioned ones at 0.6 x 0.3mm. ;-)

It seems a little odd, quoting metric dimensions when the imperial
part size is quoted. This is not necessary.

For clarity -

1210 is ~ .120 x .100 (metric part sixe is 3225 ~ 3.2 x 2.5mm)
1206 is ~ .120 x .060 (metric part size is 3216 ~ 3.2 x 1.6mm)
0805 is ~ .080 x .050 (metric part size is 2012 ~ 2.0 x 1.2mm)
0604 is ~ .060 x .040 (metric part size is 1608 ~ 1.6 x 0.8mm)
0402 is ~ .040 x .020 (metric part size is 1005 ~ 1.0 x 0.5mm)
0201 is ~ .020 x .010 (metric part size is 0603 ~ 0.6 x 0.3mm)

Do we see a useful pattern here?

Anything smaller than 0805 - check with fab house for their ability to
handle parts automatically. There may be penalties, due to tape and
reel incompatability (like trashing every second part on the tape).

Cost, performance and real estate benefits below 0604 may not be
naturally occuring either. This is particularly the case on .060 FR4.
Anyone care to guess why this is the case?

Check first.

RL