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'[OT] A final question about decoupling caps.'
1999\01\17@031438 by Lynx {Glenn Jones}

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Today as i was installing decoupling caps on a circuit board i thought to
myself: "Why dont the IC manufactures just incorperate a decoupling cap
right in the ic itself?" Any one have any ideas?

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A member of the PI-100 Club:
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058209749445923078164062862089986280348253421170679

1999\01\17@074713 by Andy In Dallas

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Today as i was installing decoupling caps on a circuit board i thought
to
myself: "Why dont the IC manufactures just incorperate a decoupling cap
right in the ic itself?" Any one have any ideas?
*********************

Andy writes:

 The largest size we can get on a die is only a few pf. And
it takes up as much space as many thousands of transistors.
Furthermore, it is not a very good cap, since there is
significant resistance in the internal track which, along
with the dielectic, limits the "Q". Additionally , any caps
that are used for coupling (RF ckts) internally also have
a stray coupling factor to substrate of about a fifth of their
value (it varies some). Lastly, the substrate for a silicon
IC is not conductive so you have to use interconnect as the
ground leads as well as the hot lead inside the device and it
can lead to undesireable intercoupling.
  Whew!!
 Actually, frequency compensated op amps do use internal
capacitors, but they are very small..... and can only be
used in special areas.

 In summary -- the technology for making acceptable capacitors
is not the same technology for making acceptable transistors...

        Andy in Dallas  (too wordy for a Sunday morning...)

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1999\01\17@082321 by uter van ooijen / floortje hanneman

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Capacitors are VERY area-expensive to integrate.

----------
> From: Lynx {Glenn Jones} <spam_OUTjones_glTakeThisOuTspamEFN.ORG>
> To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject: [OT] A final question about decoupling caps.
> Date: Sunday, January 17, 1999 09:12
>
> Today as i was installing decoupling caps on a circuit board i thought to
> myself: "Why dont the IC manufactures just incorperate a decoupling cap
> right in the ic itself?" Any one have any ideas?
>
>
----------------------------------------------------------------------------
--
> A member of the PI-100 Club:
> 3.1415926535897932384626433832795028841971693993751
> 058209749445923078164062862089986280348253421170679

1999\01\17@084817 by ryan pogge

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you can buy sockets for your chips that have decoupling caps built in.


>Capacitors are VERY area-expensive to integrate.
>
>----------
>> From: Lynx {Glenn Jones} <jones_glspamKILLspamEFN.ORG>
>> To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
>> Subject: [OT] A final question about decoupling caps.
>> Date: Sunday, January 17, 1999 09:12
>>
>> Today as i was installing decoupling caps on a circuit board i thought to
>> myself: "Why dont the IC manufactures just incorperate a decoupling cap
>> right in the ic itself?" Any one have any ideas?
>>
>>
>---------------------------------------------------------------------------
-
>--
>> A member of the PI-100 Club:
>> 3.1415926535897932384626433832795028841971693993751
>> 058209749445923078164062862089986280348253421170679

1999\01\17@125449 by dave vanhorn

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At 12:12 AM 1/17/99 -0800, Lynx {Glenn Jones} wrote:
>Today as i was installing decoupling caps on a circuit board i thought to
>myself: "Why dont the IC manufactures just incorperate a decoupling cap
>right in the ic itself?" Any one have any ideas?

because it would be hugely expensive in silicon.
Easily double the IC's cost, plus IIRC they don't have the technology to do
a lot of layers, which means it would have to be even larger.. It would be
a great thing to do, if it would work though.

1999\01\17@125700 by dave vanhorn

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At 08:35 AM 1/17/99 -0500, ryan pogge wrote:
>you can buy sockets for your chips that have decoupling caps built in.

Those are not nearly as nice as they look at first glance. Look at the long
leads on the cap!

1999\01\17@160805 by William Chops Westfield

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   >you can buy sockets for your chips that have decoupling caps built in.

   Those are not nearly as nice as they look at first glance. Look at
   the long leads on the cap!

I think you'll have to explain how to connect pin 8 to pin 16 of a typical
DIP using "leads" that are any shorter than that!

(this *is* why the newer generations of fast chips have moved the power pins
to the center of the package.)

BillW

1999\01\17@163925 by dave vanhorn

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At 01:07 PM 1/17/99 -0800, William Chops Westfield wrote:
>    >you can buy sockets for your chips that have decoupling caps built in.
>
>    Those are not nearly as nice as they look at first glance. Look at
>    the long leads on the cap!
>
>I think you'll have to explain how to connect pin 8 to pin 16 of a typical
>DIP using "leads" that are any shorter than that!
>
>(this *is* why the newer generations of fast chips have moved the power pins
> to the center of the package.)
>
>BillW


No problem. I route a solid ground to the ground pin. I mount the bypass
cap at this point. Then I run a track from here to the package's VCC pin.
The same idea is seen in the Murata and Panasonic three-lead EMI
supressors. The EMI is forced to travel to the cap, before it can get to
anything else. Distribuiting the L and C differently in the circuit forms a
Tee filter, rather than a high pass filter (well, actually a notch, but at
the wrong frequency)

The total lead length isn't any less, but look at the circuit it makes. The
noise from the IC has only one place to go, which is to the + side of the
cap.  With the capped sockets, the cap is in the middle, with lead
inductance on both sides, and nothing at all to prevent the noise from
going out to the system.

It really does make a large difference, but it's kind of like a tuneup in a
car, $100 racing plugs don't help you if your timing is off.  You have to
get a solid ground to everyone that makes any noise, and you have to
provide low Z paths between devices that exchance high speed signals.

1999\01\17@192352 by Gerhard Fiedler

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At 12:43 01/17/99 -0500, dave vanhorn wrote:
>At 12:12 AM 1/17/99 -0800, Lynx {Glenn Jones} wrote:
>>Today as i was installing decoupling caps on a circuit board i thought to
>>myself: "Why dont the IC manufactures just incorperate a decoupling cap
>>right in the ic itself?" Any one have any ideas?
>
>because it would be hugely expensive in silicon.

who's been talking about "silicon"? most ics are quite a bit more than the
small silicon chip. eg. in most but the smallest dip packages, one could
imagine that there's enough space in the package (not on the die) for some
nFarads. it's probably just too expensive.

ge

1999\01\18@042043 by Michael Rigby-Jones

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> At 08:35 AM 1/17/99 -0500, ryan pogge wrote:
> >you can buy sockets for your chips that have decoupling caps built in.
>
> Those are not nearly as nice as they look at first glance. Look at the
> long
> leads on the cap!
>
The last place I worked used some very strange decoupling caps.  They were
fitted underneath the IC, without using a socket.  They were very thin with
the same outline as the IC and thin pins that could be pushed through the
same holes as the IC.  This got around the problem of long leads with the
socket type decouplers.

Regards

Mike Rigby-Jones
EraseMEmrjonesspam_OUTspamTakeThisOuTnortelnetworks.com

1999\01\18@101749 by dave vanhorn

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>The last place I worked used some very strange decoupling caps.  They were
>fitted underneath the IC, without using a socket.  They were very thin with
>the same outline as the IC and thin pins that could be pushed through the
>same holes as the IC.  This got around the problem of long leads with the
>socket type decouplers.


Those are nice. Excellent caps, but putting a conventional cap at the GND
pin and routing the power to it first, then to the chip, actually uses the
trace inductance to help in the filtering :) At 300+ Mhz, it's significant,
and FCC scans up to 1GHz.

Still, If I HAD to use socket caps, that's what I'd use.

1999\01\18@112411 by Tom Handley

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  Mike, Q-Cap made those devices and they worked very well. They also
made bus strips and custom devices. It's been awhile. I have'nt followed
this thread very closely but now days, as speed increases, there is a
trend to use multiple low-ESR caps (ie: 10 0.01uf caps to make 1 0.1uf).
There was a good discussion of this in the comp.arch.fpga news group.

  - Tom

At 09:09 AM 1/18/99 -0000, Michael Rigby-Jones wrote:
{Quote hidden}

1999\01\18@120514 by Harold Hallikainen

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On Mon, 18 Jan 1999 09:09:25 -0000 Michael Rigby-Jones
<@spam@mrjonesKILLspamspamNORTELNETWORKS.COM> writes:
>> At 08:35 AM 1/17/99 -0500, ryan pogge wrote:
>> >you can buy sockets for your chips that have decoupling caps built
>in.
>>
>> Those are not nearly as nice as they look at first glance. Look at
>the
>> long
>> leads on the cap!
>>
>The last place I worked used some very strange decoupling caps.  They
>were
>fitted underneath the IC, without using a socket.  They were very thin
>with
>the same outline as the IC and thin pins that could be pushed through
>the
>same holes as the IC.  This got around the problem of long leads with
>the
>socket type decouplers.
>

       These are "micro-Q" capacitors, originally made by Rogers Corp.,
now called CCI at +1 602 967 0624.  The capacitors mount in the same
holes as the DIP chip or socket, making connections to the diagonal power
pins (such as 7 and 14).  My last order with them was for some 303A28 and
303A40 parts.  These are each 30nF, one for a 28 pin DIP, and the other
for a 40 pin DIP.  Cost was $1.16 for the first, $1.27 for the second.
       As Michael points out above, these parts get around lead
inductance and save board space.

Harold


Harold Hallikainen
KILLspamharoldKILLspamspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1999\01\18@120927 by Harrison Cooper

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here is another publication that might interest some.

EDN Oct 22, 1998 - "bypassing PC boards: Thumb your nose at rules of thumb"

1999\01\18@121339 by David W. Duley

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In a message dated 1/17/99 12:14:40 AM Pacific Standard Time, RemoveMEjones_glTakeThisOuTspamEFN.ORG
writes:

<<
Today as i was installing decoupling caps on a circuit board i thought to
myself: "Why dont the IC manufactures just incorperate a decoupling cap
right in the ic itself?" Any one have any ideas?
 >>

Hi All,
Since I am in the business of making machines that make Ceramic capacitors
I'll field this one.
One problem with capacitors is that the value of the capacitor is directly
related to its size.  Integrating a capacitor of sufficient value to act as a
bypass cap would require that the IC's package grow by quite a bit.. Open your
Digikey catalog and check the values VS Size.
There are IC sockets that have caps built into them as long as your chip uses
a standard Power/Gnd pin configuration (Typically the lower left pin for
ground and the upper right pin for VCC)
One of the things my company is pioneering is the use of very thin Dielectrics
(less than 5 micron) to make very small chip caps.  If you half the thickness
of the dielectric you increase capacitance by a factor of 4. You also reduce
the voltage tolerance.  Life is made of trade-offs.  We are starting to see
the use of 01-02 chip capacitors.  Thats 10 thousandths by 20 thousandths.
They look like pepper when sprinkled on a sheet of paper.  I'm not sure how
they handle them but thats not my department!

Dave Duley
Dreitek Inc.
http://www.dreitek.com

1999\01\18@125150 by Gabriel Gonzalez

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A cap requires a lot of space on an IC.

Gabriel

-----Original Message-----
From: Lynx {Glenn Jones} <spamBeGonejones_glspamBeGonespamEFN.ORG>
To: TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Date: Sunday, January 17, 1999 1:09 AM
Subject: [OT] A final question about decoupling caps.


>Today as i was installing decoupling caps on a circuit board i thought to
>myself: "Why dont the IC manufactures just incorperate a decoupling cap
>right in the ic itself?" Any one have any ideas?
>
>---------------------------------------------------------------------------
---
>A member of the PI-100 Club:
>3.1415926535897932384626433832795028841971693993751
>058209749445923078164062862089986280348253421170679

1999\01\18@132056 by John Payson

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|Today as i was installing decoupling caps on a circuit board i thought to
|myself: "Why dont the IC manufactures just incorperate a decoupling cap
|right in the ic itself?" Any one have any ideas?

Placing a cap on the die itself would be 'theoretically' possible,
but extremely expensive.  One factor which would be something of a
'killer' is that on most of the chip, one wants to minimize capac-
itive coupling between layers (since stray capacitance will slow
things down and increase power consumption).  Unfortunately, if the
chip-making process is optimized to minimize stray capacitance, that
will make it difficult to get much capacitance where it actually is
wanted.

A somewhat more plausible idea would be to incorporate a small cap
within the package, but have it external to the die.  For some app-
lications, this approach might make sense (putting multiple parts in
a package isn't all that hard as evidenced by the 12CExx parts) but
it would almost certainly cost more than an external cap and there
wouldn't generally be enough benefit to justify it.

While the need for bypass caps may be somewhat annoying, it's usual-
ly not that bad.  For proto-board projects, bypass caps can sometimes
be handy: if the board has power and ground rails running through the
center of the chip, placing the cap under the chip's socket allows
for very short lead length *AND* the cap leads can conveniently be
used to connect the power/ground rails to the PIC's power/ground pins.

1999\01\18@140929 by Gabriel Gonzalez

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Just be carefull when using low ESR caps, sometimes this can induce
oscillations.

In fact, sometimes it is necessary to add a small R to prevent this.

Gabriel


{Original Message removed}

1999\01\18@144036 by Gary Crowell

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Michael Rigby-Jones wrote:
> The last place I worked used some very strange decoupling caps.  They were
> fitted underneath the IC, without using a socket.  They were very thin with
> the same outline as the IC and thin pins that could be pushed through the
> same holes as the IC.  This got around the problem of long leads with the
> socket type decouplers.
>
> Regards
>
> Mike Rigby-Jones
> mrjonesEraseMEspam.....nortelnetworks.com


The company that does those is CCI: http://www.cci-msc.com/  They also
make some nice bus bars for board stiffening and power distribution.
Unfortunately the last time I looked at their Micro-Q caps (several
years ago), they were prohibitively expensive.  If you were to use more
than a few of them per board, it was more cost effective to add a
power/ground plane and a good bypassing structure to the pcb.  However
they are very good if you've already got a noisy design that has to be
fixed; less expensive (in the short term) than a redesign.


In response to other topics in this thread:

Its also important to remember that the clock frequency is usually not
the frequency that determines the design point for your power
distribution system.  The rule-of-thumb is:

design frequency = .3 / rise time

Where the frequency is in Hz, and "rise time" is the fastest switching
speed in use in your circuit, in seconds.  So at a rise time of 2.5ns
(not uncommon) the frequency you should be designing your bypassing for
is 120 MHz (even if your PIC is just ticking along at 4 MHz).  (Note
that the ".3" ranges between .3 and .5, depending upon whose thumb you
are using.)



The positioning of bypass capacitors is a popular subject, and a good
place for information on it is at Howard Johnson's web site:
http://signalintegrity.com , where the subject has been addressed
several times in his newsletter:

http://signalintegrity.com/news/1_6.htm
http://signalintegrity.com/news/2_3.htm
http://signalintegrity.com/news/2_1.htm
http://signalintegrity.com/news/2_26.htm

This reprint from PCB Design Mag is good too:
http://signalintegrity.com/articles/straight/bypass.htm


Note however, that I believe in all instances these discussions regard
pcb's with full power and ground planes; the recommendations there may
not be applicable to 2-layer, wire-wrap, or proto-board layouts.


This is also a popular subject on the signal integrity mail-list,
(subscription info at http://www.qsl.net/wb6tpu/si-list/info.html ), and
in its archives at http://www.qsl.net/wb6tpu/si-list/  (Threads on this
list are often addressed in Johnson's newsletter.)



Gary Crowell

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