Re: [SI-LIST] : ESR and bypass caps

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From: Douglas C. Smith (doug@dsmith.org)
Date: Sat Feb 05 2000 - 06:15:57 PST


Hi All,

I think the simulation below is missing some key ingredients, the
variable
time delay between the capacitors representing their spacing on the
board
and loading of the devices. I see below caps connected between vdd and
0.
I have observed that devices lower the Q of the whole structure.

The ground and power planes look to me like a 2 dimensional
transmission
line with random hi-Z and lo-Z connections. A simple simulation of
caps
in parallel may not give a useful answer unless the caps are right
next
to each other. I would like to see the spice model of the 2
dimensional
transmission line modeled to a few GHz. (lots of elements!!)

DougS

Doug Brooks wrote:
>
> >
> >Then we have to space capacitors close together in value. This
> >prevents the parallel resonance from getting too high. The attached
> >spice deck and .pdf file shows two combinations of capacitors. The
> >first set has 5 closely spaced values and the second set has just two
> >values. Inductance is 1nH and ESR is 100mOhms:
> >
> >xcap2 vdd 0 cap C=1.0n R=100m L=1n $npo
> >xcap3 vdd 0 cap C=1.5n R=100m L=1n $npo
> >xcap4 vdd 0 cap C=2.2n R=100m L=1n $npo
> >xcap5 vdd 0 cap C=3.3n R=100m L=1n $npo
> >xcap6 vdd 0 cap C=4.7n R=100m L=1n $npo
> >
> >...and...
> >
> >xcap2 vdd 0 cap C=1.0n R=100m L=1n $npo
> >xcap3 vdd 0 cap C=1.0n R=100m L=1n $npo
> >xcap4 vdd 0 cap C=1.0n R=100m L=1n $npo
> >xcap5 vdd 0 cap C=4.7n R=100m L=1n $npo
> >xcap6 vdd 0 cap C=4.7n R=100m L=1n $npo
> >
> > >From the spice output you can readily see the parallel resonance and
> >how dangerous it is when capacitors are not spaced closely in value.
> >(One ac amp has been forced into the parallel circuit, so volts is
> >interpreted as magnitude of impedance.) With the 5 different valued
> >capacitors, we see 4 parallel 'antiresonances', growing with increasing
> >frequency. The growth is because Q increases as omega increases. The
> >higher the resonant frequency, the more important it is to have low
> >inductance pads. The 5 capacitor-value case has a maximum impedance of
> >about 150 mOhm, but the 2 capacitor-value case has a maximum of about
> >600 mOhms. That is enough difference to make a product pass or fail.
> >
> >With 5 capacitors, we have made a nice flat 100 mOhm impedance
> >between 60MHz and 200Mhz. With 50 capacitors, we could have made a
> >10 mOhm impedance between those frequencies. If only we could get a
> >bunch of 100 mOhm capacitors...
> >
> >Members of the SI community should feel free to take this generic
> >spice deck and run it. Play around with ESR and inductance. You
> >will quickly see the value of using low ESR capacitors on low
> >inductance pads in managing a low impedance power system across
> >a broad frequency range. I believe the same results will be obtained
> >with Doug Brooks' simulator. Doug, please let us know.
>
> My pleasure. Here is what our little calculator would come up with: (Just
> like you said.)
>
> Case 1:
>
> 2/4/2000
> 4:26:12 PM
> Initial Conditions
> Input filename = C:\POLEZERO\LSMITH1.TXT
> Output filename = C:\POLEZERO\LSMITHO1.TXT
> Number of Capacitance Values = 5
> Total Capacitance = .0127
>
> Number L nH C
> uF R Resonant F (MHz)
> 1 01.00000 .001000 .1
> 159.155
> 1 01.00000 .001500 .1
> 129.949
> 1 01.00000 .002200 .1
> 107.302
> 1 01.00000 .003300 .1
> 87.612
> 1 01.00000 .004700 .1
> 73.413
>
> Frequency (MHz) Impedance Turn PhaseAngle(Deg)
> 72.023 .0675823 Min -36.5026
> 79.716 .0789909 Max -20.0925
> 86.671 .0699806 Min -7.6151
> 97.674 .1031926 Max 2.0211
> 107.66 .0733426 Min 10.7
> 119.99 .1153441 Max 18.4003
> 131.31 .0726022 Min 26.9647
> 148.33 .1355461 Max 34.9443
> 161.9 .0699509 Min 43.9007
> 4:27:00 PM
>
> Case 2:
>
> 2/4/2000
> 4:28:38 PM
> Initial Conditions
> Input filename = C:\POLEZERO\LSMITH2.TXT
> Output filename = C:\POLEZERO\LSMITHO2.TXT
> Number of Capacitance Values = 2
> Total Capacitance = .0124
>
> Number L nH C
> uF R Resonant F (MHz)
> 3 01.00000 .001000 .1
> 159.155
> 2 01.00000 .004700 .1
> 73.413
>
> Frequency (MHz) Impedance Turn PhaseAngle(Deg)
> 72.771 .0493910 Min -9.489
> 115.13 .5864965 Max -1.0232
> 159.78 .0327716 Min 9.0943
> 4:29:07 PM
>
> .
> ************************************************************
> See our updated message re in-house seminars on our web page
> .
> Doug Brooks, President doug@eskimo.com
> UltraCAD Design, Inc. http://www.ultracad.com
>
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-- 
-----------------------------------------------------------
    ___          _           Doug Smith
     \          / )          P.O. Box 1457
      =========              Los Gatos, CA 95031-1457
   _ / \     / \ _           TEL/FAX: 408-356-4186/358-3799
 /  /\  \ ] /  /\  \         Mobile:  408-858-4528
|  q-----( )  |  o  |        Email:   doug@dsmith.org
 \ _ /    ]    \ _ /         Website: http://www.dsmith.org
-----------------------------------------------------------

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