Re: [SI-LIST] : Re: Charge moving from decoupling capacitors

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From: Mike Saunders (mikesa@ia.nsc.com)
Date: Wed May 17 2000 - 12:30:30 PDT


Yu,

I disagree. There certainly do exist charge imbalances on an electrical
plane. I have attempted to attach a pdf (it may or may not come
through)
which shows simulation results, but similar results would occur if you
measured an actual PCB with a TDR. As the gates on the IC switch, the
instantaneous current requirements are first fulfilled by the on-chip
capacitance. This is next restored by the planes, then by any HF
decoupling, bulk decoupling and finally by the regulator itself. What
you
would see is that in the immediate area where the pin attaches through a
pad/via to the ground/power plane, there will be a sudden shifting of
charge on the plane itself in an attempt to restore the on-chip voltage
reservoir. Assumptions that the power or ground planes are always at a
constant potential can lead to erroneous behaviour on your circuit if
not
accounted for.

--Mike

At 07:45 AM 5/17/2000 -0700, you wrote:
>Hi, Barry,
>I think we all do agree the 1st point you said.
>But on the 2nd point, I have another opinion. Say, you
>are right,"When an electrical potential imbalance
>happens in a metallic plane, a current would flow on
>the plane for regaining the equi-potential."
>But we regard the metallic plane as a reference plane,
>then, theoretically, there is no an electrical
>potential imbalance happens in a metallic plane. In
>fact, we can never get a ideal reference plane(even
>the earth is not IDEAL). Based on this, I would say
>"yes" on your question "Does it need EM field support
>from the dielectric?". Still but, the need of the
>support is usually slight if the electical size of the
>metallic plane is big enough. because we know the
>metallic plane is a equi-potential plane.
>
>regards,
>Yu Wang
>
>--- Barry Ma <barry_ma@altavista.com> wrote:
>> Hi Andrew,
>>
>> You said: "It is just like an ordinary transmission
>> line such as stripline. "
>>
>> Please allow me to say something different.
>>
>> (1) When a signal propagates along a transmission
>> line, we could observe a current loop from source to
>> load through the transmission line. The signal
>> velocity is the same as the speed of light in the
>> dielectric. You are right.
>> (2) When an electrical potential imbalance happens
>> in a metallic plane, a current would flow on the
>> plane for regaining the equi-potential. This current
>> looks different from the signal current. There's no
>> current loop here. Does it need EM field support
>> from the dielectric? If not, should it have a
>> different velocity? That is my point. I have no
>> answer, and appreciate any input. Thanks.
>>
>> Bets Regards,
>> Barry Ma
>> bma@ANRITSU.com
>>
>> --------
>> Barry Ma wrote:
>> As the speed of digital signals gets faster and
>> faster, people begin being
>> concerned with the distance for electric charge to
>> move on power and
>> ground planes of multilayer PCB during the signal
>> rise time from a
>> decoupling capacitor (cap) to a chip it serves. I
>> would like to raise two
>> questions.
>>
>> (1) The charge is moving in a metalic plane, not
>> inside the dielectric
>> between pwr and gnd planes. Please let me know why
>> you have to use the
>> propagation velocity in the dielectric, instead of
>> that in the metal.
>> ------
>> Ingraham, Andrew wrote:
>>
>> The charge may be moving in the metal, but the
>> energy (which makes the
>> charge keep moving) is primarily in the
>> electro-magnetic field between the
>> planes, in the dielectric. The charge won't move
>> unless there is an E-M
>> field to push it.
>>
>> It is just like an ordinary transmission line such
>> as stripline. The
>> propagation velocity of a trace is that of the
>> dielectric, even though the
>> charge moves only in the metal trace and planes.
>>
>> (Edited by BM)
>>
>>
>
>
>
>=====
>Yu Wang, Ph.D
>U.T. MD Anderson Cancer Center
>1100 Holcombe Blvd., Box 217
>Houston, TX, 77030
>Tel:713-745-1671
>

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