[SI-LIST] Re: How to measure voltage drop on plane
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 19 Jul 2004 08:54:36 -0700
Zhangkun-
The measurement of power/ground noise is a differential measurement
(even if you use a single-ended probe) of the power voltage (Vdd) with
respect to the associated ground voltage (Gnd).
When making these sort of measurements you need to measure Vdd with
respect Gnd at the same XY spatial location for the measurement to make
sense. (i.e. vertically coincident). If you measure Vdd with respect to
Gnd where the Gnd node is physically offset some significant distance
(wrt lambda) then the measurement just doesn't make sense.
If the Gnd measurement point is offset from the Vdd measurement point by
only a small fraction of an inch instead exactly coincident with the
measured Vdd location that is probably an acceptably small error,
however if the offset is on the order of inches or more then the
measured voltage doesn't make sense. Why? Because you are measuring
voltages 'across time' where the time is the time-of-flight from the Gnd
node coincident with the Vdd node (which you should be using as a
reference) to the Gnd node which is distant (which you should NOT be
using as a reference). Measurements of voltage across time are
undefined, at least with respect to the measurement of power/ground
noise which you propose.
-Ray Anderson
Zhangkun wrote:
>Dear all
>The power ground noise at one location is easy to measure. How to measure
>voltage drop on plane at different location? Is it impossible? There is one
>method proposed by C. R. Paul. However, the structure of Paul's is too simple.
>
>Best Regards
>
>Zhangkun
>2004.7.19
>
>
>
.
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