Re: [SI-LIST] : voltage measurements using a current probe

Pat Zabinski (zabinski.patrick@mayo.edu)
Thu, 9 Sep 1999 07:51:29 -0500

Doug,

I checked this out and a previous "tidbit" on your wed site, and
it got me thinking about a problem I've got right now in the
lab.

I'm experimenting with some power delivery issues, where I
have a controllable (both output impedance and risetime)
SSN chip attached to a two-layer board. The hope in this
experiment is to gain insight in power delivery (i.e., charge
storage) to buffers, particularly during logic transitions.

Right now, we are measuring the VDD and VSS currents to/from
the SSN chip by placing a 1 ohm resistor in series with
the supplies, then measuring the voltage across the resistor
with a 10X (500 ohm) probe. However, the resistor adds
some unwanted parasistics (particularly inductance), and I'm
looking for a way to get around this.

Have you ever seen an "in-situ" current probe in a PCB? Meaning,
is it possible/practical to use multi-layer routing to essentially
create a current probe within the board?

My board has VSS (Ground) on the top layer, and VDD is on
the bottom layer. VDD comes up to the chip with a via. To
measure the VDD current, I'd route a track around the VDD
supply via on an inner layer. I'd then measure the
current using a standard voltage probe across the ends of
this track. For VSS, I'd use jumpers, vias, and inner layer
routes to create the same loop around the VSS supply line/pin.

Will this sort of in-situ current probe work? If so, what
sort of problems do you expect I will run into?

Thanks,
Pat

> Hi All,
>
> I thought some of you might find my technical article of the month
> interesting. It is on making voltage measurements with a current
> probe. Over much of the useful frequency range of a current probe, its
> output is not the current in the wire but rather the voltage drop
> along the wire per unit length. The waveshape of the probe output is
> that of the voltage drop along the wire (di(i)/dt), not the current in
> the wire (i(t))!
>
> If this sounds interesting, click on the current probe picture at the
> bottom of the index page on my website at http://emcesd.com for a
> technical discussion along these lines.
>
> Doug
>

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