[SI-LIST] Re: Measure ESD induced noise

Hi Jinhua,

It is better if you can show the plots and detailed settings of your
measurement.  Based on your descriptions, I think there are two things
you may want to consider in explaining these weird results:

1.  It is not true that cap impedance is always higher than short.  If
you model the short as pure inductance and capacitor as LC in series.
It is possible that at some frequency range, the impedance of capacitor
could be lower than "short".  By the way, you may want to include the
probe capacitance for more accurate model.

2. Typically true differential probe have a converter which has
relatively low bandwidth.  Because of bandwidth limitation, the output
signal could be distorted.

Best regards,

Zhiping

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of chen, jinhua
Sent: Friday, August 26, 2005 6:29 AM
To: 'Istvan Novak'
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Measure ESD induced noise

Istvan

I did those reference measurements. I also did the reference
measurements with a cap, which has the same size and value as the on
board decoupling caps. I expected the reference noise with cap should be
bigger than the reference noise with short. Because cap impedance is
always higher than short. But the result is opposite. Reference noise is
bigger with short. In general, both reference noises (background
readings) are much smaller than the real noise.

Thanks!

Jinhua

-----Original Message-----
From: Istvan Novak [mailto:istvan.novak@xxxxxxx]
Sent: Friday, August 26, 2005 8:48 AM
To: chen_jinhua@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Measure ESD induced noise

Jinhua,

There are reference measurements you can take: whether it is active
probe or coax, solder both pins (signal and
ground) to the ground point only.  With active probes, make sure the
position and orientation of probe head is the same that you have in
subsequent measurements.=20

You will find that active probes, passive probes and direct coax
connections alike pick up noise from the nearby noise sources to varying
degree.  You have to use the connection, which gives a background
reading comfortably lower than your measured data trace with your real
connections.

Regards

Istvan Novak
SUN MIcrosystems

chen, jinhua wrote:

>Hi, All
>
>I want to measure the ESD induced noise on the power and ground planes.
>First thing I need to make sure is that the probe itself did not cause=20
>any inaccuracy.
>
>Two different type of probes were used:
>1.) high impedance single ended FET probe. Signal pin solder to power=20
>side of decoupling cap, and ground pin soldered to ground side of cap.
>2.) high impedance differential FET probe. '+' pin solder to cap power,

>and '-' side solder to cap ground.
>
>I calibrated method 1 with a same size cap soldered on probe pins, but=20
>the cap was not mounted on the board. Probe was placed on the same=20
>point as
real
>measurement.=20
>
>Are those two methods are valid way to measure the ESD noise?
>
>The results from two methods had some differences. Method 1 measured=20
>bigger noise. The noise switching with 1 - 2 ns period and ~ 10 V swing

>in the beginning, then decay to zero in about 100 ns. Method 2 had=20
>similar period and 6 - 7 V swing, then decay to zero. But it had a wide

>pulse (50 + ns) after the high frequency switching noise. I don't know=20
>how to interpret the data.
>
>Thanks!
>
>Jinhua Chen
>EMC Corp.
> =20
>
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