[SI-LIST] Re: Measure ESD induced noise

David

I may did not state my problem clear. The ESD source is the ESD gun (ESD
simulator). ESD gun touched front bezel of the board. It caused link errors
in some high speed links. I try to figure out if the ESD noise get into the
signals, or the power plane. The signals were clean. But the power plane is
very noise. 

You are correct. " noise on power and ground planes"? Means the voltage
between power and ground planes.

I will try the semi-rigid coax, and compare the data with FET probes. If all
probes are carefully attached, they all should gave the same results, right?

Thanks!

Jinhua

-----Original Message-----
From: Pommerenke, David [mailto:davidjp@xxxxxxx] 
Sent: Thursday, August 25, 2005 7:03 PM
To: chen_jinhua@xxxxxxx; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Measure ESD induced noise

Jinhua Chen,

It is quite difficult to measure voltages and currents that are caused
by ESD. The main reason is the large common mode injection into the
system caused by the ESD. It is not easy to suppress them.

I have performed these measurements by a couple of methods:

  a) Optical. Convert the voltage using a small E/O converter into
     an optical signal. Then the main problem is to capture the voltage
     or current, in such a way that the current density caused by the
ESD
     will not introduce a signal.
     Usually a good "null-experiment" will tell you if this is the case
or 
     not. I an "null-experiment" you would connect both inputs of the
E/O
     converter with each other and to ground. You should not see a
signa.
  
     We have some small, remotely powered Electrical/Optical probes for
this 
     purpose.

  b) Electrical. You connect a semi-rigid cable to the ground of your
system
     and the inner conductor to the net you want to measure. If the net 
     cannot be loaded with 50 Ohm (from the scope at the other end),
then
     you insert a very small 500 Ohm resistor in series. This will give
you
     about a 1:11 voltage divider for the scope and a 550 Ohm loading 
     on the net. Most nets can handle this.

     Now you place many, many ferrites on the semi-rigid cable.

     I also place the scope into a portable shielded enclosure to avoid
     direct coupling into the scope. The semi-rigid cable will be
passing
     into this enclosure using a well grounded SMA-barrel

  c) Localizing the injection of the ESD. If you do not inject the ESD
     current into the system globally, but if you inject an ESD like
     current density locally into the system, then you do not have the 
     problem of the large common mode injection, measurements are 
     not as difficult.

The method you propose might work, but I have never been successful in
using
differential probes when applying ESD to the system. The common mode
rejection is usually not good enough, the loop areas formed at the tips
are too large and the coupling through the coax cables that connect the
probes to the scope might be a problem. But, of course, I have not done
your measurement and it might work well.

On another topic: What do you mean by " noise onpower and ground planes
"?

Is this a voltage between power and ground? I would be surprised if ESD
can 
change this directly, as the capacitances are too large (on a PCB), the
amount of charge of the ESD is too low to change the voltage a lot.

ESD might cause a latch-up, that leads to a large current consumption,
leading to a large voltage change. Or, ESD might couple into the
feed-back loop of a power supply causing a large voltage fluctuation as
the feed-back gets confused.

Is the problem you are trying to solve a system crash caused by an ESD
test?
If so, there are other methods for determining the root cause.

Regards,
   David Pommerenke



   Associate Professor 
   EMC laboratory 
   University Missouri Rolla
   



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of chen, jinhua
Sent: Thursday, August 25, 2005 4:49 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Measure ESD induced noise

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
any
inaccuracy.

Two different type of probes were used:
1.) high impedance single ended FET probe. Signal pin solder to power
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
the
cap was not mounted on the board. Probe was placed on the same point as
real
measurement. 

Are those two methods are valid way to measure the ESD noise?

The results from two methods had some differences. Method 1 measured
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 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 how to interpret
the
data.

Thanks!

Jinhua Chen
EMC Corp.
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