[SI-LIST] Re: Measuring Power supply noise

Measuring power supply noise is one of the toughest challenges out =
there. I get so many questions on this I'm thinking of writing an =
article or app note on this.

If you can use a probe, definitely use a differential probe for all the =
reasons noted.=20

Unfortunately even the lowest-noise scope probes available may have too =
much noise to measure the noise levels you need to measure. For example =
the Agilent 1168A and 1169A probes contribute only 2.5 mV RMS noise, but =
translated to p-p (whatever that is) you may well see 12-14X that much. =
Theoretically a variable with a Gaussian pdf is unbounded, or course, so =
it's much more rational to talk RMS.=20

The good news is that normally power supplies have sufficient oomph to =
drive 50 ohms, so you can often get by with just using a piece of good =
50 ohm coax as a "probe." If the power supply won't drive 50 ohms dc, or =
if the scope doesn't have sufficient offset to bring the signal to =
center screen, you can do as someone earlier suggested and solder a =
blocking cap either to your PC board or to the end of the coax. Be sure =
to select a large enough blocking cap that you don't lose significant =
low-frequency constituents of the noise, as the R part of the resultant =
RC high-pass filter will be 50 ohms.=20

I've seen people very successfully trim a piece of semi-rigid coax to =
make a 50-ohm "probe." Trim back the shield and insulator until just a =
very short piece of center conductor is exposed. Then solder a short =
piece of wire (length to match the center conductor end) to the shield =
to act as a ground.=20

There is an article on this topic on the Intel web site that is a good =
introduction to the things you have to consider: =
http://developer.intel.com/technology/itj/2005/volume09issue04/art02_powe=
rdelivery/p03_powerdelivery.htm

Art Porter
Agilent Technologies=20

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Oscar Lang
Sent: Monday, March 20, 2006 10:28 AM
To: David Instone; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Measuring Power supply noise


Well I'm trying not to use all these passives and just
ONE ADC chip to measure noise.=20

Again original question remains, Any critic to this
idea?

--- David Instone <dave.instone@xxxxxxxxxx> wrote:


---------------------------------
  Whats inside a modern scope?

Oscar Lang wrote: =20
Has anyone tried a ADC (analog to digital converter)to
measure this noise? They come with
finegranularity.Then one needs to allocate a clean /
quiet ground andmeasure the noisy ground.Any critic to
this idea?--- Andrew Ingraham <a.ingraham@xxxxxxxx>
wrote: =20
           =20
a)       By using a 50&Ugrave; probe, did I create a =20
  =20
       =20
voltage divider?Adding a probe of any kind (except
non-contactoptical, perhaps) alwaysaffects the
circuit.  There is always some loading(voltage
divider), andother things that affect what you see.  =20

         =20
To check the sanity of my measurement, Instead of    =20

       =20
my 50&Ugrave; probe, I used a   =20
         =20
standard 1M&Ugrave; probe and carried out the     =20
       =20
measurement.Check the sanity check experiment by
connecting bothprobes at the sametime.You could also
try experimenting with a passiveprobe by inserting a
smallseries resistor between the pad and your coax
cable. For example, 450 ohmsgives you a pretty good
10:1 probe with 10 times theinput impedance, ifyou're
careful with the layout.
http://www.emcesd.com/1ghzprob.htm andother places
have tips on this.Use the scope (e.g., bandwidth
controls) to see whatkind of noise you aregetting.Be
careful about the maximum input voltage at thescope.=20
Some are rated forless than 2V before damage may
occur.Regards,Andy   =20
   =20
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