[SI-LIST] Re: Measuring Power supply Ripple and Noise
- From: "Santos Fernandez, Jesus" <jsantosfernandez@xxxxxxxx>
- To: "Richard Feldman" <Richard.Feldman@xxxxxxxxxxx>, "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>, "Jaison Fernandez" <jaisonf@xxxxxxxxxxxxxxx>
- Date: Fri, 14 Nov 2008 09:09:12 +0100
Hello,
From the answers to Jaison, it seems to me that everyone has it's own little
tricks and arts. I'm surprised that there is not a single reliable method.
Some time ago, I used a (expensive) 3GHz Agilent scope with diff and single
ended probes and, I think it was OK; the results matched the simulations quite
well.
My question is: Is there a single indisputably reliable method?
J Santos
Jesús Santos Fernández
Laboratorio de EMC
-----Mensaje original-----
De: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] En
nombre de Richard Feldman
Enviado el: jueves, 13 de noviembre de 2008 21:37
Para: DAVID CUTHBERT; Jaison Fernandez
CC: si-list@xxxxxxxxxxxxx
Asunto: [SI-LIST] Re: Measuring Power supply Ripple and Noise
Jaison,
As others have mentioned, it's handy to use thin coaxial cable soldered
directly
to power & ground opposite the chip of interest. Some caveats:
1. Without 50 ohm termination, ripple voltage at cable-resonant frequencies
will be hugely exaggerated.
2. In many scopes with 1-megohm inputs, the AC-coupling time constant is
reduced to a few microseconds when you also select 50 ohm termination. That
can mask important low-frequency details, e.g. in voltage regulator step
response. If you need AC coupling, use external or no termination (or a very
large external DC-blocking capacitor) when looking at low frequencies.
3. When you probe across the terminals of a mounted chip capacitor, the
response can be significantly influenced by the RLC of that specific capacitor,
whose mounting inductance is shared with your probe. Consider removing one
capacitor if necessary to get suitable vias where you want to probe.
Rich F.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of DAVID CUTHBERT
Sent: Thursday, November 13, 2008 11:43 AM
To: Jaison Fernandez
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Measuring Power supply Ripple and Noise
Jaiso,
soldering a coaxial cable directly to the board works very well. The
common-mode noise rejection is good. Set the 'scope to 50 ohms.
Power supply boards commonly have CM noise on them and this is a real
problem with single-ended X10 probes. An active differential probe can work
well.
Dave Cuthbert
On Wed, Nov 12, 2008 at 11:46 PM, Jaison Fernandez
<jaisonf@xxxxxxxxxxxxxxx>wrote:
> Hai all,
> I am trying to characterize the power integrity of of a board (that having
> a
> system clock running at 400Mhz and DDR memory)
> What is the type of probe (Single ended or differential) is best suited for
> measuring power supply ripple and noise in the board.(I am using Tek GHz
> Oscilloscope for this purpose)
> Can any one give any document regarding measuring power integrity of a
> board.
>
>
>
>
> With Warm Regards,
>
> JAISON FERNANDEZ
>
>
>
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