[SI-LIST] Re: Overshoot/undershoot for IO buffer
- From: "Chris Padilla (cpad)" <cpad@xxxxxxxxx>
- Date: Mon, 3 Dec 2007 10:00:12 -0800
Dave,
Excellent idea! We use a lot of 10:1, 20:1 coax probes (on a
"tombstoned" 450/950-ohm 0402 resistor). In some cases, the coax seems
to be better shielded as it meanders around the board and out the front
to the scope but I like your comparison idea!
Cheers,
Chris Padilla
Cisco Systems
San Jose, CA
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of DAVID CUTHBERT
Sent: Monday, December 03, 2007 9:12 AM
To: doug@xxxxxxxxxx
Cc: weirsi@xxxxxxxxxx; shawn.h.zheng@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Overshoot/undershoot for IO buffer
Doug,
to acertain the fidelity of an active probe I parallel it with a
Tektronix 500 ohm passive probe. I figure the 500 ohm probe tells us the
truth and anything 'extra' from the active probe tells us about its
distortion. As you know the 500 ohm probe can provide lower loading than
the active probe but most people seem to be bothered by the idea of
placing a 500 ohm load across their measurement point. But they think
nothing of placing a 1 pF active probe across the same node.
Dave Cuthbert
On Dec 2, 2007 11:44 PM, Doug Smith <doug@xxxxxxxxxx> wrote:
>
> Hi Steve and the group,
>
> I wonder if the overshoot is really there? If so how much is really=20
> there andhow much is the probe?
>
> Most popular active probes whose designs are older than 5 years or so=20
> do not work very well and almost always have 100% or more excess gain=20
> in the upper octave of bandwidth with even very small probe tips. Only
> active probes (newer designs) with damping resistors can give an=20
> accurate picture of the waveform. Also some new active probes that try
> to remove probe loading from the scope display can do this also unless
> you meet the expected (by the
> probe) source impedance from the circuit the probe is connected to.
>
> What is the impedance level of the circuit? I like resistor divider=20
> probes for general purpose with the newer active probes where low DC=20
> loading is required. Even with the best active probes don't expect an=20
> input impedance ofmore than a few hundreds of Ohms in the upper octave
of bandwidth.
>
> Doug
>
> steve weir wrote:
> Shawn severe overshoot is never a good thing. The clamp diodes will=20
> try to protect the chip, but the large currents that result can cause=20
> soft error, and long term reliability problems. You will be well=20
> served to keep this problem under control by fixing the overshoot or=20
> at least limiting the current. Steve. Shawn Zheng wrote: Hi, I began=20
> to suspect something I did before wasn't right. For a simple topology=20
> of a driver and several receivers,I will add serial resistors if=20
> necessary to dampen the waveforms atreceivers. However,=20
> overshoot/undershoot at the driver's side was always ignored. I wonder
> if the driver's buffer is ok in this case when large overshoot or=20
> undershoot occurs at driver's pin or pad. Thanks. Shawn
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