[SI-LIST] Re: Overshoot/undershoot for IO buffer
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: doug@xxxxxxxxxx
- Date: Mon, 3 Dec 2007 10:11:58 -0700
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 there
> andhow much is the probe?
>
> Most popular active probes whose designs are older than 5 years or so do
> not
> work very well and almost always have 100% or more excess gain in the
> upper
> octave of bandwidth with even very small probe tips. Only active probes
> (newer designs) with damping resistors can give an 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 probes
> for general purpose with the newer active probes where low DC loading is
> required. Even with the best active probes don't expect an 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 try to
> protect the chip, but the large currents that result can cause soft error,
> and long term reliability problems. You will be well served to keep this
> problem under control by fixing the overshoot or at least limiting the
> current. Steve. Shawn Zheng wrote: Hi, I began to suspect something I did
> before wasn't right. For a simple topology of a driver and several
> receivers,I will add serial resistors if necessary to dampen the waveforms
> atreceivers. However, 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 undershoot occurs at driver's pin or pad. Thanks. Shawn
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