[SI-LIST] Re: Overshoot/undershoot for IO buffer
- From: Doug Smith <doug@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Sun, 02 Dec 2007 22:44:23 -0800
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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