[SI-LIST] Re: Overshoot/undershoot for IO buffer
- From: steve weir <weirsi@xxxxxxxxxx>
- To: doug@xxxxxxxxxx
- Date: Mon, 03 Dec 2007 04:09:58 -0800
Doug, it is a worthwhile question Shawn would have to answer for his
situation.
I agree that probes and high frequency signals can be a challenge.
While there are some active probes with damping resistors that get down
to about 0.3pF they look like the down payment on a nice car. I like to
avoid the issue by bringing out dedicated signals out to SMAs with
carefully designed launches. If we must look at a signal at a receiver,
I am with you on the 10X or 20X passive probes for the job. I find the
Tek probes are a little bit more elegant than the Agilent ones. The b/w
is wider, and the cables are definitely easier to manipulate. It
doesn't hurt that Tek only asks half the price as Agilent. The Agilent
probe has stood the test of time. It was designed back in the late 1980s.
Best Regards,
Steve.
Doug Smith wrote:
> Hi Steve and the group,
>
> I wonder if the overshoot is really there? If so how much is really
> there and how 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 of more 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 at receivers. 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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>>
>>
>>
>
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