# RE: [SI-LIST] : What speed scope should I consider? / "transient impedance"

From: Jeremy Plunkett ([email protected])
Date: Mon Nov 15 1999 - 13:57:37 PST

hmm..er..oops, thank you for bringing that up Mike.

I was measuring transmission lines with the assumption that the actual Z
would not vary with respect to I. To use the same technique for measuring
IO cells (where you can safely assume that the impedance will be nonlinear),
your desired impedance can be measured as dV/dI, if you assume that all
parasitic capacitance and inductance tend to really screw up your results).

In the scope the actual calculation you set up is dV/dt / dI/dt. And when
you are doing this make sure you have set the scale on the inputs to get
good dynamic range without exceeding the boundaries of the screen (on an HP
Infinium), or you may find some very confusing numerical errors in the

Jeremy

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Mike Degerstrom
Sent: Monday, November 15, 1999 6:34 AM
To: [email protected]
Subject: Re: [SI-LIST] : What speed scope should I consider? /
"transient impedance"

Jeremy,

You mention 'intantaneous Z=V/I'. Is this or should this really be
Z(I)=dV/dI?

Mike

On Nov 12, 7:36pm, Jeremy Plunkett wrote:
> Subject: RE: [SI-LIST] : What speed scope should I consider? / "transient
>
> A technique I've used is to measure the current using a differential probe
> across a resistor in series with the DUT, and using another probe measure
> the voltage on the pin relative to ground or vdd. You can then calculate
> instantaneous Z = V/I using algebraic functions in the scope. The
Tektronix
> probes I used had a specified bandwidth of 1.5GHz; if you needed to you
> might be able to set up a 3 or 4-channel TDR(i.e. 20GHz class scope w/
> passive probes) in one chassis to get a measurement out to much higher
> frequencies (2 channels are required to get the diff. meas).
>
> Some difficulties of this method are:
> 1) Any measurement of transient/instantaneous impedance will be affected
by
> parasitic capacitance and inductance in your measurement setup (including
> probe input capacitance).
> 2) Buffer behavior may be dependent on the load seen at the pin; if it's
> practical you may want to take measurements with several different load
> values, both resistive and capacitive.
> 3) The measurement described above includes the impedance of the IC
package
> power and ground connections in the results, therefore it will also be
> affected by the number of buffers switching, supply bypassing on the
board,
> etc.
>
> Hope you find this useful!
>
> Jeremy Plunkett
> Signal Integrity Engineer
> Reliance Computer Corp.
> [email protected]
>
>
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Muranyi, Arpad
> Sent: Friday, November 12, 1999 4:47 PM
> To: '[email protected]'
> Subject: RE: [SI-LIST] : What speed scope should I consider?
>
>
> I meant impedance vs. time, or non steady state but varying impedance,
> such as the impedance of a buffer turning on/off vs. time.
>
> ==========================================================================
>
> -----Original Message-----
> From: Ron Miller [mailto:[email protected]]
> Sent: Friday, November 12, 1999 3:54 PM
> To: [email protected]
> Subject: Re: [SI-LIST] : What speed scope should I consider?
>
>
> Try Tektronix 11801 or a similar sampling scope from HP with the
> TDR option.
>
> Ron Miller
>
>
>
>
> Does anyone know of any instrument that can measure transient impedance?
>
> Intel Corporation
> ========================================================================
>
>
> Quote from previous correspondence:
>
>
> "Network analyzers measure steady state impedance, not transient
impedance."
>
>
>
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> --
>
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
>
> (408)487-8017 (' 0-0 ') fax(408)487-8017
>
> ==========0000-(_)0000===========
>
95131
>
> [email protected], [email protected]
>
>
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>-- End of excerpt from Jeremy Plunkett

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