[SI-LIST] : Transient impedance

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Mon Nov 15 1999 - 14:46:07 PST


Adrian Shiner wrote:
>
> If there are 2 active (cmos) devices in a push pull (totem pole) output
> stage then at some point both devices will be half way between on (low
> impedance) and off (very high impedance). The impedance must vary during the
> transition. Are you going to terminate both ends of the line which this
> device is driving so that the impedance variation is minimised? Does it
> matter? Perhaps we should return to current driven logic & stop worrying
> about the voltages???

As a general rule, we design output stages to keep the crowbar current low.
Break-before-make, as it were, although it's no exactly sequenced so much
as just turning off faster than on. Necessarily, though, the output goes
through a high-impedance point.

Compounding this is the fact that MOS devices aren't just modulated resistors.
If the gate-drain voltage is greater than about the threshold voltage, then
the output is resistive. Otherwise it's more or less a constant current, and
_that_ is certainly high impedance. As the driver turns on or off, you have
a situation where the gate-source voltage is decreasing while the drain-source
voltage is increasing -- the gate-source voltage drops REALLY fast, and the
transistor spends most of its transition time in high impedance.

Therefore, the high-impedance window for a driver is quite a bit longer than
the nonoverlap interval. It's very nearly the entire edge time for a shunt
terminated line, since it starts shortly after the first device begins to
turn off and only ends when the second finishes turning on.

> ----- Original Message -----
> From: Chuck Hill <chuckh@altaeng.com>
> To: <si-list@silab.eng.sun.com>; <si-list@silab.eng.sun.com>
> Sent: 15 November 1999 02:38
> Subject: RE: [SI-LIST] : What speed scope should I consider?
>
> > Arpad,
> >
> > I have a proprietary method of measuring the transient impedance. My
> > method is used to measure the output impedance of a buffer while switching
> > between high and low states. My measurements show that the output
> > impedance during buffer switching is often much higher than the on state
> > resistance.
> >
> >
> > Charles Hill, consultant
> >
> >
> >
> >
> > At 04:47 PM 11/12/99 -0800, Muranyi, Arpad wrote:
> > >I meant impedance vs. time, or non steady state but varying impedance,
> > >such as the impedance of a buffer turning on/off vs. time.
> > >
> > >Arpad
> >
> >==========================================================================
> > >
> > >-----Original Message-----
> > >From: Ron Miller [mailto:rmiller@Brocade.COM]
> > >Sent: Friday, November 12, 1999 3:54 PM
> > >To: si-list@silab.eng.sun.com
> > >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
> > >
> > >
> > >"Muranyi, Arpad" wrote:
> > >
> > >
> > >Does anyone know of any instrument that can measure transient impedance?
> > >
> > >Arpad Muranyi
> > >Intel Corporation
> > >========================================================================
> > >
> > >
> > >Quote from previous correspondence:
> > >
> > >
> > >"Network analyzers measure steady state impedance, not transient
> impedance."
> > >
> > >
> > >
> > >**** To unsubscribe from si-list: send e-mail to
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> ****
> > >
> > >--
> > >
> > >Ronald B. Miller _\\|//_ Signal Integrity Engineer
> > >
> > >(408)487-8017 (' 0-0 ') fax(408)487-8017
> > >
> > > ==========0000-(_)0000===========
> > >
> > >Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA
> 95131
> > >
> > >rmiller@brocade.com, rbmiller@sjm.infi.net
> > >
> > >
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-- 
D. C. Sessions
dc.sessions@vlsi.com

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