# Re: [SI-LIST] : IC input impedance

Dima Smolyansky ([email protected])
Wed, 14 Oct 1998 11:16:11 -0700

I think we got a bit distracted from the original topic, which was: how do we determine the reflection coefficient of the CMOS driver for a digital signal?

Now, we are addressing a topic: how high of a bandwidth does one need to propagate digital signals without distortion? I believe the asnwer is determined by the larger of the 5-6th harmonic of the clock or about three times the 3dB
bandwidth as determined by the rise time using F(bw)=0.35/tr. The reason it is necessary to use about 3X the 3dB rise time bandwidht is that otherwise you will still see some signal distortion, while 3X the rise time bandwidth will
give you completely distortionless signal propagation.

An excellent picture illustrating these concepts, which shows the spectrum of a digital signal in relation to its rise time and clock, is given on page 3 of Howard Johnson's book "High-speed Digital Design." Tektronix also
recommends to have measurement probes that are about 3X the 3dB rise time bandwidth.

Thanks,

-Dima

=========================
Dima Smolyansky
TDA Systems, Inc.
7465 SW Elmwood St.
Portland OR 97223
503-977-3629
503-245-5684 (FAX)
[email protected]
http://www.tdasystems.com

Dennis Tomlinson wrote:

> Marin Sampaleanu wrote:
> >
> > The frequeny to be used is the highest significant component in the clock
> > signal FFT.
> > As a rule of thumb the fifth harminic would be a good choice.
> >
> > Any comments ?
>
> I was once burned by an 8 KHz clock which had 1 ns risetimes. The culprit
> wasn't a 40 kHz harmonic, but the energy up around 350 MHz.
>
> I use the rule of thumb arrived at by driving a step input to a simple first
> order R-C low pass filter whose bandwidth is:
>
> B = 1/(2*Pi*R*C)
>
> When the filter's output signal 10% -> 90% risetime (tr) is multiplied by the
> bandwidth yields:
>
> tr*B ~= 0.35
>
> >From which: B = 0.35/tr (that's where my 350 MHz came from).
>
> Note: I didn't invent this rule of thumb. I first saw it years ago on some
> Tektronix literature for a 10 MHz scope. I've also heard it from several
> other's over the years.
>
> Any other comments?
>
> Cheers,
>
> Dennis
>
> >
> > Marin Sampaleanu
> > Nortel, Wireless Development Calgary.
> > > ----------
> > > From: Dima Smolyansky[SMTP:[email protected]]
> > > Sent: Tuesday, October 13, 1998 9:18 AM
> > > To: '[email protected]'
> > > Cc: [email protected]
> > > Subject: RE: [SI-LIST] : IC input impedance
> > >
> > > I would guess, the main harmonic of the clock, as it is at this frequency
> > > that you will get the most amplitude loss.
> > >
> > > BTW, if you estimate the R in the equation I proposed, it may swarm the
> > > value of 1/jwc in the overall impedance and make it negligible.
> > >
> > > Anybody, any other good ideas?
> > >
> > > Thanks,
> > >
> > > -Dima
> > > ============================
> > > Dima Smolyansky
> > > Marketing and System Development
> > > TDA Systems, Inc.
> > > 7465 SW Elmwood St.
> > > Portland, OR 97223
> > > (503) 977-3629
> > > (503) 245-5684
> > > [email protected]
> > > http://www.tdasystems.com
> > >
> > >
> > > -----Original Message-----
> > > From: [email protected] [SMTP:[email protected]]
> > > Sent: Tuesday, October 13, 1998 6:52 AM
> > > To: Dima Smolyansky
> > > Subject: RE: [SI-LIST] : IC input impedance
> > >
> > > what is the relevant frequency w of j*w*c?
> > >
> > >
> > >
> > >
> > >
> > > Dima Smolyansky <[email protected]> on 10/13/98 05:44:33 PM
> > >
> > > To: "'[email protected]'" <[email protected]>,
> > > "[email protected]" <[email protected]>
> > > cc: (bcc: Shimon Turgeman/AME/APPLIED MATERIALS)
> > > Subject: RE: [SI-LIST] : IC input impedance
> > >
> > >
> > >
> > >
> > > I suppose, R+1/j*w*c?
> > >
> > > Thanks,
> > >
> > > -Dima
> > > ============================
> > > Dima Smolyansky
> > > TDA Systems, Inc.
> > > 7465 SW Elmwood St.
> > > Portland, OR 97223
> > > (503) 977-3629
> > > (503) 245-5684
> > > [email protected]
> > > http://www.tdasystems.com
> > >
> > >
> > > -----Original Message-----
> > > From: [email protected] [SMTP:[email protected]]
> > > Sent: Tuesday, October 13, 1998 12:55 AM
> > > To: [email protected]
> > > Subject: [SI-LIST] : IC input impedance
> > >
> > > what is the input impedance of CMOS devices (for calculating the
> > > reflection
> > > coff.)as a function of input capacitance ? how I calculate this impedance
> > > ?
> > > what is the formula ?
> > >
> > > Best Regards
> > > Shimon Turgeman
> > > Applied Materials
> > >
> > >
> > >
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