Re: [SI-LIST] : IC input impedance

Dennis Tomlinson (
Wed, 14 Oct 1998 11:55:45 -0500

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