(Fwd) RE: [SI-LIST] : IC input impedance

Pat Zabinski (zabinski.patrick@mayo.edu)
Tue, 13 Oct 1998 13:49:49 -0500

I believe the formula below (R+1/j*w*c) is correct for a series
configuration, but isn't it true that the input resistance
and capacitance are both in reference to ground, such that a
parallel configuration/formula would be more appropriate?

If so, then the formula would look like:

Z = R || 1/jwC
= R || -jX
where X = -1/jwC

= (R * -jX) / (R - jX)
= (-jRX) * (R + jX) / (R^2 + X^2)
= (R*X^2 - jX*R^2)/(R^2 + X^2)
= RX(X - jR)/(R^2 + X^2)
= RX/(X + jR)
= [R/wC]/(1/wC + jR)

(feel free to check my math)

If the frequency is very low (i.e., 1/wC >> R), then the formula
can be estimated at:

Z = [R/wC]/(1/wC)
= R

Which would agree with the concept that capacitance has little
effect at low frequencies.

At high frequencies (i.e., 1/wC << R), then the formula can be estimated
at:

Z = [R/wC]/(jR)
= -j/wC

Again, this agrees with the idea that the capacitance dominates at
high frequencies.

Given an input capacitance and an input resistance, I believe
this configuration/formula would better match the measured
impedance. Comments?

Pat

> 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
> dima@tdasystems.com
> http://www.tdasystems.com
>
>
> >
> > 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
> >

-- 
  Pat Zabinski                                     ph: 507-284-5936
  Mayo Foundation                                  fx: 507-284-9171
  200 First Street SW                     zabinski.patrick@mayo.edu
  Rochester, MN 55905       www.mayo.edu/sppdg/sppdg_home_page.html

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