Capacitance of Two Parallel Plates

[file: capac.mcd]

Formulas included in this spreadsheet:

• Capacitance of two plates CPLATE()

• Impedance magnitude of capacitor at one frequency XCF()

• Impedance magnitude of capacitor as seen by rising edge XCR()

Variables used:

• w Width of plate overlap (in.)

• x Length of plate overlap (in.)

• h Height of one plate above the other (in.)

• er Relative dielectric constant of material between plates

Capacitance of two plates (F):

• A power and ground plane separated by 0.010 in. of FR-4 dielectric (er = 4.5) share a capacitance of
  100 pF/in.²

• Halving the separation doubles the capacitance.

Impedance magnitude of capacitor at frequency f (W):

• c Capacitance (F)

• f Frequency (Hz)

The impedance, at 100 MHz, of a 100-pF capacitor is 16 W.
 

Impedance magnitude of capacitor as seen by rising edge (W):

• c Capacitance (F)

• tr 10-90% rise time (s)

        



The impedance, as seen by a 5-ns rising edge of a 100-pF capacitor is 16 W.
 

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