This chapter is from the book
Formula for Capacitance
The formula for capacitance is fairly straightforward (see Equation 3-10). It is purely a function of the geometry of the plates and the material between the plates. Some capacitors (e.g., ceramic ones) are made up of several layers of plates, and the n – 1 term accounts for this.
where:
εr |
= relative dielectric coefficient (air = 1, FR4 ≅ 4) |
A |
= area of each plate (in2) |
d |
= distance between plates (in) |
n |
= number of plates |
Applying this formula to a PCB made from FR4 with εr = 4, 3-inches square, and with power and ground planes spaced 10 mils apart would lead to a capacitance of