EP2S90F1020C5 Datasheet, PDF (740/768 Pages) Altera Corporation – Stratix II Device Handbook, Volume 1

Transmission Line Layout
Impedance Calculation
Any circuit trace on the PCB has characteristic impedance associated with
it. This impedance is dependent on the width (W) of the trace, the
thickness (T) of the trace, the dielectric constant of the material used, and
the height (H) between the trace and reference plane.
Microstrip Impedance
A circuit trace routed on an outside layer of the PCB with a reference
plane (GND or V_CC) below it, constitutes a microstrip layout. Use the
following microstrip impedance equation to calculate the impedance of a
microstrip trace layout:
( ) ^{Z0 =}
87
ln
ε_r + 1.41
5.98 × H _Ω
0.8W + T
Using typical values of W = 8 mil, H = 5 mil, T = 1.4 mil, the dielectric
constant, and (FR-4) = 4.1, with the microstrip impedance equation,
solving for microstrip impedance (Z_o) yields:
( ) ^{Z0 =}
87
ln
4.1 + 1.41
5.98 × (5)
Ω
0.8(8) + 1.4
Z₀ ~ 50 Ω
1 The measurement unit in the microstrip impedance equation is
mils (i.e., 1 mil = 0.001 inches). Also, copper (Cu) trace thickness
is usually measured in ounces for example, 1 oz = 1.4 mil).
Figure 11–3 shows microstrip trace impedance with changing trace width
(W), using the values in the microstrip impedance equation, keeping
dielectric height and trace thickness constant.
11–4
Stratix II Device Handbook, Volume 2
Altera Corporation
May 2007