EP2S130F1020I4 Datasheet, PDF (755/768 Pages) List of Unclassifed Manufacturers – Stratix II Device Handbook, Volume 1

Termination
Schemes
High-Speed Board Layout Guidelines
■ Ensure that D > 2S to minimize the crosstalk between the two
differential pairs.
■ Place the differential traces S = 3H as they leave the device to
minimize reflection noise.
■ Keep the length of the two differential traces the same to minimize
the skew and phase difference.
■ Avoid using multiple vias because they can cause impedance
mismatch and inductance.
Mismatched impedance causes signals to reflect back and forth along the
lines, which causes ringing at the load receiver. The ringing reduces the
dynamic range of the receiver and can cause false triggering. To eliminate
reflections, the impedance of the source (Z_S) must equal the impedance of
the trace (Z_o), as well as the impedance of the load (Z_L). This section
discusses the following signal termination schemes:
■ Simple parallel termination
■ Thevenin parallel termination
■ Active parallel termination
■ Series-RC parallel termination
■ Series termination
■ Differential pair termination
Simple Parallel Termination
In a simple parallel termination scheme, the termination resistor (R_T) is
equal to the line impedance. Place the R_T as close to the load as possible
to be efficient (see Figure 11–22).
Figure 11–22. Simple Parallel Termination
Stub
S
Zo = 50 Ω
L
RT = Zo
S = Source
L = Load
Altera Corporation
May 2007
The stub length from the R_T to the receiver pin and pads should be as
small as possible. A long stub length causes reflections from the receiver
pads, resulting in signal degradation. If your design requires a long
termination line between the terminator and receiver, the placement of
the resistor becomes important. For long termination line lengths, use
fly-by termination (see Figure 11–23).
11–19
Stratix II Device Handbook, Volume 2