Datasheet

SZZA016B
7–239 Basic Design Considerations for Backplanes
Optimum Termination Simulation
Figure 9 is the result of HSPICE simulation of the circuit in Figure 3, with 51 Ω used for the
pullup terminations (R
TT
) to 1.5 V. Figure 3 did not show the L-C-R values inherent in
transmission lines, but they are included in the HSPICE simulations. The transmitter (driver) is a
high-drive GTLP device operating at 50-MHz clock frequency. Because the device is operating
in the latched mode, the data signal shown is only one-half clock frequency, or 25 MHz.
Time – ns
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
–0.2
Voltage – V
Driver (Point A)
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Receiver (Point B)
Under-Terminated
Reflection Effect
Figure 9. Mismatched Line Termination
One signal in Figure 9 is the driver output at point A of the fully loaded transmission line, and the
other signal is the receiver input at point B of the fully loaded transmission line that is farthest
from the driver. The effects of the reflections due to termination mismatch can be seen clearly.
Figure 10 shows the same waveforms when the termination resistors are changed to the
calculated value of 24.2 Ω. The improvement in signal integrity is due to matching the
termination resistors to the loaded impedance of the stripline transmission line. The delay
between the two signals is measured at the 1-V threshold level for the GTLP device.
The HSPICE simulation produces the same flight time from point A to point B, as calculated
previously.