July 2005
rev 0.4
ASM2I99446
APPLICATIONS INFORMATION
impedance mismatch seen looking into the driver. The
parallel combination of the 36ꢀ series resistor plus the
output impedance does not match the parallel
combination of the line impedances. The voltage wave
launched down the two lines will equal:
Driving Transmission Lines
The ASM2I99446 clock driver was designed to drive high
speed signals in
a
terminated transmission line
environment. To provide the optimum flexibility to the
user the output drivers were designed to exhibit the
lowest impedance possible. With an output impedance of
less than 20ꢀ the drivers can drive either parallel or
series terminated transmission lines. In most high
performance clock networks point-to-point distribution of
signals is the method of choice. In a point-to-point
scheme either series terminated or parallel terminated
transmission lines can be used. The parallel technique
terminates the signal at the end of the line with a 50ꢀ
resistance to VCC÷2.
VL = VS ( Z0 ÷ (RS+R0 +Z0))
Z0 = 50ꢀ || 50ꢀ
RS = 36ꢀ || 36ꢀ
R0 = 14ꢀ
VL = 3.0 ( 25 ÷ (18+14+25)) = 1.31V
At the load end the voltage will double, due to the near
unity reflection coefficient, to 2.5V. It will then increment
towards the quiescent 3.0V in steps separated by one
This technique draws a fairly high level of DC current and
thus only a single terminated line can be driven by each
output of the ASM2I99446 clock driver. For the series
terminated case however there is no DC current draw,
thus the outputs can drive multiple series terminated
lines. Figure 1. “Single versus Dual Transmission Lines”
illustrates an output driving a single series terminated line
versus two series terminated lines in parallel. When taken
to its extreme the fanout of the ASM2I99446 clock driver
is effectively doubled due to its capability to drive multiple
lines.
round trip delay (in this case 4.0nS).
OutA
OutB
1.5
tD = 3.8956
tD = 3.9386
In
0.5
ASM2I99446
OUTPUT BUFFER
Z0=50ꢀ
RS=36ꢀ
14ꢀ
2
4
12
14
ASM2I99446
Z0=50ꢀ
Z0=15ꢀ
RS=36ꢀ
RS=36ꢀ
OUTPUT BUFFER
Figure 2. Single versus Dual Waveforms
14ꢀ
Since this step is well above the threshold region it will
not cause any false clock triggering, however designers
may be uncomfortable with unwanted reflections on the
line. To better match the impedances when driving
multiple lines the situation in Figure 3. “Optimized Dual
Line Termination” should be used. In this case the series
terminating resistors are reduced such that when the
parallel combination is added to the output buffer
impedance the line impedance is perfectly matched.
Figure 1. Single versus Dual Transmission
Lines
The waveform plots in Figure 2. “Single versus Dual Line
Termination Waveforms” show the simulation results of
an output driving a single line versus two lines. In both
cases the drive capability of the ASM2I99446 output
buffer is more than sufficient to drive 50ꢀ transmission
lines on the incident edge. Note from the delay
measurements in the simulations a delta of only 43pS
exists between the two differently loaded outputs. This
suggests that the dual line driving need not be used
exclusively to maintain the tight output-to-output skew of
the ASM2I99446. The output waveform in Figure 2
“Single versus Dual Line Termination Waveforms” shows
a step in the waveform. This step is caused by the
ASM2I99446
Z0=50ꢀ
RS=22ꢀ
OUTPUT BUFFER
14ꢀ
Z0=50ꢀ
RS=22ꢀ
14Ω + 22Ω║22Ω = 50Ω║50Ω
25Ω = 25Ω
Figure 3. Optimized Dual Line Termination
2.5V and 3.3V LVCMOS Clock Distribution Buffer
7 of 14
Notice: The information in this document is subject to change without notice.
PULSECORE [ PulseCore Semiconductor ]
