July 2005

ASM5I9653A

rev 0.2

3.3V 1:8 LVCMOS PLL Clock Generator

8 of 13

Notice: The information in this document is subject to change without notice.

Figure 6. Single versus Dual Transmission Lines

The waveform plots in Figure 7 .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 ASM5I9653A output buffer is

more than sufficient to drive 50Ω 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 ASM5I9653A. The

output waveform in Figure 7 Single versus Dual Line

Termination Waveforms shows a step in the waveform,

this step is caused by the impedance mismatch seen

looking into the driver. The parallel combination of

the

36Ω 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:

⎟ (18+14+25)

= 1.31V

At the load end the voltage will double, due to the near

unity reflection coefficient, to 2.6V. It will then increment

towards the quiescent 3.0V in steps separated by one

round trip delay (in this case 4.0nS).

Figure 7. Single versus Dual Waveforms

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 8 .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.

14

Ω + 22Ω _ 22Ω = 50Ω _ 50Ω

25

Ω = 25Ω

Figure 8. Optimized Dual Line Termination

ASM5I9653A

OUTPUT BUFFER

14Ω

IN

OUTA

ASM5I9653A

OUTPUT BUFFER

14Ω

IN

OUTB1

OUTB0

ASM5I9653A

OUTPUT BUFFER

14Ω

IN