MPC9893 REVISION 8 3/16/16

10

©2016 Integrated Device Technology, Inc.

MPC9893 Data Sheet

3.3V 1:12 LVCMOS PLL CLOCK GENERATOR

The waveform plots in Figure 9 show the simulation results

of an output driving a single line versus two lines. In both

cases the drive capability of the MPC9893 output buffer is

more than sufficient to drive 50

 transmission lines on the

incident edge. Note from the delay measurements in the

simulations a delta of only 43 ps 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 MPC9893. The output waveform

in Figure 9 shows a step in the waveform, this step is caused

by the impedance mismatch seen looking into the driver. The

parallel combination of the 36

 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:

VL =VS (Z0  (RS+R0 +Z0))

Z0 = 50  || 50 

RS = 36  || 36 

R0 = 14 

VL = 3.0 (25  (18+17+25)

= 1.31 V

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

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

towards the quiescent 3.0 V in steps separated by one round

trip delay (in this case 4.0 ns).

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 10 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 9. Single versus Dual Waveforms

Figure 10. Optimized Dual Line Termination

Figure 11. CLK0, CLK1 MPC9893 AC Test Reference for VCC = 3.3 V and VCC = 2.5 V

Time (ns)

3.0

2.5

2.0

1.5

1.0

0.5

0

2

4

6

8

10

12

14

OutB

tD = 3.9386

OutA

tD = 3.8956

In

14



MPC9893

Output

Buffer

RS = 22

ZO = 50

RS = 22

ZO = 50

14

+ 22 || 22 = 50 || 50

25

 = 25

Pulse

Generator

Z = 50



RT = 50

ZO = 50

RT = 50

ZO = 50

MPC9893 DUT

VTT

VTT