ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICES - Design and Development of Medical Electronic Instrumentation - page 195

the chassis. Of course, the level of coupling must fall well within the amount of enclosure
leakage permitted for the fl floating part.

Pulse Reflection and Termination Techniques

In a typical circuit, a driving logic element and a receiving logic element are connected by
a PCB track. In the equivalent circuit shown in Figure 4.36, a pulse with amplitude V d is
injected by the driver logic element, which presents an output impedance Z d into a PCB
track of length l t and impedance Z t . The pulse carried by the PCB track is then presented
to the input impedance Z r of the receiving element. If we suppose that a Z t

track
carries the pulse, and after looking at the data sheets for a selected 5-V family of high-
speed logic, we

100 Ω

fi find that the output and input impedances at our frequency of interest are

Z d ected pulse

starts traveling back toward the driver with amplitude that can be approximated by

50 Ω and Z r 10 k Ω , respectively, then upon reaching the receiver, a re fl

Z
Z

Z
Z

1
1

0
0

,
,

0
0

0
0

0 1

1

0
0

0
0

r t

t

V r V d (5 V) 4.9 V

which assumes a negligible attenuation of the pulse throughout its conduction, and which
takes into consideration only the real parts of the variables. This re fl ected pulse will be

rere ected back toward the receiver upon hitting the driver with an amplitude approxi-

mated by

fl

Z Z Z 5

0

0 1 0 1 0

d t

V r Z V r 5 (4.9 V) 1.63 V

0 0

d t

This negative signal will interact with the original incident pulse with a delay equiv-
alent to the time it takes for the pulse to travel back and forth along the track τ (ns)

0.31 t r .
Depending on the length of the PCB track, the ection could distort the

leading edge of the pulse so much that it will cause the false detection of a logic-low
(Figure 4.37). A di

1.63-V re fl

ff erent combination of impedances could have caused the re fl ected pulse

V d

I t

PCB Track

Z

Z d

r

Z t

GND Plane

Driver Receiver

Figure 4.36 The output of a logic element connected through a PCB track to the input of another
logic element can be modeled as an ideal voltage step generator that drives a transmission line of
impedance Z t through an output impedance Z d . The transmission line is then terminated by the
receiver's input impedance Z r .