AN-5020 Datasheet, PDF (1/3 Pages) Fairchild Semiconductor – LVDS Reduces EMI

AN-5020
LVDS Reduces EMI
Fairchild Semiconductor
Application Note
July 2002
Revised July 2002
Abstract
As the complexity and speed of electronic products
increase, the amount of Electromagnetic Interference
(EMI) emissions also grow dramatically. Increasing data
rates and tougher Electromagnetic Compatibility (EMC)
standards make electromagnetic radiation a heightened
concern. This, coupled with the proliferation of portable and
wireless products, increases the chance of interference
between systems, making EMI emissions more of a prob-
lem. This application note discusses alternatives and fea-
tures offered by Low Voltage Differential Signaling (LVDS)
to reduce EMI emissions.
What Causes EMI?
EMI radiation is fundamentally caused by any charge mov-
ing in an electrical field or change in an electrical field. Typ-
ically any electrical transitions with sharp edges -- such as
clock, data, address and control signals -- produce electro-
magnetic radiation. In digital systems, periodic clock sig-
nals are the major cause of EMI. Control and timing
signals, address and data buses, interconnect cables and
connectors also contribute to EMI emissions.
Two main modes of electromagnetic radiation are:
• “Differential mode”, caused by local current loops
between printed circuit board (PCB) interconnect traces
and the ground plane(s)
• “Common mode”, caused by the coupling of ground and
power plane noise into traces, input/output buses, and
cable lines.
Problems caused by EMI have increased with the prolifera-
tion of mobile electronic systems, wireless communication
systems and computer networks. To address problems
associated with EMI radiation, designers can use several
approaches, which may involve:
1. Developing alternative technologies, which radiate less
EMI interference.
2. Design more effective techniques with existing
technologies.
LVDS Features Reduce EMI
EMI is generated by current changes in a conductor and
increases with the rate and the amplitude of that change.
Low Voltage Differential Signaling (LVDS) features a low
swing differential constant current source configuration and
due to this small signal change, relatively little EMI is radi-
ated from the conductor. Refer to Figure 1 which illustrates
relative comparisons of signal levels for some common
data transmission standards.
FIGURE 1. Signal Level Comparisons of Common Data
Transmission Standards
Digital signals can change logic states much faster when
the signal transitions do not have as far to go to change
states. A reduced voltage swing with a small constant cur-
rent results in lower power in the transmission medium and
at the load. Reduced signal transitions that are relatively
faster than other interface technologies, such as RS-422,
not only reduce EMI but push up the frequency of operation
spectrum. (Note: The FCC continues to raise the upper fre-
quency limit for EMI compliance.)
Differential Signaling Cancels EMI
In single-ended signaling topologies such as CMOS/TTL,
PECL and RS-232, almost all the electrical field lines are
free to radiate outward from the single conductor. Other
objects can intercept these fields, but some can travel as
Transverse Electromagnetic Mode (TEM) waves, which
may escape the system and cause EMI. With balanced dif-
ferential devices signal lines, the fields around the two
electrical paths oppose each other, and the concentric
magnetic fields tend to react with one another and cancel
each other (Figure 2). The electrical coupling of the two
conductors allows cancellation of most of the low fre-
quency fields. These coupled electric fields are “tied up”
and cannot escape to propagate as TEM waves beyond
the immediate vicinity of the conductors. Only the stray
fringing fields are allowed to escape, resulting in much less
field energy available to propagate as TEM waves.
© 2002 Fairchild Semiconductor Corporation AN500496
www.fairchildsemi.com