Signal Integrity vs EMC
Whats the Difference?
Signal integrity (or SI) is getting increasing attention in the EMC
community, as evidenced by the addition of the Signal Integrity
Committee in the IEEE EMC Society several years ago.
The issue of signal integrity has become more pressing in
recent years. In years gone by, signal integrity was primarily
in the domain of high speed computers and telecom applications
most designers never ran into transmission line problems
and only occasionally ran into crosstalk problems (at least
at the circuit board level). Although the emphasis of SI has
been with high speed circuits, we need to remember that low-level
analog designers have been fighting their own version of SI
But the high speed microprocessor is leading the charge into
SI at all levels, most prominently with the personal computers
running well into the GHz range. Not to be outdone, the low
level op-amps are dipping into the nanovolt range, placing
extreme limits on any kind of noise.
Why has SI been adopted into the IEEE EMC Society? Because
there is a major overlap between SI and EMC design
we say that SI is really EMC at the local level. We usually
find that boards well designed from an SI standpoint are pretty
good from an EMC standpoint. To understand this issue, lets
do a side-by-side comparison of EMC and SI the comparison
is summarized in the table.
The focus of SI is local to the specific signal path, primarily
the circuit board. The intent is to get a signal from driver
to receiver without excess degradation. If you are dealing
with digital signals, there will be an envelope between the
minimum output signal and minimum allowed input signal, constituting
your noise budget. Your task is to keep the signal from degrading
to unacceptable levels along the way. The two primary causes
of degradation are signal reflections and crosstalk, generally
becoming important when the risetime exceeds the paths
If you have only one signal of interest, you will only be
dealing with transmission line effects. If you have more signals,
crosstalk becomes an issue. You will be interested in ground
and Vcc noise regardless of path length.
In many cases, the signal is confined to the circuit board.
In other cases, the signal must travel from one board to another,
along a header/connector path (as from mother- board to daughterboard)
or along a data cable. This means impedance needs to be controlled
over the entire path, from chip to trace to connector to cable
and so on to the receiver on another board.
Transmission lines are not an issue with low frequency analog
circuits, but there is little tolerance to ground and power
bounce even a microvolt of noise may be too high in
medical and scientific applications. So common ground paths
are a prime consideration.
The focus of EMC is the entire system that could be
a single circuit board (common in handheld devices) or it
could be a complex set of interconnected subsystems (common
in military equipment). We may be interested in the circuit
boards, the power supply, the enclosure and cables.
SI generally deals with signals in the time domain. EMC is
generally considered to be a frequency domain issue. Certainly,
emissions and RF susceptibility are purely frequency domain,
and even with the transient tests, design revolves around
knowing the frequency content.
The objective of SI is to get signals clean enough to meet
functional requirements, and involves both amplitude and timing
margins, both of which tend to disappear in actual applications.
The objective of EMC is twofold: self-compatibility within
the various elements in the system and to pass the EMC tests.
The EMC tests are intended to enhance the probability of self-compatibility,
and in many cases, particularly for manufacturers of equipment,
this is your only obligation. Where systems are involved,
notably military and vehicular applications, passing the module
test is only the first step in EMC. Once you plug the equipment
together, you need to find the vulnerable areas and fix them.
With SI, the key concerns are signal reflections, crosstalk
ground bounce and power decoupling.
With EMC, you are concerned with emissions and immunity (aka
susceptibility) and self-compatibility (making sure it works).
With EMC, you will encounter a much wider range of interference
problems than with SI, including motor controls, relays, and
other power loads. In modern electronics, you will often find
noisy loads operating close to sensitive circuits, and these
may or may not be adequately covered by a regulation or subsystem
SI solutions involve careful circuit board layout to minimize
crosstalk and reflections, attention to timing.
EMC solutions also include careful circuit board layout, but
also include attention to grounding and shielding of enclosure
and cables, I/O filtering and transient protection.
SI signal levels are in the millivolt and milliamp range
for digital circuits. As supply voltage levels drop, the noise
margins drop as well. Analog circuits may have voltage levels
as low as a microvolt or less, and currents in the picoamp
range. EMC levels cover a much wider range. For emissions,
the levels will be in the microvolt and microamp range. These
levels are much too low to detect with an oscilloscope set
for logic levels.
For immunity, interference levels for transients are in the
kilovolt range, as evidenced by the IEC surge and EFT, and
currents are in the ampere range.
While EMC and SI needs go hand-in-hand, the levels are much
different. So when you see an application note from your chip
supplier that tells you to decouple for Vcc droop and EMI,
they are really thinking SI. Decoupling that is satisfactory
for SI is entirely inadequate for EMI.
We see that, while EMC and Signal Integrity design requirements
overlap to a significant extent, there are still major differences
in emphasis. In most digital applications, EMC requirements
will be more demanding than those of signal integrity. Signal
integrity in analog applications are usually more demanding
than EMC requirements. But the good news is that the requirements
are usually not in conflict boards that are well designed
from a signal integrity standpoint are well on their way to
being good EMC boards. And, in fact, we think that signal
integrity design will be enhanced if the designers would adopt
some of the EMC design guidelines edge rate control
helps to quiet the board in addition to controlling emissions.