Signal Integrity and Interconnects
3.3 Where Signal Integrity Lives
net each terminal connects, define the connectivity. An example of a schematic for wiring up an Atmega 328 microcontroller is shown in Figure 3.6.
Figure 3.6 An example of a schematic showing the components and how their terminals are connected together.
The wires we draw in the schematic are perfectly transparent. They have no electrical properties other than 0 ohm resistance and 0 time delay. As Shawn Hailey, one of the founders of MetaSoft (acquired by Synopsis) and a cocreator of HSPICE, is fond of saying, “Wires in a schematic are Tachyonic superconductors.”
This means the wires drawn in a schematic will never create noise of any sort. If the connectivity is correct, they will never contribute to electrical problems. They are perfectly transparent. A schematic says absolutely nothing about signal integrity.
The problems with the interconnect arise when the schematic is translated into the physical features of traces on a circuit board in the layout process.
Noise is always due to an aggressor signal on an aggressor
interconnect generating noise that appears on a victim interconnect.
When the noise distorts the aggressor signal on the aggressor interconnect due to its own interconnect, we refer to this as self-
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aggression noise. When the noise from the aggressor is generated on another interconnect, we refer to this as mutual-aggression noise.
Self-aggression noise is generated on the same net as where the aggressor signal propagates. Self-aggression noise on a net is
generated by the physical implementation of the wires that make up the net.
Mutual-aggression noise is transmitted through the space between the aggressor and victim nets, literally through the insulating dielectrics or air between the conductors. It is transmitted by the electric and magnetic fields of the aggressor signal from the
aggressor interconnect, coupling to the victim interconnect through the space (air) between them.
Even though there is no DC current path between the aggressor and victim nets, currents can flow literally through the air or insulation by either displacement current (the dV/dt across capacitance) or through induced currents between a mutual inductance. These currents flow because of the changing electric and magnetic fields between the aggressor and victim loops.
We call the field lines that span between the aggressor and victim signal-return loops fringe fields because they extend beyond just the local vicinity of the aggressor conductors. An example of these fringe electric and magnetic fields from an aggressor signal-return path to a victim signal-return path is illustrated in Figure 3.7.
Figure 3.7 A cross-section view of an aggressor and victim signal conductor and their return path, showing their fringe electric and magnetic field lines that create cross talk.
The shared field lines are referred to as mutual field lines.
Signal integrity problems lurk in the invisible spaces in a schematic: in the wires and in the white space of the schematic. They are only brought to life in the layout.
It is an unfortunate fact that once connectivity is defined by the wires, the performance of the circuit will only be degraded by the interconnects.
The electrical properties of the interconnects can never improve the performance of the circuit over what the components themselves are capable. The interconnects can only degrade performance and screw things up.
When we design interconnects, one of our goals is to engineer them to limit how much they screw up the signals and keep the noise they generate below an acceptable level.
There are some best design features we can implement, which are free, that will always contribute to less noise. If they really are free to implement (no cost), and they reduce noise (add value), they offer a very good ROI and should always be implemented. We call these habits.
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Practice including these features in your boards even if you don’t need them in your current design for it to work. They may not be necessary in your current design, but guaranteed, there will be a future design where these design guidelines are important.
Experience incorporating these habits in all your designs will make you a better designer.
For example, the most important feature we can add to a board to reduce one type of switching noise is to use a continuous ground plane, or return path, on a copper layer adjacent to the signal traces.
We can use a signal trace in the vicinity of the aggressor trace on which we purposefully keep the driven signal at a low level. This trace becomes a sense line for any noise. If there is no signal on the trace, but we measure a voltage, this voltage must be due only to noise. We call this sort of sense line a quiet line. It is a very important way of measuring just the noise that would appear on a trace. Of course, when a signal is imposed on the trace, the voltage measured would be the signal plus the noise.
Figure 3.8 shows the measured noise on a nearby victim trace when signals propagate on an aggressor interconnect over a ground plane and in a separate region of the circuit board where there is no
ground plane. The measured noise on a similar quiet trace is reduced by a factor of 30 using a continuous ground plane underneath, acting as the return path in this example.
Figure 3.8 Measured noise on two different quiet lines when an aggressor line switches in two identical circuits with different layouts. When there is a continuous return plane, the noise is less than 20 mV. When there is no continuous return plane, the noise is 600
mV.
The schematics for these two circuits are identical. Their only difference is in how the layout was implemented. One circuit has a continuous return plane under all the signal traces while the other layout just has a trace as the return path.
This specific noise is generated because of a changing current in the aggressor interconnect passing through the mutual inductance shared with the victim interconnect. It is an example of switching noise. Since it arises due to a screwed-up return path, it is also a form of ground bounce. And because it scales with the number of signals switching simultaneously with their return currents sharing this screw-up return path it is also a form of simultaneous switching noise.