Download Microelectronics : Systems and Devices

Planning the system
97
Noise
Any unwanted electrical signals present in a circuit are
referred to as noise. If the amplitude of the noise is great
enough, it degrades the wanted signals. In logic circuits, it
may cause a logic low level to be read as a logic high level
or the other way about, so that data is corrupted. It may
have a similar effect on control signals causing a device to
reset, for example, or to latch its input at the wrong time.
Similar effects can result from noise on the supply lines.
It is usually possible to minimise noise to an acceptable
level but impossible to remove it altogether. Noise may be
generated in the circuit itself, especially in resistances and
semiconductors. It may be picked up from other parts of
the same circuit, especially where tracks on the circuit
board run side-by-side (crosstalk). It may also be picked
up from outside as electromagnetic interference (EMI).
For the reasons stated above, minor variations in power supply,
ambient temperature, and component values have little effect, if any,
on the operations of a digital circuit. For the same reasons, noise in the
form of voltage spikes is largely ignored by digital circuits. This is
why digital circuits have become so widely used, even for applications
such as audio recording and playback in which the original input and
final output are necessarily analogue.
In spite of the essential robustness of digital circuits, there can be
problems with these too. The problems are more severe when we try to
push the circuits to the limits of their capabilities, particularly as we
increase processing speeds to their maximum. In this chapter, we look
at some of the problems that may arise with digital circuits in
microelectronic systems and how we may try to overcome them.
Many of the problems arise because of two related reasons:
•
The signals in microelectronic systems are mostly of radio
frequency.
•
The signals being of high frequency, their rise times and fall times
are extremely short. Putting this the other way round, there are
high rates of change of voltage and current.
A third source of difficulty is electromagnetic interference (EMI).