handbook of modern sensors

218 5 Interface Electronic Circuits
teristics (multiplicative noise) or they may result in the generation of spurious signals
(additive noise) by a sensor. If a sensor incorporates certain mechanical elements,
vibration along some axes with a given frequency and amplitude may cause resonant
effects. For some sensors, acceleration is a source of noise. For instance, most
pyroelectric detectors also possess piezoelectric properties. The main function of the
detector is to respond to thermal gradients. However, such environmental mechanical
factors as fast changing air pressure, strong wind, or structural vibration cause the sensor
to respond with output signals which often are indistinguishable from responses
to normal stimuli.
5.9.7 Ground Planes
For many years, ground planes have been known to electronic engineers and printed
circuit designers as a “mystical and ill-defined” cure for spurious circuit operation
[16]. Ground planes are primarily useful for minimizing circuit inductance. They
do this by utilizing the basic magnetic theory. Current flowing in a wire produces an
associated magnetic field (Section 3.3 of Chapter 3). The field’s strength is proportional
to the current i and inversely related to the distance r from the conductor:
B = µ 0 i
2πr . (5.85)
Thus, we can imagine a current carrying wire surrounded by a magnetic field. Wire
inductance is defined as energy stored in the field set up by the wire’s current. To
compute the wire’s inductance requires integrating the field over the wire’s length and
the total area of the field. This implies integrating on the radius from the wire surface
to infinity. However, if two wires carrying the same current in opposite directions
are in close proximity, their magnetic fields are canceled. In this case, the virtual
wire inductance is much smaller. An opposite flowing current is called return current.
This is the underlying reason for ground planes. A ground plane provides a return
path directly under the signal-carrying conductor through which return current can
flow. Return current has a direct path to ground, regardless of the number of branches
associated with the conductor. Currents will always flow through the return path
of the lowest impedance. In a properly designed ground plane, this path is directly
under the signal conductor. In practical circuits, a ground plane is on one side of
the board and the signal conductors are on the other. In the multilayer boards, a
ground plane is usually sandwiched between two or more conductor planes. Aside
from minimizing parasitic inductance, ground planes have additional benefits. Their
flat surface minimizes resistive losses due to the "skin effect” (ac current travel along
a conductor’s surface). Additionally, they aid the circuit’s high-frequency stability by
referring stray capacitance to the ground. Some practical suggestions are as follows:
• Make ground planes of as much area as possible on the component side (or inside
for the multilayer boards). Maximize the area especially under traces that operate
with high frequency or digital signals.
• Mount components that conduct fast transient currents (terminal resistors, ICs,
transistors, decoupling capacitors, etc.) as close to the board as possible.