handbook of modern sensors

5.9 Noise in Sensors and Circuits 211
Since f(s 0 ) is constant, the ratio is not temperature dependent. It should be emphasized,
however, that the ratiometric technique is useful only when the anticipated
noise has a multiplicative nature, whereas a differential technique works only for
additive transmitted noise. Neither technique is useful for inherent noise, which is
generated internally in sensors and circuits.
Although inherent noise is mostly Gaussian, the transmitted noise is usually less
suitable for conventional statistical description. Transmitted noise may be periodic,
irregularly recurring, or essentially random, and it ordinarily may be reduced substantially
by taking precautions to minimize electrostatic and electromagnetic pickup
from power sources at line frequencies and their harmonics, radio broadcast stations,
arcing of mechanical switches, and current and voltage spikes resulting from switching
in reactive (having inductance and capacitance) circuits. Such precautions may
include filtering, decoupling, shielding of leads and components, use of guarding
potentials, elimination of ground loops, physical reorientation of leads, components,
and wires, use of damping diodes across relay coils and electric motors, choice of low
impedances where possible, and choice of power supply and references having low
noise. Transmitted noise from vibration may be reduced by proper mechanical design.
A list outlining some of the sources of transmitted noise, their typical magnitudes,
and some ways of dealing with them is shown in Table 5.4.
The most frequent channel for the coupling of electrical noise is a “parasitic”
capacitance. Such a coupling exists everywhere. Any object is capacitively coupled
to another object. For instance, a human standing on isolated earth develops a capacitance
to ground on the order of 700 pF, electrical connectors have a pin-to-pin
capacitance of about 2pF, and an optoisolator has an emitter-detector capacitance of
about 2 pF. Figure 5.48A shows that an electrical noise source is connected to the
sensor’s internal impedance Z through a coupling capacitance C S . That impedance
Table 5.4. Typical Sources of Transmitted Noise
External Source Typical Typical Cure
Magnitude
60/50 Hz power 100 pA Shielding; attention to ground loops;
isolated power supply
120/100 Hz supply ripple 3 µV Supply filtering
180/150 Hz magnetic pickup from
saturated 60/50-Hz transformers 0.5 µV Reorientation of components
Radio broadcast stations 1 mV Shielding
Switch arcing 1 mV Filtering of 5–100-MHz components;
attention to ground loops and shielding
Vibration 10 pA Proper attention to mechanical coupling;
(10–100 Hz) elimination of leads with large voltages
near input terminals and sensors
Cable vibration 100 pA Use a low-noise (carbon-coated dielectric)
cable
0.01-10 pA/Hz Clean board thoroughly; use Teflon insulation
Circuit boards below 10 Hz where needed and guard well
Source: Adapted from Ref. [13].