handbook of modern sensors

5.9 Noise in Sensors and Circuits 215
Fig. 5.51. Equivalent circuit of a capacitor.
supply impedance can be quite high.As the frequency increases, the inductive parasitic
becomes troublesome and may result in circuit oscillation or ringing effects. Even if
the circuit operates at lower frequencies, the bypass capacitors are still important, as
high-frequency noise may be transmitted to the circuit and power-supply conductors
from external sources, (e.g., radio stations). At high frequencies, no power supply
or regulator has zero output impedance. What type of capacitor to use is determined
by the application, frequency range of the circuit, cost, board space, and some other
considerations. To select a bypass capacitor, one must remember that a practical
capacitor at high frequencies may be far from the idealized capacitor described in
textbooks.’
Ageneralized equivalent circuit of a capacitor is shown in Fig. 5.51. It is composed
of a nominal capacitance C, leakage resistance r l , lead inductances L, and resistances
R. Further, it includes dielectric absorption terms r and c a , which are manifested in
the capacitor’s “memory”. In many interface circuits, especially amplifiers, analog
integrators, and current (charge)-to-voltage converters, dielectric absorption is a major
cause for errors. In such circuits, film capacitors should be used whenever possible.
In bypass applications, r l and dielectric absorption are second-order terms, but
series R and L are of importance. They limit the capacitor’s ability to damp transients
and maintain a low-power supply output impedance. Often, bypass capacitors must be
of large values (10 µF or more) so they can absorb longer transients; thus, electrolytic
capacitors are often employed. Unfortunately, these capacitors have large series R
and L. Usually, tantalum capacitors offer better results; however, a combination of
aluminum electrolytic with nonpolarized (ceramic or film) capacitors may offer even
further improvement. A combination of the wrong types of bypass capacitor may lead
to ringing, oscillation, and cross-talk between data communication channels. The best
way to specify a correct combination of bypass capacitors is to first try them on a
breadboard.
5.9.5 Magnetic Shielding
Proper shielding may dramatically reduce noise resulting from electrostatic and electrical
fields. Unfortunately, it is much more difficult to shield against magnetic fields
because it penetrates conducting materials.Atypical shield placed around a conductor
and grounded at one end has little, if any, effect on the magnetically induced voltage
in that conductor. As a magnetic field B 0 penetrates the shield, its amplitude drops
exponentially (Fig. 5.52B). The skin depth δ of the shield is the depth required for
the field attenuation by 37% of that in the air. Table 5.5 lists typical values of δ for