ACTIVE_FILTERS_Theory_and_Design

Introduction 19
1.5.1 TONE SIGNALING
The “touch-tone” telephone systems use active filters to decode the dual tone generated
at the telephone into the characters 0–9, “*”, and “#”.
1.5.2 BIOFEEDBACK
The four commonly recognized brainwaves, delta, theta, alpha, and beta, can be
segregated using active band-pass filtering techniques. Various graphical techniques
may be employed to monitor transitions between wave-ground as a response to
stimuli. Careful attention must be paid to the amplifier’s low-frequency noise characteristic.
The OP-07 (PMI) has the lowest noise of any monolithic op-amp.
1.5.3 INSTRUMENTATION
Applications of active filters to instrumentation is the most diverse of all fields. The
low-pass filter is often used as a signal conditioner or noise filter. Because low-pass
filters are designated to operate down to dc, low op-amp offset voltages, such as those
offered by the OP-07, are of prime importance. In harmonic distortion measurements,
notch or bandstop filters allow harmonic interference to be accurately determined.
1.5.4 DATA ACQUISITION SYSTEMS
The noise caused by input switches or high-speed logic is removed with low-pass
active filters. Signals reproduced from digital information through a digital-to-analog
converter often appear as staircases. In more extreme cases, because of limited
sampling, the reproduction appears at only a few discrete levels; however, sophisticated
filters can accurately reconstruct the input signal. Remote sensing, in noisy
environments, requires the noise-rejecting properties of simple active filters.
1.5.5 AUDIO
Electronic music and audio equalizers use a large number of active filters. Synthesizers
combine various low-pass, high-pass, and bandpass functions to generate waveforms
that have spectral densities similar to orchestra instruments. Symmetrical positive and
negative slew rates make the OP-11 (PMI) well suited for audio applications.
1.5.6 LAB SIGNAL SOURCES
Because of the active filter characteristics, high-purity oscillators are easily designed
with very few components.
1.6 THE VOLTAGE-CONTROLLED VOLTAGE
SOURCE (VCVS)
In many applications where a high-impedance source must be interfaced to a filter,
a noninverting VCVS op-amp filter may be used. Typical input impedances are
greater than tens of megaohms, and output impedances are typically less than a few
ohms. This, of course, depends on the amplifier being used.