Page 338 - Analog and Digital Filter Design

HAPTER 14

FILTER INTEGRATED CIRCUITS

This chapter gives an introduction to integrated circuit (IC) filters. Switched
capacitor and continuous time filters are covered. Many semiconductor manu-
facturers produce filter ICs, and filter ICs from three companies, Maxim. Texas
Instruments (formerly Burr-Brown), and Linear Technology, are described and
some practical design examples are given. Problems encountered with these
types of filter are also described. The benefits are. for example, being able to
make the filter cutoff programmable or adjustable.

A frequency synthesizer will be described that derives a sine wave signal from a
clock-driven logic circuit. The reason for including this description is that the
filtering of its output can be carried out in one of two ways: (1) with a fixed fre-
quency filter, if the clock frequency is fixed; (2) if the clock frequency is tunable,
by using a tuned switched capacitor filter that tracks the frequency of the output
signal. Method (2) shows the advantage of having a filter cutoff frequency that
is clock dependent.

Continuous Time Filters

Continuous time fiiters are the same as some of the active filters described earlier
in Chapters 4-7 of this book. Generally, a continuous time filter IC has a fixed
frequency response that is determined at the design stage and set by external
resistors. The MAX274, the MAX275, and the UAF42 are IC filters of this type.
However there are some digitally programmable continuous time filters, such
as the MAX270 and the MAX271, that require no external components for
frequency selection: the frequency is determined by data programmed into an
internal latch.

The MAX274, MAX275, and UAF42 all have a state variable architecture.
Internal capacitors and resistors are provided in all three types.