Page 347 - Analog and Digital Filter Design

344 Analog and Digital Filter Design

Other Switched Capacitor Filters
Only a small range of filter ICs have been described. Manufacturers of these
and other popular devices include National Semiconductor, Linear Technology,
and Maxim.

One type of switch capacitor filter IC, which is available from more than one
company, provides DC accurate lowpass filtering by providing an active shunt.
Examples are the Maxim parts MAX280, MAX281, and MXL1062. This type
of Uter only requires a resistor in the signal path, the filter input is connected
to one side of the resistor and the filter output is connected to the other side.
The filter IC is connected between the filter output and ground, and causes
signals to be attenuated by the equivalent of a fifth-order response. The circuit
for this is illustrated in Figure 14.4.

Passband
= 100 x CLOCK
Switched
-
Capacitor IC

-

Figure 14.4
Lowpass Filter with DC Accurate I
output GROUND

There are other devices that are programmed to have, say, an eighth-order
Butterworth, Bessel, or Cauer response. The cutoff frequency is usually 1/100
or 1/50 of the clock frequency. Although these devices are not flexible, they
are very simple to use and apply.

An Application of Switched Capacitor Filters

One of the most useful applications of switched capacitor filters relies on the
cutoff frequency being proportional to the clock frequency.

A sinusoidal frequency synthesizer can be produced from a digital circuit known
as a Walking Ring, two designs are shown in Figure 14.5. The Walking Ring
uses counter circuits to produce a pattern of logic states that repeats after a
certain number of clock pulses. Using summing resistors at the input of an op-