Page 342 - Analog and Digital Filter Design

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Filter Integrated Circuits

Integrated Circuit Filter MAX270/MAX27 1
The MAX270 and MAX271 are second-order lowpass active filters. These filter
ICs are unusual because, although they are continuous time devices, they are
digitally programmed for their cutoff frequency. The filters need no external
components for their frequency selection, but cutoff frequencies are limited to
the range 1 kHz to 25 kHz. Internally the devices use a Sallen and Key lowpass
circuit, with variable shunt and feedback capacitors; these are varicap diodes
and have a capacitance dependant upon the reverse bias potential across them.
An internal digital-to-analog converter provides a bias voltage to tune the filter
and give it the required cutoff frequency.

The MAX270 and the MAX271 are different. The MAX270 has an uncom-
mitted op-amp that has a 2MHz gain-bandwidth product. This op-amp can be
used to produce another filter stage or for other applications.

The MAX27 1 has a track-and-hold amplifier that can select, as its input, either
lowpass filter’s output. It is possible to multiplex the two signals by switching
the track-and-hold circuit from one input to the other, since the filter will prevent
aliasing if the switching rate is high enough; a sampling clock of 50kHz to
200 kHz would be suitable for most applications. The track and hold output is
disabled if the enable pin is at logic 0. The output from several devices can be
connected in parallel and, by only enabling one at a time, all the signals can
be multiplexed onto one circuit for (perhaps) carrying out analog to digital
conversion .

Switched Capacitor Filters

Switched capacitor filters are generally considerably noisier than their
continuous-time counterparts. This is mainly due to the switching process;
signals at the switching frequency and other spurious signals appear at the
filter’s output. There is also a risk of aliasing; this is where a signal outside the
band of interest appears in-band due to nuxing with the sampling clock
(which causes frequency shifting).
A switched capacitor filter uses the principle that, by switching a capacitor
between the source and the load, the equivalent of a high resistor value is created
between the two. Thus instead of resistors and capacitors there are just capa-
citors and switches. This serves two purposes: (1) high value resistors are diffi-
cult to produce on a semiconductor wafer; (2) by varying the switching rate the
effective resistance value changes. The basic switched capacitor circuit is shown
in Figure 14.3.