Page 346 - Analog and Digital Filter Design

Filter Integrated Circuits 34
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Mode 4 produces an all-pass function, as well as lowpass and bandpass outputs.
The all-pass response, you may remember from earlier chapters, provides group
delay equalization or phase-shift networks.

In each mode it is possible to program the pole’s Q factor: setting certain regis-
ters in the device’s memory does this. The lowest Q is 0.5 in modes I, 3, and 4,
and 0.707 in mode 2. The highest Q is 64 in modes 1, 3, and 4. and 90.5 in mode
2. The possible Q values are given by the equation:

64
Q=- 128-Ai” in modes 1, 3 and 4. In mode 2 these values are
multiplied by a.

Maxim supplies a comprehensive data sheet with plenty of applications, if
further information is required.

Pin Programmable ICs MAX263lMAX264lMAX2671MAX268
These devices are very similar to those previously described, except that the reg-
isters containing the pole frequency and Q setting data are externally accessible.
This allows a simple circuit that is suitable for an analog design. As before there
are four standard modes of operation, with an extra mode (3A) that us- bs an
external summing circuit.

The clock inputs have TTL logic level thresholds of 0.8V and 2.4V. This is not
the case for the frequency and Q programming inputs; these have thresholds
within 0.5V of the supply rails. Thus the positive threshold is V’ - 0.5V and
the negative threshold is V- + 0.5V. The supply rails are nominally +5 V (V+) and
-5 V (V-). Programming is achieved by wire-strapping the programming inputs
to the positive and negative rails, rather than by using logic gates.

The MAX263 and MAX267 can have pole frequencies up to 57 kHz. In modes
1, 3, and 4 the clock to pole frequency ratio can be programmed to be between
100.53: I and 197.92: 1, and the Q can be programmed to be between 0.504 and
64. In mode 2 the frequency ratio can be set between 71.09 : 1 and 139.95 : 1, and
the Q can be set between 0.713 and 90.5.

The MAX264 and MAX268 have lower clock to pole frequency ratios and can
therefore work up to 140kHz. In modes 1, 3, and 4 the clock to pole frequency
ratio range is from 40.84: 1 to 138.23: I. In mode 2 the frequency ratio can be
set to between 28.88 and 97.74. The same range of Q values applies as in the
MAX263 and bfAX267.