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Maxim > Design Support > Technical Documents > Application Notes > Filter Circuits (Analog) > APP 23
Keywords: analog filter, microprocessor supervisor, uP supervisor, reset circuit, power fail comparator
APPLICATION NOTE 23
µP-Supervisor Chip Controls Programmable Filter
Jul 09, 1998
Abstract: An application showing how the use of a microprocessor supervisor can be used to program a dual
section filter. Using the time delay from the reset circuit and strapping it to the power-fail comparator with a
delay circuit, the filter can easily be programmed to provide the correct cutoff frequencies during power up
using this reset circuit.
Certain dual-section filter ICs have a common 7-bit port for programming the two cutoff frequencies (f
C
). If
both sections require the same f
C
, you can strap an appropriate code to the port pins, but other applications
require a different f
C
for each section. In such cases, a microprocessor is the obvious tool for sequentially
programming the two filter sections, but lacking a µP, you can do the job without the circuit of Figure 1.
Figure 1. A µP-supervisory chip (IC1) directs the sequential loading of f
C
data into the dual, programmable
lowpass filter IC2. The circuit reloads this f
C
data following each power-up.
IC
2
is a continuous, dual-lowpass filter containing identical 2nd-order sections A and B. To program desired f
C
values, obtain corresponding codes from the data sheet and connect each pin of A0-A6 and B0-B6 to 5V ("1")
or GND ("0") accordingly. (The latches internal to inputs D0-D6 remain "transparent" because inputs active-low
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