lmf380 National Semiconductor Corporation, lmf380 Datasheet - Page 8

• Manufacturer Part Number: lmf380
• Description: Triple One-third Octave Switched-capacitor Active Filter
• Manufacturer: National Semiconductor Corporation

Typical Applications
THIRD-OCTAVE ANALYZER FILTER SET
The circuit shown in Figure 3 uses the LMF380 to imple-
ment a
gram analyzers Ten LMF380s provide all of the bandpass
filtering for the full audio frequency range The power supply
connections are not shown but each power supply pin
should be bypassed with a 0 1 mF ceramic capacitor in par-
allel with a 1 mF tantalum capacitor
The first LMF380 at the top of Figure 3 handles the highest
octave with center frequencies of 20 kHz 16 kHz and
12 6 kHz It also contains the 1 MHz master clock oscillator
for the entire system Its Clock Out pin provides a 500 kHz
clock for the second LMF380 which supplies 250 kHz to
the third LMF380 and so on
If the audio input signal were applied to all of the LMF380
input pins aliasing might occur in the lower frequency filters
due to audio components near their clock frequencies For
example the LMF380 at the bottom of Figure 3 has a clock
frequency equal to 1 953125 kHz An input signal at
1 93 kHz will be aliased down to 23 125 Hz which is near
the band center of the 24 4 Hz bandpass filter and will ap-
pear at the output of that filter
This problem is solved by two LMF60 – 100 6th order Butter-
worth low-pass filters serving as anti-aliasing filters as
shown in Figure 3 The first LMF60 – 100 is connected to the
input signal The clock for this LMF60 is 250 kHz and comes
from pin 10 of the second LMF380 The cutoff frequency is
therefore 2 5 kHz The output of this first LMF60 – 100 drives
the inputs of the fifth sixth and seventh LMF380s The sev-
enth LMF380 has a 15 625 kHz clock so aliasing will begin
to become a problem around 15 2 kHz With a sixth-order
2 5 kHz low-pass filter preceding this circuit the attenuation
at 15 2 kHz is theoretically about 94 dB which prevents
aliasing from occuring at this bandpass filter
The output of the first LMF60 also drives the input of the
second LMF60 which provides anti-aliasing filtering for the
three LMF380s that handle the lowest part of the audio fre-
quency spectrum
Note that no anti-aliasing filtering is provided for the four
LMF380s at the top of Figure 3 These devices will not en-
counter aliasing problems for frequencies below about
120 kHz if higher input frequencies are expected an addi-
tional low-pass filter at V
DETECTORS
In a real-time analyzer the amplitude of the signal at the
output of each filter is displayed usually in ‘‘bar-graph’’
form The AC signal at the output of each bandpass filter
must be converted to a unipolar signal that is appropriate for
driving the display circuit
The detector can take any of several forms It can respond
to the peaks of the input signal to the average value or to
the rms value The best type of detector depends on the
application For example peak detectors are useful when
monitoring audio program signals that are likely to overdrive
an amplifier Since the output of the peak detector is propor-
-octave filter set for use in ‘‘real time’’ audio pro-
IN
may be required
(Continued)
8
tional to the peak signal voltage it provides a good indica-
tion of the voltage swing Generally the output of the peak
detector must have a moderately fast (about 1 ms) attack
time and a much slower (tens or hundreds of milliseconds)
decay time The actual attack and decay times depend on
the expected application An average detector responds to
the average value of the rectified input signal and provides a
good solution when measuring random noise An average
detector will normally respond relatively slowly to a rapid
change in input amplitude An rms detector gives an output
that is proportional to signal power and is therefore useful
in many instrumentation applications especially those that
involve complex signals
Peak detectors and average-responding detectors require
precision rectifiers to convert the bipolar input signal into a
unipolar output Half-wave rectifiers are relatively inexpen-
sive but respond to only one polarity of input signal there-
fore they can potentially ignore information Full-wave recti-
fiers need more components but respond to both polarities
of input signal Examples of half- and full-wave peak- and
average-responding detectors are shown in Figure 4 The
component values shown may need to be adjusted to meet
the requirements of a particular application For example
peak detector attack and decay times may be changed by
changing the value of the ‘‘hold’’ capacitor
The input to each detector should be capacitively-coupled
as shown in Figure 4 This prevents any errors due to volt-
age offsets in the preceding circuitry The cutoff frequency
of the resulting high-pass filter should be less than half the
center frequency of the band of interest
Note that a passive low-pass filter is shown at the input to
each detector in Figure 4 These filters attenuate any clock-
frequency signals at the outputs of the third-octave
switched-capacitor filters The typical clock feedthrough at a
filter output is 10 mV rms or 40 dB down from a nominal
1 Vrms signal amplitude When more than 40 dB dynamic
range is needed a passive low-pass filter with a cutoff fre-
quency about three times the center frequency of the band-
pass will attenuate the clock feedthrough by about 24 dB
yielding about 64 dB dynamic range The component values
shown produce a cutoff frequency of 1 kHz changing the
capacitor value will alter the cutoff frequency in inverse pro-
portion to the capacitance
The offset voltage of the operational amplifier used in the
detector will also affect the detector’s dynamic range The
LF353 used in the circuits in Figure 3 is appropriate for sys-
tems requiring up to 40 dB dynamic range
DISPLAYS
The output of the detector will drive the input of the display
circuit An example of an LED display driver using the
LM3915 is shown in Figure 5 The LM3915 drives 10 LEDs
with 3 dB steps between LEDs the total display range for an
LM3915 is therefore 27 dB Two LM3915s can be cascaded
to yield a total range of 57 dB See the LM3915 data sheet
for more information