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LMF380CIN Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMF380CIN Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Typical Applications (Continued) THIRD-OCTAVE ANALYZER FILTER SET The circuit shown in Figure 3 uses the LMF380 to imple- ment a -octave filter set for use in ‘‘real time’’ audio pro- 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 01 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 126 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 1953125 kHz An input signal at 193 kHz will be aliased down to 23125 Hz which is near the band center of the 244 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 25 kHz The output of this first LMF60 – 100 drives the inputs of the fifth sixth and seventh LMF380s The sev- enth LMF380 has a 15625 kHz clock so aliasing will begin to become a problem around 152 kHz With a sixth-order 25 kHz low-pass filter preceding this circuit the attenuation at 152 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 VIN may be required 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- 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 8 |
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