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參數(shù)資料
| 型號(hào)： | LM1837M |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2 CHANNEL, AUDIO PREAMPLIFIER, PDSO16 |
| 封裝： | DIP-16 |
| 文件頁(yè)數(shù)： | 13/14頁(yè) |
| 文件大?。?/td> | 450K |
| 代理商： | LM1837M |
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Application Hints (Continued)
DISTORTION MEASUREMENT METHOD
In order to clearly interpret and compare specifications and
measurements for low noise preamplifiers it is necessary to
understand several basic concepts of noise An obvious ex-
ample is the measurement of total harmonic distortion at
very low input signal levels Distortion analyzers provide out-
puts which allow viewing of the distortion products on an
oscilloscope The oscilloscope often reveals that the ‘‘dis-
tortion’’ being measured contains 1) distortion 2) noise and
3) 50 or 60 cycle AC line hum
Line hum can be detected by using the ‘‘line sync’’ on the
oscilloscope (horizontal sync selector) The triggering of a
constant waveform indicates that AC line pick-up is present
This is usually the result of electro-magnetic coupling into
the preamplifiers input or improper test equipment ground-
ing which simply must be eliminated before making further
measurements
Input coupling problems can usually be corrected by any
one of the following solutions 1) shielding the source of the
magnetic field (using mu metal or steel) 2) magnetically
shielding the preamplifier 3) physically moving the pream-
plifier far enough away from the magnetic field or 4) using a
high pass filter (fo e 200 Hz–1 kHz) at the output of the
preamplifier to prevent any line signal from entering the dis-
tortion analyzer Ground loop problems can be solved by
rearranging ground connections of the circuit and test
equipment
Separating noise from distortion products is necessary
when it is desired to find the actual distortion and not the
signal-to-noise ratio of an amplifier The distortion produced
by the LM1837 is predominantly a second harmonic It is for
this reason that the third and higher order harmonics can be
filtered without resulting in any appreciable error in the mea-
surement The filter also reduces the amount of noise in the
measured data Another more tedious technique for mea-
suring THD is to use a wave analyzer Each harmonic is
measured and then summed in an rms calculation A typical
curve is plotted for distortion vs frequency using this meth-
od A typical curve is also included using a 20 Hz to 20 kHz
4th order filter
To specify the distortion of the LM1837 accurately and also
not require unusual or tedious measurements the following
method is used The output level is set to 1 Vrms at 1 kHz
(approximately 5 mV at the input) The output is filtered with
the circuit of
Figure 4 to limit the bandwidth of the noise and
measured with a standard distortion analyzer The analyzer
has a filter that is switched in to remove line hum and
ground loop pick-up as well as unrelated low frequency
noise The resulting measurement is fast and accurate
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio misinterpre-
tations of the numbers actually measured are common One
amplifier may sound much quieter than another but due to
improper testing techniques they appear equal in measure-
ments This is often the case when comparing integrated
circuit to discrete preamplifier designs Discrete transistor
preamps often ‘‘run out of gain’’ at high frequencies and
therefore have small bandwidths to noise as indicated in
Figure 5
TLH7902 – 13
FIGURE 5
Integrated circuits have additional open loop gain allowing
additional feedback loop gain in order to lower harmonic
distortion and improve frequency response It is this addi-
tional bandwidth that can lead to erroneous signal-to-noise
measurements if not considered during the measurement
process In the typical example above the difference in
bandwidth appears small on a log scale but the factor of 10
in bandwidth (200 kHz to 2 MHz) can result in a 10 dB
theoretical difference in the signal-to-noise ratio (white
noise is proportional to the square root of the bandwidth in a
system)
In comparing audio amplifiers it is necessary to measure the
magnitude of noise in the audible bandwidth by using a
‘‘weighting’’ filter 1 A ‘‘weighting’’ filter alters the frequency
response in order to compensate for the average human
ear’s sensitivity to certain undesirable frequency spectra
The weighting filters at the same time provide the bandwidth
limiting as discussed in the previous paragraph
The 32 Hz to 12740 Hz filter shown in
Figure 4 is a simple
two pole one zero filter approximately equivalent to a
‘’brick wall’’ filter of 20 Hz to 20 kHz This approximation is
absolutely valid if the noise has a flat energy spectrum over
the frequencies involved In other words a measurement of
a noise source with constant spectral density through either
of the two filters would result in the same reading The out-
put frequency response of the two filters is shown in
Figure
6
8
Obsolete
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| LM1851N | POWER SUPPLY SUPPORT CKT, PDIP8 |
| LM1851M | POWER SUPPLY SUPPORT CKT, PDSO8 |
| LM1865MX | FM, AUDIO DEMODULATOR, PDSO20 |
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM1837N | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Low Noise PREAMPLIFIER FOR AUTOREVERSING TAPE PLAYBACK SYSTEMS |
| LM1837N/A+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Preamp/Equalizer for Audio Tape Recorder |
| LM1837N/B+ | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Preamp/Equalizer for Audio Tape Recorder |
| LM185 | 制造商:Texas Instruments 功能描述: |
| LM185_08 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Adjustable Micropower Voltage References |
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