LM4782TABD/NOPB National Semiconductor, LM4782TABD/NOPB Datasheet - Page 20

• Manufacturer Part Number: LM4782TABD/NOPB
• Manufacturer: National Semiconductor

Specifications of LM4782TABD/NOPB

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Application Information
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet
the design targets of an application. The choice of external
component values that will affect gain and low frequency
response are discussed below.
The gain of each amplifier is set by resistors R
non-inverting configuration shown in Figure 1. The gain is
found by Equation (5) below:
For best noise performance, lower values of resistors are
used. A value of 1kΩ is commonly used for R
setting the value of R
the gain should be set no lower than 10V/V and no higher
than 50V/V. Gain settings below 10V/V may experience
instability and using the LM4782 for gains higher than 50V/V
will see an increase in noise and THD.
The combination of R
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation (6) shown below:
If an input coupling capacitor is used to block DC from the
inputs as shown in Figure 5, there will be another high pass
filter created with the combination of C
using a input coupling capacitor R
bias point on the amplifier’s input terminal. The resulting
-3dB frequency response due to the combination of C
R
With large values of R
outputs when the inputs are left floating. Decreasing the
value of R
oscillations. If the value of R
C
frequency response.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such
as C
performance. However, electrolytic capacitors are less linear
than other premium capacitors. Higher THD+N performance
may be obtained by using high quality polypropylene capaci-
tors in the signal path. A more cost effective solution may be
the use of smaller value premium capacitors in parallel with
the larger electrolytic capacitors. This will maintain signal
quality in the upper audio band where any degradation is
most noticeable while also coupling in the signals in the
lower audio band for good bass response.
Distortion is introduced as the audio signal approaches the
lower -3dB point, determined as discussed in the section
above. By using larger values of capacitors such that the
-3dB point is well outside of the audio band will reduce this
distortion and improve THD+N performance.
Increasing the value of the large supply bypass capacitors
will improve burst power output. The larger the supply by-
pass capacitors the higher the output pulse current without
supply droop increasing the peak output power. This will also
increase the headroom of the amplifier and reduce THD.
IN
IN
can be found from Equation (7) shown below:
will need to increase in order to maintain the same -3dB
IN
and C
IN
or not letting the inputs float will remove the
i
(see Figures 1 - 5) will result in very good
f
IN
A
f
V
i
= 1 / (2πR
f
= 1 / (2πR
i
IN
= 1 + R
for the desired gain. For the LM4782
with C
oscillations may be observed on the
IN
i
is decreased then the value of
f
(see Figure 1) creates a high
IN
/ R
i
C
C
i
IN
) (Hz)
i
IN
(V/V)
is needed to set the DC
) (Hz)
IN
(Continued)
and R
f
and R
i
IN
and then
. When
i
for the
IN
and
(5)
(6)
(7)
20
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 designs to discrete amplifier designs. Discrete transis-
tor amps often “run out of gain” at high frequencies and
therefore have small bandwidths to noise as indicated below.
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 10in
bandwidth, (200kHz to 2MHz) can result in a 10dB theoreti-
cal 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 (Note 19). A “weighting” filter alters the
frequency response in order to compensate for the average
human ear’s sensitivity to the frequency spectra. The weight-
ing filters at the same time provide the bandwidth limiting as
discussed in the previous paragraph.
Note 19: CCIR/ARM: A Practical Noise Measurement Method; by Ray
Dolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3).
In addition to noise filtering, differing meter types give differ-
ent noise readings. Meter responses include:
1. RMS reading,
2. average responding,
3. peak reading, and
4. quasi peak reading.
Although theoretical noise analysis is derived using true
RMS based calculations, most actual measurements are
taken with ARM (Average Responding Meter) test equip-
ment.
Typical signal-to-noise figures are listed for an A-weighted
filter which is commonly used in the measurement of noise.
The shape of all weighting filters is similar, with the peak of
the curve usually occurring in the 3kHz–7kHz region.
LEAD INDUCTANCE
Power op amps are sensitive to inductance in the output
leads, particularly with heavy capacitive loading. Feedback
to the input should be taken directly from the output terminal,
minimizing common inductance with the load.
Lead inductance can also cause voltage surges on the sup-
plies. With long leads to the power supply, energy is stored in
the lead inductance when the output is shorted. This energy
can be dumped back into the supply bypass capacitors when
the short is removed. The magnitude of this transient is
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