LM4782 datasheet(20/27 Pages) NSC

20 / 27 page

Application Information (Continued)

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

f and Ri for the

non-inverting configuration shown in Figure 1. The gain is

found by Equation (5) below:

V =1+Rf /Ri (V/V)

(5)

For best noise performance, lower values of resistors are

used. A value of 1k

Ω is commonly used for R

i and then

setting the value of R

f for the desired gain. For the LM4782

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

i with Ci (see Figure 1) creates a high

pass filter. The low frequency response is determined by

these two components. The -3dB point can be found from

Equation (6) shown below:

i =1/(2

πR

iCi) (Hz)

(6)

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

IN and RIN. When

using a input coupling capacitor R

IN is needed to set the DC

bias point on the amplifier’s input terminal. The resulting

-3dB frequency response due to the combination of C

IN and

IN can be found from Equation (7) shown below:

IN =1/(2

πR

INCIN) (Hz)

(7)

With large values of R

IN oscillations may be observed on the

outputs when the inputs are left floating. Decreasing the

value of R

IN or not letting the inputs float will remove the

oscillations. If the value of R

IN is decreased then the value of

IN will need to increase in order to maintain the same -3dB

frequency response.

HIGH PERFORMANCE CONSIDERATIONS

Using low cost electrolytic capacitors in the signal path such

as C

IN and Ci (see Figures1-5) will result in very good

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.

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.

20081199

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:

RMS reading,

average responding,

peak reading, and

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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