Datasheet

LM4782, LM4782TABD

SNAS231B –FEBRUARY 2004–REVISED MARCH 2013

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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

bypass 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, misinterpretations 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 measurements. This is often the case when comparing integrated circuit designs to discrete

amplifier designs. Discrete transistor 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 additional 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 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 (see Note below). A “weighting” filter alters the frequency response in order to

compensate for the average human ear's sensitivity to the frequency spectra. The weighting filters at the same

time provide the bandwidth limiting as discussed in the previous paragraph.

NOTE

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 different 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 equipment.

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.

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