Datasheet

LM2876

SNAS088C –AUGUST 1995–REVISED MARCH 2013

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Equation 1 and Equation 4 are the only equations needed in the determination of the maximum heat sink thermal

resistance. This is of course given that the system designer knows the required supply voltages to drive his rated

load at a particular power output level and the parameters provided by the semiconductor manufacturer. These

parameters are the junction to case thermal resistance, θ

, T

Jmax

= 150°C, and the recommended Thermalloy

Thermacote thermal compound resistance, θ

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 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 (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 3 kHz–7 kHz

region as shown below.

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