Datasheet

TPA3101D2

SLOS473C –DECEMBER 2005–REVISED SEPTEMBER 2007

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BASIC MEASUREMENT SYSTEM

This application note focuses on methods that use the basic equipment listed below:

• Audio analyzer or spectrum analyzer

• Digital multimeter (DMM)

• Oscilloscope

• Twisted-pair wires

• Signal generator

• Power resistor(s)

• Linear regulated power supply

• Filter components

• EVM or other complete audio circuit

Figure 35 shows the block diagrams of basic measurement systems for class-AB and class-D amplifiers. A sine

wave is normally used as the input signal because it consists of the fundamental frequency only (no other

harmonics are present). An analyzer is then connected to the APA output to measure the voltage output. The

analyzer must be capable of measuring the entire audio bandwidth. A regulated dc power supply is used to

reduce the noise and distortion injected into the APA through the power pins. A System Two audio measurement

system (AP-II) (Reference 1) by Audio Precision includes the signal generator and analyzer in one package.

The generator output and amplifier input must be ac-coupled. However, the EVMs already have the ac-coupling

capacitors, (C

), so no additional coupling is required. The generator output impedance should be low to avoid

attenuating the test signal, and is important because the input resistance of APAs is not high. Conversely, the

analyzer-input impedance should be high. The output resistance, R

OUT

, of the APA is normally in the hundreds of

milliohms and can be ignored for all but the power-related calculations.

Figure 35(a) shows a class-AB amplifier system. It takes an analog signal input and produces an analog signal

output. This amplifier circuit can be directly connected to the AP-II or other analyzer input.

This is not true of the class-D amplifier system shown in Figure 35(b), which requires low-pass filters in most

cases in order to measure the audio output waveforms. This is because it takes an analog input signal and

converts it into a pulse-width modulated (PWM) output signal that is not accurately processed by some

analyzers.

Product Folder Link(s): TPA3101D2