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BASIC MEASUREMENT SYSTEM
TPA3008D2
SLOS435A – MAY 2004 – REVISED JULY 2004
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 21 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
IN
), 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 impedance, R
OUT
, of the APA is normally in the hundreds
of milliohms and can be ignored for all but the power-related calculations.
Figure 21(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 21(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.
20
