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Typically bench measurements are performed using Audio Precision Analyzers. In the case of the AP2700 family of analyzers, there will be a Programmable Serial Interface Adapter (PSIA) which is used to drive and receive I2S data. When monitoring the Class-D outputs of audio amplifiers, it is also necessary to use a filter to assist the analyzer by resolving the output switching waveform.
When selecting a load, it is critical to use a resistor capable of easily handling the full-scale output voltage of the amplifier. Any changes in nominal resistance due to heating will reduce measurement accuracy. Additionally, wires used to connect the load need to be selected to minimize resistive losses at high output power.
Please refer to Figure 1-1 for a typical hardware setup:
After connecting the AP analyzer as shown above, configure the analyzer software to use the following settings:
Once the analyzer is setup, configure the amplifier to generate IV sense data. This requires you to enable IV sense and then configure the I2S outputs to transmit both Isense and Vsense data. This can be easily done using the Pure Path Console 3 software associated with the EVM.
It is critical to the reliability of this test that the load resistance be a known calibrated value. It is best to measure this using a 4-wire connection for the most accurate value. Several calculations will depend on the accuracy of this value.
The test will proceed in a stepwise manner across output power. At each power level of concern, the following data should be recorded:
Measuring both Voltages and Digital inputs require changing the AP Input source between the HiRes A/D Converter and the Digital inputs.
Proceed to capture each of the above values, step the output power to the next amplitude, and repeat until the full output range under test is captured. However, in this configuration, the pilot tone amplitude is a scalar ratio of the fundamental. You only want to increase the magnitude of the fundamental, so this ratio will require adjustment at each step. Setting the ratio and the amplitude in units of dB will allow for easy adjustments. As long as the sum of the ratio and amplitude is kept constant, the pilot tone amplitude should remain fixed.
Once data collection is complete, you can perform calculations for each output level to determine accuracy.
It is necessary to determine the full scale voltage and current values from the data sheet of the amplifier being used. For example, you can pull this information for TAS2562 in Figure 2-3: