SLOS358C September 2011 – April 2020 THS4531
At 1 MHz and above, the circuit shown in Figure 67 is used to measure harmonic, intermodulation distortion, and output impedance of the amplifier.
A signal generator is used as the signal source and the output is measured with a spectrum analyzer. The output impedance of the signal generator is 50 Ω and is AC coupled. RIT and RG are chosen to impedance match to 50 Ω and maintain the proper gain. To balance the amplifier, a 0.22 µF cap and 49.9 Ω resistor to ground is inserted across RIT on the alternate input. A low-pass filter is inserted in series with the input to reduce harmonics generated at the signal source. The level of the fundamental is measured and then a high-pass filter is inserted at the output to reduce the fundamental so it does not generate distortion in the input of the spectrum analyzer.
Distortion in the audio band is measured using an audio analyzer. Refer to audio measurement section for detail.