Measure an amplifier's THD without external filters

摘要

Measuring an op amp's THD (total harmonic distortion) at frequencies below 100 kHz is problematic because modern amplifiers have a THD rating that is much lower than that of most test equipment. The circuit in this Design Idea uses active noise amplification to eliminate the need for external filters. This approach allows you to measure THD at several orders of magnitude lower than the resolution of the test fixture you are using. Many THD test circuits use passive-amplification techniques to force the DUT (device under test) to correct itself. This approach can generate considerable error by terminating the distortion signal with resistive loads. A preferred way to generate accurate data is to isolate the DUT and measure it using a high-impedance buffer. This method requires active amplification of the distortion at the amplifier's output. You can use a secondary operational amplifier for active amplification (Figure 1).