Harmonic Distortion: Part II Understanding The Effects Of Closed-Loop Gain And Common-Mode Errors On THD+N
by Jorge Vega, Characterization Engineer, Texas Instruments
High-precision, low-noise amplifiers are often used in numerous audio and sensor applications. These applications benefit from the ultra-low distortion values these amplifiers provide. The ultra-low distortion benefits, however, can decrease depending on the amplifier’s closed-loop gain configuration and the common-mode signal applied. Applications using a closed-loop gain greater than one will see degraded total harmonic distortion plus noise (THD N). Likewise, applications using a large common-mode signal may also have degraded THD N because of common-mode rejection ratio error.
This TechNote presents the relationship between closed-loop gain and THD N. This in turn explains the differences between THD N versus frequency traces in closed-loop gains of 1 V/V, –1 V/V and 10 V/V. This relationship is best presented through the concept of noise gain because the higher the noise gain, the less the amplifier’s negative feedback can correct for distortion factors or non-linear terms, resulting in worsened THD N. To this effect, a brief review on the concept of noise gain is given. Furthermore, the effects of a large common-mode signal are also explained.