Out-of-band noise from sources like DACs and PWM outputs can increase the noise floor of an APA (audio power amplifier). Noise like this includes high frequency, high slew rate components that can be down-shifted into the audio band by switching modulation or intermodulation through amplifier nonlinearities, and this can increase the noise floor of an APA. When this happens, low-pass filtering is required to attenuate the out-of-band noise to eliminate the added noise.
- A TI applications paper, SLOA128, explains how to design both low- and high-pass filters using TPA2016D2 as an example. The techniques described in this paper can be applied to all APAs.
Various TI APAs like TPA2013D1 and TPA2100P1 include internal low-pass input filters to reject out-of-band noise. Generally these filters eliminate any need for external filtering and reduce design, material and assembly costs.
There are still a few cases in which passive RC filters may be needed.
- Out-of-band noise of sigma-delta DACs is among the highest. Occasionally, in severe cases, it may be necessary to add passive RC low-pass filters to avoid added noise.
- Passive RC low-pass filters generally must be used with PWM sources, even with devices with internal filters. Slew rates of PWM outputs are so high that they generally must be attenuated before the internal filters can handle them.