A class AD audio
amplifier system (10) with DC output detection logic (26) is disclosed. The
amplifier system (10) includes multiple audio channels (20), each of which includes a pulse-width-modulator (PWM) (24). The DC detection logic (26) includes a sigma-
delta modulator (60) and a digital low-pass filter (62) that monitors the PWM output signals from the PWM modulators (24). The sigma-
delta modulator (60) operates at a first
clock frequency, while the low-pass filter (62) operates at a much lower
clock frequency, so that AC audio components, PWM
harmonics, and sigma-
delta quantization error is suppressed from the DC detection. The modulated filtered
signal is compared against a threshold level (THRSH) to determine whether the amplitude of a DC component at the PWM output is sufficiently high to constitute a fault. If so, a fault detection
signal (DC_DET) is issued, and the PWM modulators (24) are disabled to prevent unsafe conditions in the
system (10).