3.4.7 ClassD Audio Interface

The SAM9X75 Curiosity Pro features a Class D audio amplifier output connected to 3-pin header J82.

The Class D output drives a speaker through an LC low-pass filter. The design includes a high-pass filter capacitor (100 µF), a low-pass filter capacitor (470 nF) and a low-pass filter inductor (56 µH).

Design parameters:

  • Speaker impedance: RSPK = 8Ω
  • High-pass filter cutoff frequency: fHP = 1/(2×π×100 µF×8Ω) ≈ 199 Hz
  • Low-pass filter cutoff frequency: fC = 1/(2×π×√(56 µH×470 nF)) ≈ 31 kHz
  • Quality factor of the LC filter: Q = 8Ω×√(470 nF/56 µH) ≈ 0.73
Figure 3-31. Class D Audio