10 Input

As mentioned in the modulation section, PWM is fundamental to a Class-D amplifier. However, some amplifiers do not include a modulator and instead receive an already-modulated audio signal as input, making them PWM-input Class-D amplifiers. Often these amplifiers are referred to as digital-input amplifiers, because they are to be paired with modulators that have an integrated DAC. Alternatively, some amplifiers integrate a DAC and PWM internally to take direct digital-input. Of course, between these two are analog-input amplifiers which integrate the PWM but not a DAC. See Figure 10-1 to understand the different input types of Class-D audio amplifiers.

Figure 10-1 Class-D Amplifier Input Topologies

Digital-input formats vary, but the most common areinter-IC-sound (I2S), time division multiplexing (TDM), pulse-density modulation (PDM) and SoundWire. The most common amongst those is I2S, which has universal standards unlike other formats such as TDM, which can vary. The different formats handle data transfer and data clocking differently, making some better for systems with multiple sources, for example.

Analog-input devices are a bit simpler, but still have options. Analog-input can be in the form of single-ended or differential signals. Differential input uses two pins for the high (+) and low (-) of the signal. The differential signal is then the difference between the two inputs, thus canceling out any common voltage on the two pins. This effectively reduces any EMI or noise imposed on both signals. Single-ended input on the other hand has just one input pin and then a common pin, making it more susceptible to passing EMI and noise to the amplification stage.