Silicon Laboratories SI2494/39, SI2493/57/34/15/04 manual Audio Filter Response, Audio Quality

AN93

4.5.2. Audio Quality

The mulipole filter illustrated in this diagram is designed to shape the response for a pleasant sound and remove interference, but note that, when PWM is demodulated in this way, it carries all the audio spectrum noise that is present in the power supply of the modem minus 6 dB. This requires VCC to be as clean as one wants the call progress audio to be. An alternative is for the AOUT signal to be buffered to a clean supply domain using a logic gate or transistor buffer.

The 3-pole low-pass filter, with a 3 dB point at approximately 2 kHz, filters the 32 kHz square wave from AOUT and allows only audio signals below 2 kHz to pass. See Figure 24 below. The amplifier provides differential speaker drive, eliminating the need for a large coupling capacitor. Some additional design work and optimization must be done to select the optimum gain and frequency response of this circuit, depending on speaker efficiency, final product enclosure, and performance requirements. A two- or even one-pole filter may be adequate in some applications.

Keep this audio circuitry well away from digital signals and use generous ground fill in the PCB layout.

Figure 24. Audio Filter Response

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