SI2457-D-FT Silicon Laboratories Inc, SI2457-D-FT Datasheet - Page 55

• Manufacturer Part Number: SI2457-D-FT
• Description: IC ISOMODEM SYSTEM-SIDE 24TSSOP
• Manufacturer: Silicon Laboratories Inc

Specifications of SI2457-D-FT

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

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
Rev. 1.3
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