W681513 Datasheet, PDF (30/33 Pages) Winbond – 5V SINGLE-CHANNEL VOICEBAND CODEC FOR USB APPLICATIONS

W681513
earphone (150 Ώ impedance) in a typical handset, the output gain from the Power Amp is set to ~0.16
for a satisfactory listening level.
In the VoIP telephone, or small wireless phones, the plastic case is typically too small to provide a
reasonable acoustic chamber. Thus the output from the microphone is less than in the previous
example. This results in having to set the input gain of the CODEC to ~75 to 90 and in a comparable
signal level to the receive telephone handset but, because of the increased gain, the Signal-to-Noise
Ratio (SNR) has decreased and the signal sounds noisier. On the receive side, the gain is set as in
the previous example. When the Power Amp gain is as low as 0.16 a 32 ohm load speaker can be
driven.
Resistor R9 sets the sidetone level (the signal fed back to the earpiece from the microphone so the
telephone sounds “live”) to the level desired by the designer.
Capacitors C6 and C7 are introduced for external compensation to keep the input amplifier stable at
such high gain figures and prevent oscillation. These capacitors are not needed when the gain is
close to unity or less than unity.
- 30 -