SAA7712H Datasheet, PDF (13/44 Pages) NXP Semiconductors – Sound effects DSP

Philips Semiconductors
Sound effects DSP
Preliminary specification
SAA7712H
8.1.5 PERFORMANCE
The signed-magnitude noise-shaped DAC has a dynamic
range in excess of 100 dB. The signal-to-noise ratio of the
audio output at full-scale is determined by the word length
of the converter. The noise at low outputs is fully
determined by the noise performance of the DAC. Since it
is a signed-magnitude type, the noise at digital silence is
also low. As a disadvantage, the total THD is higher than
conventional DACs. The typical total harmonic
distortion-plus-noise to signal ratio as a function of the
output level is shown in Fig.6.
8.1.6 POWER-ON MUTE (POM)
To avoid any uncontrolled noise at the audio outputs after
power-on of the IC, the reference current source of the
DAC is switched off. The capacitor on pin POM
determines the time after which this current has a soft
switch-on. So at power-on the current audio signal outputs
are always muted. The loading of the external capacitor is
done in two stages via two different current sources.
The loading starts at a current level that is 9 times lower
than the current loading after the voltage on pin POM has
passed the 1 V level. This results in an almost dB linear
behaviour.
handbook, half^−p²ag⁰e
(THD + N)/S
(dB)
−40
−60
MGS211
8.1.7 POWER-OFF PLOP SUPPRESSION
Power should still be provided to the analog part of the
DAC, while the digital part is switching off. As a result, the
output voltage will decrease gradually allowing the power
amplifier some extra time to switch-off without audible
plops. If a 5 V power supply is present, the supply voltage
of the analog part of the DAC can be fed from the 5 V
power supply via a 1.8 V zener diode. A capacitor,
connected to the 3.3 V power supply, provides power to
the analog part when the 5 V power supply is switching off
fast.
−80
−80
−60
−40
−20
0
output level (dB)
Fig.6 Typical (THD + N)/S curve as a function of
the output level.
1999 Aug 05
13