SAA7712 datasheet(13/44 Pages) PHILIPS

13 / 44 page

1999 Aug 05

Philips Semiconductors

Preliminary speciﬁcation

Sound effects DSP

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.

Fig.6

Typical (THD + N)/S curve as a function of

the output level.

handbook, halfpage

−80

−60

−40

MGS211

−20

−60

−80

−40

−20

(THD

+ N)/S

(dB)

output level (dB)

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.

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.

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