SAA7715 Datasheet, PDF (10/36 Pages) NXP Semiconductors – Digital Signal Processor

Philips Semiconductors
Digital Signal Processor
Preliminary specification
SAA7715H
8.3 The Filter Stream DAC (FSDAC)
The FSDAC is a semi-digital reconstruction filter that
converts the 1-bit data stream of the noise shaper to an
analog output voltage. The filter coefficients are
implemented as current sources and are summed at
virtual ground of the output operational amplifier. In this
way very high signal-to-noise performance and low clock
jitter sensitivity is achieved. A post-filter is not needed due
to the inherent filter function of the DAC. On-board
amplifiers convert the FSDAC output current to an output
voltage signal capable of driving a line output.
The output voltage of the FSDAC scales proportionally
with the power supply voltage.
8.3.1 INTERPOLATION FILTER
The digital filter interpolates from 1 to 64f_s by means of a
cascade of a recursive filter and an FIR filter.
Table 5 Digital interpolation filter characteristics
ITEM
Pass band ripple
Stop band
Dynamic range
Gain
CONDITIONS
0 to 0.45f_s
>0.55f_s
0 to 0.45f_s
DC
VALUE (dB)
±0.03
−50
116.5
−3.5
8.3.2 NOISE SHAPER
The 5th-order noise shaper operates at 64f_s. It shifts
in-band quantization noise to frequencies well above the
audio band. This noise shaping technique enables high
signal-to-noise ratios to be achieved. The noise shaper
output is converted into an analog signal using a filter
stream digital-to-analog converter.
8.3.4 POWER OFF PLOP SUPPRESSION
To avoid plops in a power amplifier, the supply voltage of
the analog part of the DAC and the rest of the chip can be
fed from a separate supply of 3.3 V. A capacitor connected
to this supply enables to provide power to the analog part
at the moment the digital voltage is switching off fast. In
this event the output voltage will decrease gradually
allowing the power amplifier some extra time to switch off
without audible plops.
8.3.5 PIN VREFDA FOR INTERNAL REFERENCE
With two internal resistors half the supply voltage V_DDA1 is
obtained and used as an internal reference. This reference
voltage is used as DC voltage for the output operational
amplifiers and as reference for the DAC. In order to obtain
the lowest noise and to have the best ripple rejection, a
filter capacitor has to be added between this pin and
ground, preferably close to the analog pin V_SSA1.
8.3.6 SUPPLY OF THE ANALOG OUTPUTS
The entire analog circuitry of the DACs and the OPAMPS
are supplied by 2 supply pins, V_DDA1 and V_SSA1. The
V_DDA1 must have sufficient decoupling to prevent THD
degradation and to ensure a good Power Supply Rejection
Ratio (PSRR). The digital part of the DAC is fully supplied
from the chip core supply.
8.4 External control pins
The flags DSP_INOUT5 to DSP_INOUT7 are available as
external pins. The flags can be used by the DSP
depending on the downloaded software.
8.3.3 FUNCTION OF PIN POM
With pin POM it is possible to switch off the reference
current of the DAC. 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 lower than the current loading after
the voltage on pin POM has passed a particular level. This
results in an almost dB-linear behaviour. This prevents
‘plop’ effects during power on/off.
2001 May 07
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