Philips CD721 - Page 33

Noise shaper
The 2nd-order digital noise shaper converts the 18bit
data at 96fs into a 5-bit bitstream, while shifting in-band
quantisation noise to frequencies well above the audio
band. For low sinal levels the noise shaper output is a
one-bitstream. This noise shaping technique used in
combination with a special data code and bitstream
converter enables extremely high signal-to-noise ratios to
be achieved.
Data encoder
The data encoder converts the five-bit 2’s complement
output data from the noise shaper to a 32-bit thermome-
ter code.
In traditional unidirectional current converters, half the
full-scale current flows to the output during small signal
reproduction. The thermal noise and substrate crosstalk
components, present in this current, severely restrict the
dynamic range which can be attained. In this BCC-DAC
true low-noise performance is achieved using a special
data code and bidirectional current sources. The special
data code guarantees that only small values of current
flow to the output during small-signal passages while
larger. positive or negative signals are generated using
the bidirectional sources. Furthermore, for every change
in the 18 bit input sample only one current source or
current sink is switched on. This intrinsically monotonic
thermometer code ensures the high differential linearity,
zero crossover distortion and superior signal-to-noise
ratio associated with bitstream conversion.
Continuous Calibration D/A converter.
The DAC currents(16 sources and 16 sinks) of each
channel are repeatedly generated from one single refe-
rence current. See description of CC-DAC TDA1545.
Operational amplifiers
High precision, low-noise amplifiers together with the
internal conversion resistors RCONV1 and RCONV2
convert the converter output current to a voltage capable
of driving a line output. This voltage is available at VOL
and VOR(1.5Vrms typical). Connecting external capaci-
tors CEXT1 and CEXT2 between resp. IOL and VOL,
IOR and VOR provides the required first order postfilte-
ring for the left and right channels. The RC combinations
determine the first-order fall-off frequencies.
Internal reference circuitry
Internal reference circuitry ensures that the output volta-
ge signal is proportional to the supply voltage, thereby
maintaining maximum dynamic range for supply voltages
from 3.4 to 5.5V.
PCS 60 968