Cirrus Logic AN306 - Manual (Page 4)

4 AN306REV1
AN306
6. OUT-OF-BAND PERFORMANCE
Converters that rely on an external Master Clock signal are often only able to operate with an internal over-sampling
rate of 128 times the sample rate (128 x Fs). This can limit the performance of the modulator by requiring the mod-
ulator noise shaping to begin rising at a lower frequency than would be possible if a higher modulator rate were avail-
able.
Because the CS4350’s circuitry has access to the internal high-frequency output of the PLL, it is able to achieve a
4x higher modulator rate than typical audio converters. This allows for much lower out-of-band noise than is typically
achievable in a 128x over-sampling DAC. See
Figure 4 and Figure 5 .
As a result, the CS4350 is able to achieve lower noise within much wider frequency band than many similar typical
DACs that require an external Master Clock signal. This can provide significant benefits for applications that are sen-
sitive to noise outside of the typical 20 kHz audio band. Achieving similar out-of-band performance using a typical
128x over-sampling DAC would require high-order off-chip analog filtering.
7. CS4350 ANALOG ARCHITECTURE
The CS4350 implements a current-based DAC with on chip current-to-voltage (I-to-V) conversion and a continuous
time summing amplifier. It offers both differential and single-ended analog output options with an analog supply of
3.3 V or 5 V. For design flexibility, the full-scale output reference level is set by an internal fixed reference to allow
operation with a 2 Vrms differential, or 1 Vrms single-ended output, regardless of changes to the VA supply.
8. CONCLUSIONS
The CS4350 PLL DAC offers a number of advantages relative to conventional digital-to-analog converters, each of
which can be instrumental in simplifying the design of audio systems. From flexible output options, to localized Mas-
ter Clock recovery and distribution, to low out-of-band noise performance, to system clock jitter independence, to
the easing of PCB layout and EMI concerns, the CS4350 can be called upon to solve many real-world problems of
audio system designers.
For more information about the CS4350 PLL DAC, please refer to the CS4350 datasheet available at www.cir-
rus.com .
-150
+0
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
V
20k 120k 40k 60k 80k 100k
Hz
Figure 4. Typical 128x DAC Out-of-Band Noise Figure 5. CS4350 Out-of-Band Noise
-150
+0
-140
-130
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
d
B
V
20k 120k 40k 60k 80k 100k
Hz