CS42406
4.5 Analog Connections
The analog modulator samples the input at 6.144 MHz. The digital filter will reject signals within the stop-
band of the filter. However, there is no rejection for input signals which are multiples of the input sampling
frequency (n * 6.144 MHz), where n=0, 1, 2, ... Refer to Figure 36 which shows the suggested filter that
will attenuate any noise energy at 6.144 MHz, in addition to providing the optimum source impedance for
the modulators. The use of capacitors which have a large voltage coefficient (such as general purpose
ceramics) must be avoided since these can degrade signal linearity.
634 Ω
CS42406
470pF
C0G
VA
-
100kΩ
91 Ω
4.7 µF
AINL
+
Input1
*
2200pF
C0G
100k Ω
634 Ω
470pF
C0G
VA
-
100kΩ
91 Ω
4.7 µF
AINR
+
Input2
*
2200pF
C0G
100kΩ
*Place as close to
the CS42406 as
possible.
Figure 36. CS42406 Recommended Analog Input Buffer
4.5.1
Capacitor Size on the Reference Pin (FILT+)
The CS42406 requires an external capacitance on the internal reference voltage pin, ADC_FILT+. The
size of this decoupling capacitor will affect the low frequency distortion performance as shown in Figure
37, with larger capacitor values used to optimize low frequency distortion performance.
1 uF
2.2 uF
3.3 uF
4.7 uF
5.6 uF
6.8 uF
10 uF
22 uF
47 uF
100 uF
Figure 37. CS42406 ADC: THD+N versus Frequency
34
DS614PP5
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