CS42435
8. EXTERNAL FILTERS
8.1
ADC Input Filter
The analog modulator samples the input at 6.144 MHz (internal MCLK=12.288 MHz). The digital filter will
reject signals within the stopband of the filter. However, there is no rejection for input signals which are mul-
tiples of the digital passband frequency (n × 6.144 MHz), where n=0,1,2,... Refer to Figures 19 and 20 for
a recommended analog input filter that will attenuate any noise energy at 6.144 MHz, in addition to providing
the optimum source impedance for the modulators. Refer to Figures 21 and 22 for low-cost, low-component-
count passive input filters. The use of capacitors that have a large voltage coefficient (such as general-pur-
pose ceramics) must be avoided since these can degrade signal linearity
634 Ω
470 pF
ADC1-2
AINx+
C0G
-
91 Ω
4.7 μF
+
634 Ω
634 Ω
2700 pF
C0G
100 kΩ
470 pF
C0G
VA
10 kΩ
0.1 μF
91 Ω
100 kΩ
-
AINx-
+
332 Ω
100 μF
100 kΩ
Figure 19. Single-to-Differential Active Input Filter
634 Ω
VA
470 pF
ADC1-2
AIN1+,2+,3+,4+
C0G
100 kΩ
4.7 μF
-
91 Ω
+
2700 pF
C0G
100 kΩ
100 kΩ
AIN1-,2-,3-,4-
4.7 μF
Figure 20. Single-Ended Active Input Filter
DS685F1
49
CIRRUS [ CIRRUS LOGIC ]
