7 APPENDIX A: EXTERNAL FILTERS
7.1 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
multiples of the digital passband frequency (n × 6.144 MHz), where n=0,1,2,... Refer to Figures 26 and 27
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 28 and 29 for low cost, low
component count passive input filters. 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
470pF
ADC1-3
AINx+
C0G
-
91 Ω
4.7 µF
+
634 Ω
634 Ω
2700pF
C0G
100kΩ
470pF
C0G
VA
10kΩ
0.1 µF
100kΩ
91 Ω
-
AINx-
+
332 Ω
100 µF
100kΩ
Figure 26. Single to Differential Active Input Filter
634 Ω
VA
470 pF
ADC1-2
C0G
100 kΩ
4.7 µF
-
91 Ω
AIN1+,2+,3+,4+
+
2700 pF
C0G
100 kΩ
100 kΩ
AIN1-,2-,3-,4-
4.7 µF
634 Ω
91 Ω
VA
470 pF
C0G
ADC3
100 kΩ
4.7 µF
-
AIN5A,6A
+
2700 pF
C0G
100 kΩ
100 kΩ
634 Ω
91 Ω
VA
470 pF
C0G
100 kΩ
4.7 µF
-
AIN5B,6B
+
2700 pF
C0G
100 kΩ
100 kΩ
Figure 27. Single-Ended Active Input Filter
56
DS648PP2
CIRRUS [ CIRRUS LOGIC ]
