8 APPENDIX A: 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
multiples 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 which have a large voltage coefficient (such
as general-purpose 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
470pF
ADC1-2
C0G
100k Ω
4.7 µF
-
91 Ω
AIN1+,2+,3+,4+
+
2700pF
C0G
100k Ω
100k Ω
AIN1-,2-,3-,4-
4.7 µF
Figure 20. Single-Ended Active Input Filter
DS673PP2
49
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