CS42436 datasheet(52/66 Pages) CIRRUS

52 / 66 page

DS647PP2

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 20 and 21

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 22 and 23 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.

4.7

µF

100 k

Ω

10 k

Ω

100 k

Ω

100 k

Ω

0.1

µF

100

µF

470pF

470 pF

C0G

634

Ω

634

Ω

634

Ω

2700 pF

C0G

332

Ω

AINx+

AINx-

ADC1-3

Figure 20. Single to Differential Active Input Filter

470 pF

C0G

634

Ω

2700 pF

C0G

4.7

µF

100 k

Ω

100 k

Ω

100 k

Ω

4.7

µF

AIN1+,2+,3+,4+

AIN1-,2-,3-,4-

ADC1-2

ADC3

470 pF

C0G

634

Ω

2700 pF

C0G

4.7

µF

100 k

Ω

100 k

Ω

100 k

Ω

AIN5A,6A

470 pF

C0G

634

Ω

2700 pF

C0G

4.7

µF

100 k

Ω

100 k

Ω

100 k

Ω

AIN5B,6B

Figure 21. Single-Ended Active Input Filter

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