CS4270_06 Datasheet, PDF (28/49 Pages) Cirrus Logic – 24-Bit, 192 kHz Stereo Audio CODEC

5.4 Analog Connections
CS4270
5.4.1 Input 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 15 which shows the recommended topology
of the analog input network. The capacitor values chosen not only provide the appropriate filtering of noise
at the modulator sampling frequency, but also act as a charge source for the internal sampling circuits.
The use of capacitors which have a large voltage coefficient (such as general purpose ceramics) must be
avoided since these can degrade signal linearity.
Analog
Input
R1
10 µF
R2
AINx
220 pF
CS4270
Figure 15. CS4270 Recommended Analog Input Network
Three parameters determine the values of resistors R1 and R2 as shown in Figure 15 source impedance,
attenuation, and input impedance. Table 6 shows the design equation used to determine these values.
Source Impedance: Source impedance is defined as the impedance as seen from the ADC looking back
into the signal network. The ADC achieves optimal THD+N performance when source impedance is min-
imized and THD+N degrades for source impedance greater than 1 kΩ. See Figure 16 and 17 below.
Figure 16. A/D THD+N Performance vrs. Input Source Resistance
28
DS686PP1