CS4270 Datasheet, PDF (28/48 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 stopband 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 12 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 lin-
earity.
Analog
Input
R1
10 µF
R2
AINx
2200 pF
CS4270
Figure 12. CS4270 Recommended Analog Input Network
Three parameters determine the values of resistors R1 and R2 as shown in Figure 12: source impedance,
attenuation, and input impedance. Source impedance is defined as the impedance as seen from the ADC
looking back into the signal network. Analog performance is optimized for small source impedance and a
source impedance above 2.5 kΩ results in degraded THD+N.
The required attenuation factor depends on the magnitude of the input signal. The full-scale input voltage
scales with VA; for VA = 5 V, the CS5344 full-scale input magnitude is 1 Vrms. R1 and R2 should be set
such that an input signal greater than the full-scale input should be attenuated to the appropriate magni-
tude. Typical line-level voltage in audio applications is 2 Vrms, in which case R1 and R2 must combine to
form an attenuation factor of 2, thus giving the CS5344 a 1 Vrms input.
Input impedance is the impedance from the signal source to the ADC analog input pins. The target input
impedance depends on the overall system specifications, but typical audio systems require an input im-
pedance of 10 kΩ. Table 6 shows the input parameters and the associated design equations. Figure 13
illustrates an example configuration for a source impedance of 46 Ω, an attenuation factor of 1, and input
impedance of 9.8 kΩ.
Source Impedance
(_---R_----1----×_-----R_----2----)
R1 + R2
Attenuation Factor
R^----1---R--×^--2---R^----2-
Input Impedance
(R1 + R2)
Table 6. Analog Input Design Parameters
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