CS42438_06 Datasheet, PDF (51/62 Pages) Cirrus Logic – 108 dB, 192 kHz 6-in, 8-out TDM CODEC

8.1.1
CS42438
Passive Input Filter
The passive filter implementation shown in Figure 22 will attenuate any noise energy at 6.144 MHz but
will not provide optimum source impedance for the ADC modulators. Full analog performance will there-
fore not be realized using a passive filter. Figure 22 illustrates the unity gain, passive input filter solution.
In this topology the distortion performance is affected, but the dynamic range performance is not limited.
100 kΩ
150 Ω
10 µF
2700 pF
C0G
ADC1-2
AIN1+,2+,3+,4+
4.7 µF
AIN1-,2-,3-,4-
100 kΩ
150 Ω
10 µF
2700 pF
C0G
ADC3
AIN5A,6A
100 kΩ
150 Ω
10 µF
2700 pF
C0G
AIN5B,6B
Figure 22. Passive Input Filter
8.1.2
Passive Input Filter w/Attenuation
Some applications may require signal attenuation prior to the ADC. The full-scale input voltage will scale
with the analog power supply voltage. For VA = 5.0 V, the full-scale input voltage is approximately
2.8 Vpp, or 1 Vrms (most consumer audio line-level outputs range from 1.5 to 2 Vrms).
Figure 23 shows a passive input filter with 6 dB of signal attenuation. Due to the relatively high input im-
pedance on the analog inputs, the full distortion performance cannot be realized. Also, the resistor divider
circuit will determine the input impedance into the input filter. In the circuit shown in Figure 23, the input
impedance is approximately 5 kΩ. By doubling the resistor values, the input impedance will increase to
10 kΩ. However, in this case the distortion performance will drop due to the increase in series resistance
on the analog inputs.
DS646F1
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