Cirrus Logic AN249 - Manual (Page 8)

AN249
8
Some applications may require signal attenuation prior to the converter. The full-scale input voltage of the
CS5340 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. Typical consumer audio line level outputs range from 1.5 to 2 Vrms.
Figure 6 shows a passive input filter for the CS5340 and the resulting THD+N over the audio bandwidth.
This filter provides 6 dB of signal attenuation. Due to the relatively high input impedance on the CS5340
analog inputs, the full distortion performance of the CS5340 cannot be realized. Also, the combination of
the series resistor and the biasing resistor-divider circuit will determine the input impedance into the input
filter. In the circuit shown in Figure 6, the input impedance is approximately 5 k
Ω. By doubling the resistor
values, the input impedance will increase to 10 k
Ω. However, in this case the distortion performance will
drop to approximately 70 dB due to the increase in series resistance on the CS5340 analog inputs.
Figure 6. CS5340 Passive Input Filter, Solution 2
-105
-60
-
102.5
-100
-97.5
-95
-92.5
-90
-87.5
-85
-82.5
-80
-77.5
-75
-72.5
-70
-67.5
-65
-62.5
d
B
F
S
20 20k 50 100 200 500 1k 2k 5k 10k
Hz
CS5340
VA
AIN
2200 pF
C0G
10 µF
5 k Ω
5 k Ω
100 k Ω
2.5 k
Ω