Cirrus Logic AN311 - Manual (Page 2)

2 AN311REV1
AN311
Figure 2 illustrates each Sinc filter function from Sinc 1 to Sinc 5 with the frequency axis scaled logarithmically and
the output word rate normalized to 1.
Figure 2. First-order Sinc Filter Response
While a Sinc 1 filter has a single zero at the output rate (normalized to 1 in Figure 1), a Sinc 2 filter would have two
zeroes at the output word rate. A Sinc
5
filter will have five zeroes at the output word rate. Notice in Figure 2 that
each time the order of the Sinc filter increases, the magnitude response has 6 dB more of attenuation as the fre-
quency doubles. Therefore a Sinc
1 filter has a slope of 6 dB per octave (20 dB per dacade); a Sinc 5 filter has a
slope of 30 dB per octave (100 dB per decade).
3. The CS5530/31/32/33/34 Filter:
Attenuating 60 Hz at 50 Sps Output Word Rate
The CS5530/31/32/33/34 series ADCs use Sinc filters. There are two stages of filtering. The first filter is a Sinc
5
filter. This filter outputs words at the converter's highest rate. The second stage of the digital filter is a programmable
Sinc
3 filter. It accepts the output of the Sinc 5 filter stage and provides the remainder of the availableword rates.
The actual output rate depends upon the master clock (nominally 4.9152 MHz) and the setting of the FRS (Filter
Rate Selection) bit in the CS5530/31/32/33/34 configuration register. Refer to Table 1.
.
If the FRS bit is set to logic 0, the converter can provide output word rates of 3840, 1920, 960, 480, 240, 120, 60,
30, 15, and 7.5 samples per second (Sps). When set to output word rates of 60, 30, 15, or 7.5 Sps, the Sinc
3 filter
output will provide zeroes in the transfer function that will yield excellent rejection of 60 Hz line interference compo-
nents.
Table 1. Crystal Frequency vs. Output Word Rate
Crystal Freq. FRS Available Word Rates
4.9152 MHz
0 3840, 1920, 960, 480, 240, 120, 60, 30, 15, 7.5
1 3200, 1600, 800, 400, 200, 100, 50, 25, 12.5, 6.25
Frequency (Hz)
Attenuation (d
B)
Sinc
1
-6 dB/Octave
Sinc
2
-12 dB/Octave
Sinc
3
-18 dB/Octave
Sinc 4 -24 dB/Octave
Sinc 5 -30 dB/Octave