CS5531 View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'CS5531' PDF : 52 Pages View PDF
CS5531/32/33/34
2.9. Digital Filter
The CS5531/32/33/34 have linear phase digital fil-
ters which are programmed to achieve a range of
output word rates (OWRs) as stated in the Channel-
Setup Register Descriptions section. The ADCs use
a Sinc5 digital filter to output word rates at 3200
Sps and 3840 Sps (MCLK = 4.9152 MHz). Other
output word rates are achieved by using the Sinc5
filter followed by a Sinc3 filter with a programma-
ble decimation rate.Figure 16 shows the magnitude
response of the 60 Sps filter, while Figures 17 and
18 show the magnitude and phase response of the
filter at 120 Sps. The Sinc3 is active for all output
0
word rates except for the 3200 Sps and 3840 Sps
(MCLK = 4.9152 MHz) rate. The Z-transforms of
the two filters are shown in Figure 19. For the Sinc3
filter, “D” is the programmable decimation ratio,
which is equal to 3840/OWR when FRS = 0 and
3200/OWR when FRS = 1.
The converter’s digital filters scale with MCLK.
For example, with an output word rate of 120 Sps,
the filter’s corner frequency is at 31 Hz. If MCLK
is increased to 5.0 MHz, the OWR increases by
1.0175 percent and the filter’s corner frequency
moves to 31.54 Hz. Note that the converter is not
specified to run at MCLK clock frequencies greater
than 5 MHz.
-40
-80
-120
0
60 120 180 240 300
Frequency (Hz)
Figure 16. Digital Filter Response (Word Rate = 60 Sps)
0
-40
-80
-120
Flatness
Frequency dB
2
-0.01
4
-0.05
6
-0.11
8
-0.19
10
-0.30
12
-0.43
14
-0.59
16
-0.77
19
-1.09
32
-3.13
0
40
80
120
Frequency (Hz)
Figure 17. 120 Sps Filter Magnitude Plot to 120 Hz
180
90
0
-90
-180
0
30
60
90
120
Frequency (Hz)
Figure 18. 120 Sps Filter Phase Plot to 120 Hz
Sinc5
=
(---1----–-----z--–---8--0---)--5-
(1 – z–16 )5
×
(---1----–-----z--–---1--6---)--3-
(1 – z–4)3
×
(---1----–-----z---–--4---)--2-
(1 – z–2)2
×
(---1----–-----z--–---2---)--3-
(1 – z–1)3
Sinc3 = (---1----–-----z--–---D----)--3-
(1 – z–1)3
Note:
See the text regarding the Sinc3 filter’s
decimation ratio “D”.
Figure 19. Z-Transforms of Digital Filters
40
DS289PP5
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