Pre-Production
WM8940
NOTCH FILTERS AND LOW PASS FILTER
The WM8940 supports four programmable notch filters. The fourth notch filter can be configured as
a low pass filter. The following illustrates three digital notch filters, followed by a single low pass filter
in the ADC filter path. Both the centre frequency and -3dB bandwidth are programmable for the
notch filters. The cut off frequency is programmable for the low pass filter. The following graphs
show the responses of 1) a single notch filter at three chosen centre frequencies, with three
bandwidths for each, 2) the low pass filter at three chosen cut off frequencies and 3) the cascade of
three notch filters followed by the low pass filter, each with a different centre / cut off frequency with
three bandwidths for each.
+0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
R
E
S
P
O
N
S
E
(dB)
-60
20
50
100
200
500
1k
2k
5k
10k
20k
Frequency (Hz)
Figure 46 ADC Notch Filter Responses (48kHz); fc=100Hz, 1kHz, 10kHz; fb = 100Hz, 600Hz, 2kHz
+0
-5
T
-10
R
E
S
P
-15
-20
-25
-30
-35
-40
-45
-50
-55
O
N
S
E
(dB)
-60
20
50
100
200
500
1k
2k
5k
10k
20k
Frequency (Hz)
Figure 47 ADC Low Pass Filter Responses (48kHz); fc= 1kHz, 5kHz, 10kHz
Pre-Production, Rev 3.0, February 2007
81
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
