Pre-Production
WM8904
HIGH PASS FILTER
A digital high pass filter is applied by default to the ADC path to remove DC offsets. This filter can
also be programmed to remove low frequency noise in voice applications (e.g. wind noise or
mechanical vibration). This filter is controlled using the ADC_HPF and ADC_HPF_CUT register bits.
In hi-fi mode the high pass filter is optimised for removing DC offsets without degrading the bass
response and has a cut-off frequency of 3.7Hz at fs=44.1kHz.
In voice mode the high pass filter is optimised for voice communication and it is recommended to
program the cut-off frequency below 300Hz (e.g. ADC_HPF_CUT=11 at fs=8kHz or
ADC_HPF_CUT=10 at fs=16kHz).
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
R38 (26h)
ADC Digital High Pass Filter Cut-Off
Frequency (fc)
6:5
ADC_HPF_C
UT [1:0]
00
ADC Digital
0
00 = Hi-fi mode (fc=4Hz at fs=48kHz)
01 = Voice mode 1 (fc=127Hz at
fs=16kHz)
10 = Voice mode 2 (fc=130Hz at fs=8kHz)
11 = Voice mode 3 (fc=267Hz at fs=8kHz)
(Note: fc scales with sample rate. See
Table 13 for cut-off frequencies at all
supported sample rates)
ADC Digital High Pass Filter Enable
0 = disabled
4
ADC_HPF
1
1 = enabled
Table 12 ADC Digital 0 Register
Sample
CUT-OFF FREQUENCY (Hz)
Frequency
ADC_HPF_CUT
ADC_HPF_CUT
=01
ADC_HPF_CUT
ADC_HPF_CUT
=11
(kHz)
=00
=10
130
178
258
354
386
514
707
770
8.000
11.025
16.000
22.050
24.000
32.000
44.100
48.000
0.7
0.9
1.3
1.9
2.0
2.7
3.7
4.0
64
267
367
88
127
175
190
253
348
379
532
733
798
1063
1464
1594
Table 13 ADC High Pass Filter Cut-Off Frequencies
The high pass filter characteristics are shown in the “Digital Filter Characteristics” section.
PP, Rev 3.3, September 2012
51
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
