WM8991 datasheet(56/174 Pages) WOLFSON

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WM8991

Pre-Production

PP, May 2008, Rev 3.1

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_ENA 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).

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

ADC_HPF_ENA

ADC Digital High Pass Filter Enable

0 = disabled

1 = enabled

R14 (0Eh)

6:5

ADC_HPF_CUT

[1:0]

00b

ADC Digital High Pass Filter Cut-Off

Frequency (fc)

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 18 for cut-off frequencies at all

supported sample rates)

Table 17 ADC High Pass Filter Control Registers

CUT-OFF FREQUENCY (Hz)

SAMPLE

FREQUENCY

(kHz)

ADC_HPF_CUT

=00

ADC_HPF_CUT

=01

ADC_HPF_CUT

=10

ADC_HPF_CUT

=11

8.000

0.7

130

267

11.025

0.9

178

367

16.000

1.3

127

258

532

22.050

1.9

175

354

733

24.000

2.0

190

386

798

32.000

2.7

253

514

1063

44.100

3.7

348

707

1464

48.000

4.0

379

770

1594

Table 18 ADC High Pass Filter Cut-Off Frequencies

The high pass filter characteristics are shown in the "Digital Filter Characteristics" section.

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