WM8944BECSSLASHR datasheet(30/165 Pages) CIRRUS

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WM8944B

Rev 4.3

ADC HIGH PASS FILTER

A digital high-pass filter is enabled by default to the ADC path to remove DC offsets. This filter can

also be used to remove low frequency noise in handheld applications (e.g. wind noise, handling noise

or mechanical vibration).

The ADC High Pass filter is enabled by ADC_HPF. Note that this filter must always be set when the

Dynamic Range Controller (DRC) is enabled. The DRC will not function correctly if ADC_HPF is not

set.

The filter operates in one of two modes, selected by ADC_HPF_MODE.

The ADC_HPF_SR register should b

e set according to the selected ADC sample rate. See “Clocking

and Sample Rates

” for details of the ADC sample rate.

In Hi-Fi mode (ADC_HPF_MODE = 0), the high-pass filter is optimised for removing DC offsets

without degrading the bass response and has a cut-off frequency of 3.5Hz when the sample rate (fs) =

44.1kHz.

In Application mode (ADC_HPF_MODE = 1), the HPF cut-off frequency is set using ADC_HPF_CUT.

This mode is intended for voice communication; it is recommended to set the cut-off frequency below

300Hz (e.g. ADC_HPF_CUT = 101 when fs = 8kHz or fs = 16kHz).

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R26 (1Ah)

ADC Control 2

ADC_HPF_MOD

ADC Digital HPF Mode

0 = Hi-Fi mode (1st order)

1 = Application mode (2nd order)

5:4

ADC_HPF_SR

[1:0]

ADC Digital HPF Sample Rate

00 = 8kHz to 12kHz

01 = 16kHz to 24kHz

10 = 32kHz to 48kHz

11 = Reserved

3:1

ADC_HPF_CUT

[2:0]

000

ADC Digital High Pass Filter Cutoff

Note that the cut-off frequency

scales with sample rate. See Table

11 for cut-off frequencies at all

supported sample rates.

ADC_HPF

ADC Digital High Pass Filter Enable

0 = Disabled

1 = Enabled

Table 10 ADC High-pass Filter Control Registers

SAMPLE

FREQUENCY

(kHz)

CUT-OFF FREQUENCY (Hz)

HI-FI

MODE

APPLICATION MODE / ADC_HPF_CUT[2:0]

000

001

010

011

100

101

110

111

8.000

3.0

80.5

100.5

129.0

160.5

200.5

256.5

320.5

400.5

11.025

4.5

111.0

138.5

177.5

221.0

276.5

353.0

442.0

551.5

12.000

4.5

120.5

151.0

193.0

240.5

300.5

384.5

481.0

600.5

16.000

3.0

80.0

101.0

129.0

161.0

200.5

256.5

321.0

401.0

22.050

4.5

110.5

139.0

177.5

221.5

276.0

353.5

442.0

552.5

24.000

4.5

120.5

151.5

193.5

241.5

300.5

385.0

481.5

601.0

32.000

2.5

80.0

101.5

129.0

160.5

200.0

257.5

320.0

401.0

44.100

3.5

110.0

139.5

177.5

221.0

275.5

355.0

441.0

552.5

48.000

4.0

119.5

152.0

193.0

240.5

300.0

386.5

480.0

601.0

Table 11 ADC High-pass Filter Cut-off Frequencies

Filter response plots for the ADC high-pass

filter are shown in “Digital Filter Characteristics”.

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