WM8940CGEFLSLASHV datasheet(82/88 Pages) CIRRUS

82 / 88 page

WM8940

Rev 4.4

HIGHPASS FILTER

The WM8940 has a selectable digital high pass filter in the ADC filter path. This filter has two modes,

audio and applications. In audio mode the filter is a 1st order IIR with a cut-off of around 3.7Hz. In

applications mode the filter is a 2nd order high pass filter with a selectable cut-off frequency.

-40

-35

-30

-25

-20

-15

-10

-5

Frequency (Hz)

-60

-50

-40

-30

-20

-10

200

400

600

800

1000

1200

Frequency (Hz)

Figure 42 ADC High pass Filter Response, HPFAPP=0

Figure 43 ADC High pass Filter Responses (48kHz),

HPFAPP=1, all cut-off settings shown.

-80

-70

-60

-50

-40

-30

-20

-10

200

400

600

800

1000

1200

Frequency (Hz)

-90

-80

-70

-60

-50

-40

-30

-20

-10

200

400

600

800

1000

1200

Frequency (Hz)

Figure 44 ADC High pass Filter Responses (24kHz),

HPFAPP=1, all cut-off settings shown.

Figure 45 ADC Highpass Filter Responses (12kHz),

HPFAPP=1, all cutoff settings shown.

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.

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