WM8976 datasheet(27/108 Pages) WOLFSON

27 / 108 page

Pre-Production

WM8976

PP Rev 3.0 April 2006

PROGRAMMABLE NOTCH FILTER

A programmable notch filter is provided. This filter has a variable centre frequency and bandwidth,

programmable via two coefficients, a0 and a1.

a0 and a1 are represented by the register bits

NFA0[13:0] and NFA1[13:0]. Because these coefficient values require four register writes to setup

there is an NFU (Notch Filter Update) flag which should be set only when all four registers are setup.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

6:0

NFA0[13:7]

Notch Filter a0 coefficient, bits [13:7]

NFEN

Notch filter enable:

0=Disabled

1=Enabled

R27

Notch Filter 1

NFU

Notch filter update. The notch filter

values used internally only update

when one of the NFU bits is set high.

6:0

NFA0[6:0]

Notch Filter a0 coefficient, bits [6:0]

R28

Notch Filter 2

NFU

Notch filter update. The notch filter

values used internally only update

when one of the NFU bits is set high.

6:0

NFA1[13:7]

Notch Filter a1 coefficient, bits [13:7]

R29

Notch Filter 3

NFU

Notch filter update. The notch filter

values used internally only update

when one of the NFU bits is set high.

0-6

NFA1[6:0]

Notch Filter a1 coefficient, bits [6:0]

R30

Notch Filter 4

NFU

Notch filter update. The notch filter

values used internally only update

when one of the NFU bits is set high.

Table 15 Notch Filter Function

The coefficients are calculated as follows:

)

tan(

)

tan(

−

)

cos(

)

(

−

Where:

fc = centre frequency in Hz, fb = -3dB bandwidth in Hz, fs = sample frequency in Hz

The actual register values can be determined from the coefficients as follows:

NFA0 = -a0 x 2

NFA1 = -a1 x 2

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