WM8950 datasheet(20/52 Pages) WOLFSON

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WM8950

Preliminary Technical Data

PTD Rev 2.1 June 2005

PROGRAMMABLE IIR FILTER

An IIR filter with fully programmable coefficients is provided, typically used as a notch filter for

removing narrow band noise at a given frequency. This notch filter has a variable centre frequency

and bandwidth, programmable via two coefficients, a0 and a1. a0 and a1 are represented by the

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.

6:0

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 12 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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