WM8940CGEFLSLASHV datasheet(87/91 Pages) WOLFSON

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Production Data

WM8940

PD, Rev 4.3, November 2011

Figure 48 Cumulative Notch + Low Pass Filters Responses (48kHz);

NF0 fc = 1kHz; NF1 fc = 5kHz; NF2 fc = 10kHz; LPF fc = 11kHz; fb = 100Hz, 600Hz, 2kHz

NOTCH FILTER WORKED EXAMPLE

The following example illustrates how to calculate the a0 and a1 coefficients for a desired centre frequency and -3dB

bandwidth.

fc = 1000 Hz

fb = 100 Hz

fs = 48000 Hz







x (1000 / 48000) = 0.1308996939 rads







x (100 / 48000) = 0.01308996939 rads

)

tan(

)

tan(







)

0130899693

tan(

)

0130899693

tan(





= 0.9869949627

)

cos(

)

(







)

1308996939

cos(

)

9869949627

(





= -1.969995945

NFn_A0 = -a0 x 2

13 = -8085 (rounded to nearest whole number)

NFn_A1 = -a1 x 2

12 = 8069 (rounded to nearest whole number)

These values are then converted to a 14-bit sign / magnitude notation:

NFn_A0[13] = 1; NFn_A0[12:0] = 13’h1F95; NFn_A0 = 14’h3F95 = 14’b11111110010101

NFn_A1[13] = 0; NFn_A1[12:0] = 13’h1F85; NFn_A1 = 14’h1F85 = 14’b01111110000101

-40

-35

-30

-25

-20

-15

-10

-5

(dB)

20k

100

200

500

10k

Frequency (Hz)

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