Pre Production
WM8941
The notch filter coefficients are calculated as follows:
1− tan(w / 2)
b
a
=
0
1+ tan(w / 2)
b
a = −(1+ a )cos(w )
1
0
0
Where:
w
= 2πf / f
c
0
b
s
w
= 2πf / f
b
s
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:
NFn_A0 = -a0 x 213
NFn_A1 = -a1 x 212
These values are then converted to a 14-bit sign / magnitude notation.
To configure Notch Filter 3 as a 1st order low pass filter, set the NF3_LP bit to 1 and calculate the
coefficients as follows:
a
= 0
0
tan(w / 2) −1
c
a
=
1
tan(w / 2) +1
c
Where:
= 2πf / f
s
w
c
c
fc = cut-off frequency in Hz, fs = sample frequency in Hz
The actual register values can be determined from the coefficients as follows:
NF3_A0 = 0
NF3_A1 = -a1 x212
These values are then converted to a 14-bit sign / magnitude notation.
DIGITAL ADC VOLUME CONTROL
The output of the ADCs can be digitally attenuated over a range from –127dB to 0dB in 0.5dB steps.
The gain for a given eight-bit code X is given by:
Gain = 0.5 x (x–255) dB for 1 ≤ x ≤ 255, MUTE for x = 0
REGISTER
ADDRESS
BIT
7:0
LABEL
DEFAULT
DESCRIPTION
R15
ADCVOL
[7:0]
11111111
( 0dB )
ADC Digital Volume Control
0000 0000 = Digital Mute
0000 0001 = -127dB
0000 0010 = -126.5dB
... 0.5dB steps up to
1111 1111 = 0dB
ADC Digital
Volume
Table 19 ADC Volume
PP, Rev 3.3, December 2007
29
w
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
