WM8940CGEFLSLASHV datasheet(29/88 Pages) CIRRUS

29 / 88 page

WM8940

Rev 4.4

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

NFn_A0 = -a0 x 213

NFn_A1 = -a1 x 212

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:

0 =

)

tan(

)

tan(

−

Where:



fc = cutoff 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 x2

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

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R15

ADC Digital

Volume

7:0

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

Table 19 ADC Volume

INPUT LIMITER / AUTOMATIC LEVEL CONTROL (ALC)

The WM8940 has an automatic PGA gain control circuit, which can function as an input peak limiter or

as an automatic level control (ALC).

The Automatic Level Control (ALC) provides continuous adjustment of the input PGA in response to

the amplitude of the input signal. A digital peak detector monitors the input signal amplitude and

compares it to a register defined threshold level (ALCLVL).

If the signal is below the threshold, the ALC will increase the gain of the PGA at a rate set by

ALCDCY. If the signal is above the threshold, the ALC will reduce the gain of the PGA at a rate set by

ALCATK.

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