WM8950 datasheet(21/52 Pages) WOLFSON

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Preliminary Technical Data

WM8950

PTD Rev 2.1 June 2005

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 13 ADC Volume

INPUT LIMITER / AUTOMATIC LEVEL CONTROL (ALC)

The WM8950 has an automatic pga gain control circuit, which can function as an input peak limiter

or as an automatic level control (ALC).

In input peak limiter mode (ALCMODE bit = 1), a digital peak detector detects when the input signal

goes above a predefined level and will ramp the pga gain down to prevent the signal becoming too

large for the input range of the ADC. When the signal returns to a level below the threshold, the pga

gain is slowly returned to its starting level. The peak limiter cannot increase the pga gain above its

static level.

Figure 11 Input Peak Limiter Operation

In ALC mode (ALCMODE bit = 0) the circuit aims to keep a constant recording volume irrespective

of the input signal level. This is achieved by continuously adjusting the PGA gain so that the signal

level at the ADC input remains constant. A digital peak detector monitors the ADC output and

changes the PGA gain if necessary.

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