WM8940 datasheet(22/85 Pages) WOLFSON

22 / 85 page

WM8940

Pre-Production

Pre-Production, Rev 3.0, February 2007

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

Power

management 1

MICBEN

Microphone Bias Enable

0 = OFF (high impedance output)

1 = ON

Table 9 Microphone Bias Enable

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R44

Input Control

MBVSEL

Microphone Bias Voltage Control

0 = 0.9 * AVDD

1 = 0.65 * AVDD

Table 10 Microphone Bias Voltage Control

The internal MICBIAS circuitry is shown in Figure 9.

Note that the maximum source current

capability for MICBIAS is 3mA. The external biasing resistors therefore must be large enough to limit

the MICBIAS current to 3mA.

Figure 9 Microphone Bias Schematic

ANALOGUE TO DIGITAL CONVERTER (ADC)

The WM8940 uses a multi-bit, over sampled sigma-delta ADC channel. The use of multi-bit

feedback and high over sampling rates reduces the effects of jitter and high frequency noise. The

ADC Full Scale input level is proportional to AVDD. With a 3.3V supply voltage, the full scale level is

1.0Vrms. Any voltage greater than full scale may overload the ADC and cause distortion.

ADC DIGITAL FILTERS

The ADC filters perform true 24 bit signal processing to convert the raw multi-bit over sampled data

from the ADC to the correct sampling frequency to be output on the digital audio interface. The

digital filter path is illustrated in .

AGND

MBVSEL=0

MICBIAS

= 1.8 x VMID

= 0.9 X AVDD

VMID

internal

resistor

internal

resistor

MBVSEL=1

MICBIAS

= 1.3 x VMID

= 0.65 X AVDD

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