WM8978GEFLSLASHV datasheet(23/78 Pages) WOLFSON

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WM8978

PP Rev 1.7 January 2005

MICROPHONE BIASING CIRCUIT

The MICBIAS output provides a low noise reference voltage suitable for biasing electret type

microphones and the associated external resistor biasing network. Refer to the Applications

Information section for recommended external components. The MICBIAS voltage can be altered via

the MBVSEL register bit.

When MBVSEL=0, MICBIAS=0.9*AVDD and when MBVSEL=1,

MICBIAS=0.6*AVDD. The output can be enabled or disabled using the MICBEN control bit.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

Power

management 1

MICBEN

Microphone Bias Enable

0 = OFF (high impedance output)

1 = ON

Table 8 Microphone Bias Enable Control

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R44

Input control

MBVSEL

Microphone Bias Voltage Control

0 = 0.9 * AVDD

1 = 0.6 * AVDD

Table 9 Microphone Bias Voltage Control

The internal MICBIAS circuitry is shown in Figure 8.

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 8 Microphone Bias Schematic

ANALOGUE TO DIGITAL CONVERTER (ADC)

The WM8978 uses stereo multi-bit, oversampled sigma-delta ADCs. The use of multi-bit feedback

and high oversampling 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 oversampled data

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

digital filter path for each ADC channel is illustrated in Figure 9.

AGND

MBVSEL=0

MICBIAS

= 1.8 x VMID

= 0.9 X AVDD

VMID

internal

resistor

internal

resistor

MBVSEL=1

MICBIAS

= 1.2 x VMID

= 0.6 X AVDD

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