WM8940CGEFLSLASHV datasheet(25/88 Pages) CIRRUS

25 / 88 page

WM8940

Rev 4.4

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 Error! Reference source not found..

ADC

DIGITAL

DECIMATOR

DIGITAL

FILTERS

GAIN

HIGH PASS

FILTER

NOTCH

FILTERS

DIGITAL

AUDIO

INTERFACE

ADC DIGITAL FILTERS

Figure 10 ADC Digital Filter Path

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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