Pre Production
WM8941
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.
MBVSEL=0
MICBIAS
VMID
MB
= 1.8 x VMID
= 0.9 X AVDD
internal
resistor
MBVSEL=1
MICBIAS
= 1.3 x VMID
= 0.65 X AVDD
internal
resistor
AGND
Figure 9 Microphone Bias Schematic
ANALOGUE TO DIGITAL CONVERTER (ADC)
The WM8941 uses a multi-bit, oversampled sigma-delta ADC channel. 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 is illustrated in Figure 10.
Figure 10 ADC Digital Filter Path
PP, Rev 3.3, December 2007
25
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
