WM8978 Wolfson Microelectronics Ltd., WM8978 Datasheet - Page 34

• Manufacturer Part Number: WM8978
• Description: The WM8978 is a low power, high quality stereo codec designed for portable applications such as Digital still camera or Digital Camcorde
• Manufacturer: Wolfson Microelectronics Ltd.

WM8978
ANALOGUE TO DIGITAL CONVERTER (ADC)
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MICROPHONE BIASING CIRCUIT
Table 12 Microphone Bias Enable Control
Table 13 Microphone Bias Voltage Control
Figure 17 Microphone Bias Schematic
ADC DIGITAL FILTERS
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.
MICBIAS=0.65*AVDD. The output can be enabled or disabled using the MICBEN control bit.
The internal MICBIAS circuitry is shown in Figure 17. 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.
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 1.0V
Any voltage greater than full scale may overload the ADC and cause distortion.
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 18.
R1
Power
management 1
R44
Input control
REGISTER
ADDRESS
REGISTER
ADDRESS
8
BIT
4
BIT
MBVSEL
LABEL
MICBEN
When MBVSEL=0, MICBIAS=0.9*AVDD and when MBVSEL=1,
LABEL
0
DEFAULT
0
DEFAULT
Microphone Bias Voltage Control
0 = 0.9 * AVDD
1 = 0.65 * AVDD
Microphone Bias Enable
0 = OFF (high impedance output)
1 = ON
DESCRIPTION
DESCRIPTION
PTD Rev 2.6 November 2005
Preliminary Technical Data
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