WM8731L WOLFSON [Wolfson Microelectronics plc], WM8731L Datasheet - Page 25

• Manufacturer Part Number: WM8731L
• Description: Portable Internet Audio CODEC with Headphone Driver and Programmable Sample Rates
• Manufacturer: WOLFSON [Wolfson Microelectronics plc]

WM8731 / WM8731L
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R1 and R2 form part of the biasing network (refer to Microphone Bias section below). R1 connected
to MICBIAS is necessary only for electret type microphones that require a voltage bias. R2 should
always be present to prevent the microphone input from charging to a high voltage which may
damage the microphone on connection. R1 and R2 should be large so as not to attenuate the signal
from the microphone, which can have source impedance greater than 2k. C1 together with the
source impedance of the microphone and the input impedance of MICIN forms an RF filter. C2 is a
DC blocking capacitor to allow the microphone to be biased at a different DC voltage to the MICIN
signal.
MICROPHONE BIAS
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 Microphone Input
section for an application drawing and further description.
The scheme for MICBIAS is shown in Figure 14. Note that there is a maximum source current
capability of 3mA available for the MICBIAS. This limits the smallest value of external biasing
resistors that can safely be used.
Note that the MICBIAS output is not active in standby mode.
Figure 14 Microphone Bias Schematic
ADC
The WM8731/L uses a multi-bit oversampled sigma-delta ADC. A single channel of the ADC is
illustrated in the Figure 15.
Figure 15 Multi-Bit Oversampling Sigma Delta ADC Schematic
FROM MICROPHONE
FROM LINE INPUT
VMID
INPUT
INSEL
INTEGRATOR
ANALOG
AGND
MULTI
BITS
PD Rev 4.3 August 2006
TO ADC DIGITAL FILTERS
MICBIAS
Production Data
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