WM8731 / WM8731L
Production Data
Or alternatively to calculate the value of Rmic to achieve a given gain, G1.
Rmic = (50k/G1) – 10k
The internal 50k and 10k resistors have a tolerance of 15%. For Rmicext = 90k G = 0.5 (-6dB) and
for Rmicext = 0 G = x10 (14dB).
The 2nd stage comprises a 0dB gain stage that can be software configured to provide a fixed 20dB of
gain for low sensitivity microphones.
The microphone input can therefore be configured with a variable gain of between -6dB and 14dB on
the 1st stage, and an additional fixed 0dB or 20dB on the 2nd stage. This allows for all gains to the
input signal in the range –6dB to 34dB to be catered for.
The ADC Full Scale input is 1.0V rms at AVDD = 3.3 volts. Any voltage greater than full scale will
possibly overload the ADC and cause distortion. Note that the full scale input tracks directly with
AVDD. Stage 1 and Stage 2 gains should be configured so that the ADC receives a maximum signal
equal to its full scale for maximising the signal to noise.
The software control for the MICIN is shown in Table 4. Note that the Microphone Mute only mutes
the input to the ADC, this will still allow the Microphone Input signal to pass to the line output in
Sidetone Mode.
REGISTER
ADDRESS
BIT
LABEL
DEFAULT
DESCRIPTION
0000100
0
MICBOOST
0
Microphone Input Level Boost
1 = Enable Boost
Analogue Audio
Path Control
0 = Disable Boost
1
MUTEMIC
1
Line Input Mute to ADC
1 = Enable Mute
0 = Disable Mute
Table 4 Microphone Input Software Control
The microphone input is biased internally through the operational amplifier to VMID. Whenever the
line inputs are muted the MICIN input is kept biased to VMID using special anti-thump circuitry. This
reduces any audible clicks that may otherwise be heard when re-activating the input.
The application drawing for the microphone is shown in Figure 13.
MICBIAS
R1
Rmic
C2
FROM
MICIN
MICROPHONE
C1
R2
AGND
AGND AGND
Figure 13 Microphone Input and Bias Application Drawing
Recommended component values are C1 = 220pF (npo ceramic), C2 = 1μF, R1 = 680 Ω, R2 = 47k.
Rmic values depends on gain setting (see above).
PD, Rev 4.8, April 2009
24
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
