Specifications

Bluegiga Technologies Oy
Page 37 of 64
9.1.3 Microphone Input
Figure 18 shows the recommended microphone biasing. The microphone bias, MIC_BIAS, derives its power
from the VDD_BAT and requires 1uF capacitor on its output (C1).
The input impedance at AUDIO_IN_P_LEFT and AUDIO_IN_N_LEFT is typically 6kohm and C5 and C4 are
typically 1uF. If bass roll-off is required to limit the wind noise on the microphone then C4 and C5 should be
150 nF.
R2 sets the microphone load impedance and is normally in the range of 1kΩ to 2kΩ
R1, C2 and C3 improve the supply rejection by decoupling supply noise from the microphone. Values should
be selected as required. R1 can be connected or to the MIC_BIAS output (which is ground referenced and
provides good rejection of the supply) as shown in Figure 18. MIC_BIAS is configured to provide bias only
when the microphone is required. R1 may also be connected to a convenient supply, in which case the bias
network is permanently enabled.
WT32i
MIC_BIAS
C1
C2
C3
MIC
C4
C5
AUDIO_IN_N_LEFT
AUDIO_IN_P_LEFT
R1
R2
Figure 18: Microphone connection to audio input
The MIC_BIAS is like any voltage regulator and requires a minimum load to maintain regulation. The
MIC_BIAS maintains regulation within the limits 0.200mA to 1.230mA. If the microphone sits below these
limits, then the microphone output must be pre-loaded with a large value resistor to ground.
The audio input is intended for use in the range from 1μA @ 94dB SPL to about 10μA @ 94dB SPL. With
biasing resistors R1 and R2 equal to 1kΩ, this requires microphones with sensitivity between about –40dBV
and –60dBV.
Table 21 lists the possible voltage and current setting in iWRAP for the MIC_BIAS.
Setting in iWRAP
Voltage (V)
Current (mA)
0
1.71
0.200
1
1.76
0.280
2
1.82
0.340
3
1.87
0.420