9.2.13 Microphone Input
CSR8640 BGA contains an independent low-noise microphone bias generator. The microphone bias generator is
recommended for biasing electret condensor microphones. Figure 9.4 shows a biasing circuit for microphones with
a sensitivity between about ‑40 to ‑60dB (0dB = 1V/Pa).
Where:
■
The microphone bias generator derives its power from VBAT or 3V3_USB and requires no capacitor on its
output.
■
The microphone bias generator maintains regulation within the limits 70μA to 2.8mA, supporting a 2mA
source typically required by 2 electret condensor microphones. If the microphone sits below these limits,
then the microphone output must be pre-loaded with a large value resistor to ground.
■
Biasing resistors R1 and R24 equal 2.2kΩ.
■
The input impedance at MIC_AN, MIC_AP, MIC_BN and MIC_BP is typically 6kΩ.
■
C1, C2, C3 and C4 are 100/150nF if bass roll-off is required to limit wind noise on the microphone.
■
R1 and R2 set the microphone load impedance and are normally around 2.2kΩ.
G-
T
W
-000
8073.2.2
C2
R1
C1
Microphone Bias
(MIC_BIAS)
MIC_AP
MIC_AN
MIC1
+
Input
Amplifier
C4
R2
C3
Microphone Bias
(MIC_BIAS)
MIC_BP
MIC_BN
MIC2
+
Input
Amplifier
Figure 9.4: Microphone Biasing
The microphone bias characteristics include:
■
Power supply:
■
CSR8640 BGA microphone supply is VBAT or 3V3_USB
■
Minimum input voltage = Output v drop-out voltage
■
Maximum input voltage is 4.3V
■
Drop-out voltage:
■
300mV maximum
■
Output voltage:
■
1.8V or 2.6V
■
Tolerance 90% to 110%
■
Output current:
■
70μA to 2.8mA
■
No load capacitor required
Advance Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2011
Page 42 of 110
CS-209182-DSP1
CSR8640 BGA
Data Sheet
