Datasheet
LM49370
SNAS356D –FEBRUARY 2007–REVISED MARCH 2012
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The AGC algorithm should not be used where very large background noise is present. If the type of input data,
application and microphone is known then the AGC will typically not be required for good performance, it is
intended for use with inputs with a large dynamic range or unknown nominal level. When setting
NOISE_GATE_THRESHOLD be aware that in some mobile phone scenarios the ADC SNR will be dictated by
the microphone performance rather than the ADC or the signal. Gain changes to the microphone are performed
on zero crossings. To eliminate DC offsets, wind noise, and pop sounds from the output of the ADC, the ADC's
HPF should always be enabled.
MIC_1 CONFIGURATION REGISTER
This register is used to control the microphone configuration.
Table 17. MIC_1 (0x0Bh)
Bits Field Description
3:0 PREAMP_GAIN This programs the gain applied to the microphone preamplifier if the AGC is not in use.
PREAMP_GAIN Gain
0000
2
6 dB
0001
2
8 dB
0010
2
10 dB
0011
2
12 dB
0100
2
to 1100
2
14 dB to 30 dB
1101
2
32 dB
1110
2
34 dB
1111
2
36 dB
4 MIC_MUTE If set, the microphone preamplifier is muted.
5 INT_SE_DIFF If set, the internal microphone is assumed to be single ended and the negative connection is connected to the
ADC common mode point internally. This allows a single-ended internal microphone to be used.
6 INT_EXT If set, the single ended external microphone is used and the negative microphone input is grounded internally,
otherwise internal microphone operation is assumed.
(1)
(1) On changing INT_EXT from internal to external note that the dc blocking cap will not be charged so some time should be taken (300ms
for a 1µF cap) between the detection of an external headset and the switching of the output stages and ADC to that input to allow the
DC points on either side of this cap to stabilize. This can be accomplished by deselecting the microphone input from the audio outputs
and ADC until the DC points stabilize. An active MIC path to CPOUT or the ADC may result in the microphone DC blocking caps
causing audio pops under the following situations:1) Switching between internal and external microphone operation while in chip modes
'10' or '11'.2) Toggling in and out of powerdown/standby modes.3) Toggling between chip modes '10' and '11' whenever external
microphone operation is selected.4) The insertion/removal of a headset while in chip modes '10' or '11' whenever external microphone
operation is selected. To avoid these potential pop issues, it is recommended to deselect the microphone input from CPOUT and ADC
until the DC points stabilize.
MIC_2 CONFIGURATION REGISTER
This register is used to control the microphone configuration.
Table 18. MIC_2 (0x0Ch)
Bits Field Description
0 OCL_ This selects the voltage used as virtual ground (HP_VMID pin) in OCL mode. This will depend on the
VCM_ available supply and the power output requirements of the headphone amplifiers.
VOLTAGE
OCL_VCM_VOLTAGE Voltage
0 1.2V
1 1.5V
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