Datasheet

Analog
Noise
Cancelling
Processor
Optimized
Audio
Ouput
OUT+
Post Amp
Gain
( 6
- 18
dB
)
Pre Amp
Gain
( 6
- 36
dB
)
Mic1
or
Mic2
Gain
(
Max.
9
dB
)
OUT-
Maximum
AC Input
Voltage
<1.4 Vpp
Maximum
AC Output
Voltage
<2.8 Vpp
Maximum
AC Input
Voltage
<300 mVpp
Maximum
AC Intput
Voltage
<1.4 Vpp
LMV1089
www.ti.com SNAS441H –SEPTEMBER 2008–REVISED MAY 2010
Figure 22. Maximum Signal Levels
Example 1
An application using microphones with 50mV
P-P
maximum output voltage, and a baseband chip after the
LMV1089 with 1.5V
P-P
maximum input voltage.
For optimum noise performance, the gain of the input stage should be set to the maximum.
1. 50mV
P-P
+36 dB = 3.1V
P-P
.
2. 3.1V
P-P
is higher than the maximum 1.4V
P-P
allowed for the Noise Cancelling Processor (NCP). This means a
gain lower than 28.9dB should be selected.
3. Select the nearest lower gain from the gain settings shown in Table 1, 28dB is selected. This will prevent the
NCP from being overloaded by the microphone. With this setting, the resulting output level of the Pre
Amplifier will be 1.26V
P-P
.
4. The NCP can have a maximum processing gain of 9dB (depending on the calibration result) which will result
in 3.5V
P-P
at the output of the LMV1089. This level is higher then maximum level that is allowed at the input
of the post amp of the LMV1089. Therefore the preamp gain has to be reduced, to 1.4V
P-P
minus 9dB =
0.5V
P-P
. This limits the preamp gain to a maximum of 20dB.
5. The baseband chip limits the maximum output voltage to 1.5V
P-P
with the minimum of 6dB post amp gain,
this results in requiring a lower level at the input of the post amp of 0.75V
P-P
. Now calculating this for a
maximum NCP gain of 9dB the output of the preamp must be <266mV
P-P
.
6. Calculating the new gain for the preamp will result in <1.4dB gain.
7. The nearest lower gain will be 14dB.
So using preamp gain = 14dB and postamp gain = 6dB is the optimum for this application.
Example 2
An application using microphones with 10mV
P-P
maximum output voltage, and a baseband chip after the
LMV1089 with 3.3V
P-P
maximum input voltage.
For optimum noise performance we would like to have the maximum gain at the input stage.
1. 10mV
P-P
+ 36dB = 631mV
P-P
.
2. This is lower than the maximum 1.4V
P-P
so this is OK.
3. The NCP can have a maximum processing gain of 9dB (depending on the calibration result) which will result
in 3.5V
P-P
at the output of the LMV1089. This level is higher then maximum level that is allowed at the input
of the Post Amp of the LMV1089. Therefore the Pre Amp gain has to be reduced, to 1.4V
P-P
minus 9dB =
0.5V
P-P
. This limits the Pre Amp gain to a maximum of 34dB.
4. With a Post Amp gain setting of 6dB the output of the Post Amp will be 2.8V
P-P
which is OK for the
baseband.
5. The nearest lower Post Amp gain will be 6dB.
So using preamp gain = 34dB and postamp gain = 6dB is optimum for this application.
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