Application Data
Gain Balance and Gain Budget
In systems where input signals have a high dynamic range,
critical noise levels and where the dynamic range of the output
voltage is also limited, careful gain balancing can be essential
for the best performance. Having not enough gain in the Pre
Amplifier can result in higher noise levels while to much gain
in the Pre Amplifier will result in clipping and saturation in the
noise cancelling processor and output stages.
The gain ranges and maximum signal levels for the different
functional blocks is shown in Figure 2. Two examples are giv-
en as a guideline how to select proper gain settings.
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FIGURE 2. Maximum Signal Levels
Example1 :
An application using microphones with 50mV
pp
maximum
output voltage, and a baseband chip after the LMV1088 with
1.5V
pp
maximum input voltage.
For optimum noise performance we would like to have the
maximum gain at the input stage.
So using Pre Amp gain =14dB and Post Amp gain = 6dB is
the optimum for this application.
1.
50mV
pp
+ 36 dB = 3.1V
pp
.
2.
This is higher than the maximum 1.4V
pp
allowed for the
Noice cancelling Processor (NCP). This means a gain
lower then 28.9dB should be selected.
3.
Select the nearest lower gain from the gain setting table
to be 28dB. 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
pp
.
4.
The NCP can have a maximum processing gain of 9dB
(depending on the calibration result) which will result in
3.5V
pp
at the output of the LMV1088. This level is higher
then maximum level hat is allowed at the input of the Post
Amp of the LMV1088. Therefore the Pre Amp gain has
to be reduced, to 1.4V
pp
minus 9dB = 0.5V
pp
. This limits
the Pre Amp gain to a maximum of 20dB.
5.
The baseband chip limits the maximum output voltage to
1.5V
pp
with the minimum of 6dB Post Amp gain, this
results in having a lower level at the input of the Post Amp
of 0.75V
pp
. Now calculating this for a maximum NCP gain
of 9dB the output of the Pre Amp must be <266mV
pp
.
6.
Calculating the new gain for the Pre Amp will result in
<1.4 dB gain.
7.
The nearest lower gain will be 14dB.
So using Pre Amp gain =14dB and Post Amp gain = 6dB is
the optimum for this application.
Example 2:
An application using microphones with 10mV
pp
maximum
output voltage, and a baseband chip after the LMV1088 with
3.3V
pp
maximum input voltage.
For optimum noise performance we would like to have the
maximum gain at the input stage.
1.
10mV
pp
+ 36dB = 631mV
pp
.
2.
This is lower than the maximum 1.4V
pp
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
pp
at the output of the LMV1088. This level is higher
then maximum level hat is allowed at the input of the Post
Amp of the LMV1088. Therefore the Pre Amp gain has
to be reduced, to 1.4V
pp
minus 9dB = 0.5V
pp
. 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
pp
which is OK for the baseband.
5.
The nearest lower Post Amp gain will be 6dB.
So using Pre Amp gain =14dB and Post Amp gain = 6dB is
the optimum for this application.
I
2
C Compatible Interface
I
2
C SIGNALS
The LMV1088 pin Serial Clock (SCL) is used for the I
2
C clock
SCL and the pin Serial Data (SDA) is used for the I
2
C data
signal SDA. Both of these signals need a pull-up resistor ac-
cording to I
2
C specification. The LMV1088 can be controlled
through two slave addresses. The two I
2
C slave address for
LMV1088 are given inTable 2 .
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LMV1088
