Example 1
6. Calculating the new gain for the preamp will result in
<23.5dB gain.
An application using microphones with 50mVP-P maximum
output voltage, and a baseband chip after the LMV1091 with
1.5VP-P maximum input voltage.
7. The nearest lower gain will be 22dB.
So using preamp gain = 22dB and postamp gain = 6dB is the
optimum for this application.
For optimum noise performance, the gain of the input stage
should be set to the maximum.
Example 2
1. 50mVP-P +36dB = 3.1VP-P
.
An application using microphones with 10mVP-P maximum
output voltage, and a baseband chip after the LMV1091 with
3.3VP-P maximum input voltage.
2. 3.1VP-P is higher than the maximum 1.5VP-P allowed for
the Noise Cancelling Block (NCB). This means a gain
lower than 29.5dB should be selected.
For optimum noise performance we would like to have the
maximum gain at the input stage.
3. Select the nearest lower gain from the gain settings
shown in Table 2,28dB is selected. This will prevent the
NCB from being overloaded by the microphone. With this
setting, the resulting output level of the Pre Amplifier will
1. 10mVP-P + 36dB = 631mVP-P
.
2. This is lower than the maximum 1.5VP-P, so this is OK.
be 1.26VP-P
.
3. The NCB has a gain of 0dB which will result in 1.5VP-P at
the output of the LMV1091. This level is lower than the
maximum level that is allowed at the input of the Post
Amp of the LMV1091.
4. The NCB has a gain of 0dB which will result in 1.26VP-P
at the output of the LMV1091. This level is less than
maximum level that is allowed at the input of the post amp
of the LMV1091.
4. With a Post Amp gain setting of 6dB the output of the
Post Amp will be 3VP-P which is OK for the baseband.
5. The baseband chip limits the maximum output voltage to
1.5VP-P with the minimum of 6dB post amp gain, this
results in requiring a lower level at the input of the post
amp of 0.75VP-P. Now calculating this for a maximum
preamp gain, the output of the preamp must be no more
5. The nearest lower Post Amp gain will be 6dB.
So using preamp gain = 36dB and postamp gain = 6dB is
optimum for this application.
than 0.75mVP-P
.
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