Analog

Noise

Cancelling

Block

Optimized

Audio

Ouput

OUT+

Post Amp Gain

(6 dB - 18 dB)

Pre Amp

Gain

(6 dB - 36 dB)

Mic1

or

Mic2

Gain

(Max. 0 dB)

OUT-

Maximum

AC Input

Voltage

<1.6 Vpp

Maximum

AC Output

Voltage

<3.2 Vpp

Maximum

AC Input

Voltage

<440 mVpp

Maximum

AC Intput

Voltage

<1.6 Vpp

LMV1091

www.ti.com

SNAS481C – OCTOBER 2009 – REVISED MAY 2013

Figure 19. Maximum Signal Levels

Example 1

An application using microphones with 50mVP-P maximum output voltage, and a baseband chip after the

LMV1091 with 1.5VP-P maximum input voltage.

For optimum noise performance, the gain of the input stage should be set to the maximum.

1. 50mVP-P +36dB = 3.1VP-P.

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.

3. Select the nearest lower gain from the gain settings shown in Table 1, 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 be 1.26VP-P.

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.

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 than 0.75mVP-P.

6. Calculating the new gain for the preamp will result in <23.5dB gain.

7. The nearest lower gain will be 22dB.

So using preamp gain = 22dB and postamp gain = 6dB is the optimum for this application.

Example 2

An application using microphones with 10mVP-P maximum output voltage, and a baseband chip after the

LMV1091 with 3.3VP-P maximum input voltage.

For optimum noise performance we would like to have the maximum gain at the input stage.

1. 10mVP-P + 36dB = 631mVP-P.

2. This is lower than the maximum 1.5VP-P, so this is OK.

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. 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 nearest lower Post Amp gain will be 6dB.

So using preamp gain = 36dB and postamp gain = 6dB is optimum for this application.

Pre-Amp/Post-Amp Gains

The Pre-amplifier gain of the LMV1091TM can be controlled using the GA0-GA3 pins. See Table 1 below for

Pre-amplifier gain control. The Post-Amp gain can be controlled using the GB0-GB2 pins. See Table 2 below for

Post-amplifier gain control.

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