Analog

Noise

Cancelling

Block

Optimized

Audio

Ouput

OUT+

Post Amp Gain

(6 dB / 12 dB)

Pre Amp Gain

(12 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

Analog

Noise

Cancelling

Block

Optimized

Audio

Ouput

OUT+

Post Amp Gain

(6 dB / 12 dB)

Preamp Gain

(12 dB - 36 dB)

Mic1

OUT-

Mic2

LMV1099

www.ti.com

SNAS490D – JULY 2010 – REVISED MAY 2013

APPLICATION DATA

UPLINK FAR-FIELD NOISE REDUCTION OVERVIEW

The uplink portion of the LMV1099 is a fully analog solution to reduce the far field noise picked up by

microphones in a communication system. A simplified block diagram is provided in Figure 20.

Figure 20. Simplified Block Diagram of the LMV1099 Uplink path

The output signal of the microphones is amplified by a pre-amplifier with adjustable gain between 12dB and

36dB. The matched signals are then routed through the Analog Noise Cancelling block which suppresses the far-

field signal. The output of the analog noise cancelling processor is amplified in the post amplifier with selectable

gain, 6dB or 12dB. For optimum noise and EMI immunity, the microphones have a differential connection to the

LMV1099 and the uplink output is also differential. The adjustable gain functions can be controlled via I2C.

POWER SUPPLY CIRCUITS

A low drop-out (LDO) voltage regulator in the LMV1099 allows the device to be independent of supply voltage

variations.

The Power On Reset (POR) circuitry in the LMV1099 requires the supply voltage to rise from 0V to VDD in less

than 100ms.

The Mic Bias output is provided as a low noise supply source for the electret microphones. The noise voltage on

the Mic Bias microphone supply output pin depends on the noise voltage on the internal the reference node. The

de-coupling capacitor on the VREF pin determines the noise voltage on this internal reference. This capacitor

should be larger than 1nF; having a larger capacitor value will result in a lower noise voltage on the Mic Bias

output.

GAIN BALANCE AND GAIN BUDGET

In systems where input signals have a high dynamic range, critical noise levels or where the dynamic range of

the output voltage is also limited, careful gain balancing is essential for the best performance. Too low of a gain

setting in the preamplifier can result in higher noise levels, while too high of a gain setting in the preamplifier will

result in saturation of the noise cancelling processor and output stages.

The gain ranges and maximum signal levels for the different functional blocks are shown in Figure 21. Two

examples are given as a guideline on how to select proper gain settings.

Figure 21. Maximum Signal Levels

Copyright © 2010–2013, Texas Instruments Incorporated

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