INPUT

OUTPUT

10 pF

33 pF

10

1k

1M

FREQUENCY (Hz)

-5

5

20

100k

10k

100

15

10

0

-40°C

25°C

85°C

LMV1015

SNAS291B – MARCH 2005 – REVISED APRIL 2013

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Example: Busy traffic is 70 dB SPL.

Since the LMV1015-15 has a gain of 6 (15.6 dB) over the JFET, the output voltage of the microphone is 6.78

LOW FREQUENCY CUT OFF FILTER

To reduce noise on the output of the microphone a low frequency cut off filter has been implemented. This filter

reduces the effect of wind and handling noise.

It's also helpful to reduce the proximity effect in directional microphones. This effect occurs when the sound

source is very close to the microphone. The lower frequencies are amplified which gives a bass sound. This

amplification can cause an overload, which results in a distortion of the signal.

Figure 16. LMV1015-15 Gain vs. Frequency Over Temperature

The LMV1015 is optimized to be used in audio band applications. By using the LMV1015, the gain response is

flat within the audio band and has linearity and temperature stability Figure 16.

NOISE

Noise pick-up by a microphone in cell phones is a well-known problem. A conventional JFET circuit is sensitive

for noise pick-up because of its high output impedance, which is usually around 2.2 k

Ω.

RF noise is amongst other caused by non-linear behavior. The non-linear behavior of the amplifier at high

frequencies, well above the usable bandwidth of the device, causes AM-demodulation of high frequency signals.

The AM modulation contained in such signals folds back into the audio band, thereby disturbing the intended

microphone signal. The GSM signal of a cell phone is such an AM-modulated signal. The modulation frequency

of 216 Hz and its harmonics can be observed in the audio band. This kind of noise is called bumblebee noise.

RF noise caused by a GSM signal can be reduced by connecting two external capacitors to ground, see

Figure 17. One capacitor reduces the noise caused by the 900 MHz carrier and the other reduces the noise

caused by 1800/1900 MHz.

Figure 17. RF Noise Reduction

8

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