lmv1015xrx-25 National Semiconductor Corporation, lmv1015xrx-25 Datasheet - Page 8

• Manufacturer Part Number: lmv1015xrx-25
• Description: Built-in Gain Ic S For High Sensitivity 2-wire Microphones
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Section
SPL needs to be converted to the absolute sound pressure
in dBPa. This is the sound pressure level in decibels referred
to 1 Pascal (Pa).
The conversion is given by:
Translation from absolute sound pressure level to a voltage
is specified by the sensitivity of the microphone. A conven-
tional microphone has a sensitivity of -44 dBV/Pa.
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 mV
implementing the LMV1015-15, the sensitivity of the micro-
phone is −28.4 dBV/Pa (−44 + 15.6).
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
dBPa = dB SPL + 20*log 20 µPa
dBPa = dB SPL - 94 dB
V
This is equivalent to 1.13 mV
OUT
= 70 −94 −44 = −68 dBV
FIGURE 4. dB SPL to dBV Conversion
PP
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PP
. By
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amplified which gives a bass sound. This amplification can
cause an overload, which results in a distortion of the signal.
The LMV1015 is optimized to be used in audio band appli-
cations. By using the LMV1015, the gain response is flat
within the audio band and has linearity and temperature
stability Figure 5.
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 kΩ.
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 6.
One capacitor reduces the noise caused by the 900 MHz
carrier and the other reduces the noise caused by 1800/
1900 MHz.
FIGURE 5. LMV1015-15 Gain vs. Frequency Over
Temperature
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