lmv1032-06 National Semiconductor Corporation, lmv1032-06 Datasheet - Page 9

• Manufacturer Part Number: lmv1032-06
• Description: Amplifiers For 3 Wire Analog Electret Microphones
• Manufacturer: National Semiconductor Corporation

Application Section
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 LMV1032-06 has a gain of 2 (6 dB) over the JFET,
the output voltage of the microphone is 2.26 mV
implementing the LMV1032-06, the sensitivity of the micro-
phone is −38 dBV/Pa (−44 + 6).
LOW FREQUENCY CUT OFF FILTER
To reduce noise on the output of the microphone a low cut
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.
dBPa = dB SPL - 94 dB
V
This is equivalent to 1.13 mV
dBPa = dB SPL + 20*log 20 µPa
OUT
= 70 −94 −44 = −68 dBV
FIGURE 4. dB SPL to dBV Conversion
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The LMV1032 is optimized to be used in audio band appli-
cations. By using the LMV1032, the gain response is flat
within the audio band and has the linearity and temperature
stability.
ADVANTAGE OF THREE PINS
The LMV1032 ECM solution has three pins instead of two
pins as in the case of a JFET solution. The third pin provides
the advantage of a low supply current, high PSRR and
eliminates the need for additional components.
Noise pick-up by a microphone in a cell phone is a well-
known problem. A conventional JFET circuit is sensitive for
noise pick-up because of its high output impedance. The
output impedance is usually around 2.2 kΩ. By providing
separate output and supply pins a much lower output imped-
ance is achieved and therefore is less sensitive to noise
pick-up.
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 type of noise is called
bumblebee noise.
EXTERNAL PRE-AMPLIFIER APPLICATION
The LMV1032 can also be used outside of an ECM as a
space saving external pre-amplifier. In this application, the
LMV1032 follows a phantom biased JFET microphone in the
circuit (Figure 6). The input of the LMV1032 is connected to
the microphone via the 2.2 µF capacitor. The advantage of
this circuit versus one with only a JFET microphone are the
additional gain and the high pass filter of the LMV1032. The
high pass filter makes the output signal more robust and less
sensitive to low frequency disturbances. The LMV1032
should be placed as close as possible to the microphone.
FIGURE 5. Gain vs. Frequency Over Temperature
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