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LMV1015XR-15 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LMV1015XR-15 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 11 page  Application Section (Continued) 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: dBPa = dB SPL + 20*log 20 µPa dBPa=dBSPL -94dB 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 V OUT = 70 −94 −44 = −68 dBV This is equivalent to 1.13 mV PP 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 PP.By 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 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 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 6. One capacitor reduces the noise caused by the 900 MHz carrier and the other reduces the noise caused by 1800/ 1900 MHz. 20128911 FIGURE 4. dB SPL to dBV Conversion 20128912 FIGURE 5. LMV1015-15 Gain vs. Frequency Over Temperature www.national.com 8 |
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