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LMV1032UPX-06 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LMV1032UPX-06 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 12 page  Application Section (Continued) 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 LMV1032-06 has a gain of 2 (6 dB) over the JFET, the output voltage of the microphone is 2.26 mV PP.By 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. 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 k Ω. 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. 20084211 FIGURE 4. dB SPL to dBV Conversion 20084212 FIGURE 5. Gain vs. Frequency Over Temperature www.national.com 9 |
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Descripción similar - LMV1032UPX-06 |
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