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LMV1012XP-25 Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LMV1012XP-25 Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 15 page  Application Section HIGH GAIN The LMV1012 series provides outstanding gain versus the JFET and still maintains the same ease of implementation, with improved gain, linearity and temperature stability. A high gain eliminates the need for extra external components. BUILT IN GAIN The LMV1012 is offered in 0.3 mm height space saving small 4-pin micro SMD packages in order to fit inside the different size ECM canisters of a microphone. The LMV1012 is placed on the PCB inside the microphone. The bottom side of the PCB usually shows a bull’s eye pattern where the outer ring, which is shorted to the metal can, should be connected to the ground. The center dot on the PCB is connected to the V DD through a resistor. This phantom biasing allows both supply voltage and output sig- nal on one connection. A-WEIGHTED FILTER The human ear has a frequency range from 20 Hz to about 20 kHz. Within this range the sensitivity of the human ear is not equal for each frequency. To approach the hearing re- sponse weighting filters are introduced. One of those filters is the A-weighted filter. The A-weighted filter is usually used in signal to noise ratio measurements, where sound is compared to device noise. This filter improves the correlation of the measured data to the signal to noise ratio perceived by the human ear. MEASURING NOISE AND SNR The overall noise of the LMV1012 is measured within the frequency band from 10 Hz to 22 kHz using an A-weighted filter. The input of the LMV1012 is connected to ground with a 5 pF capacitor, as in Figure 3. Special precautions in the internal structure of the LMV1012 have been taken to reduce the noise on the output. The signal to noise ratio (SNR) is measured witha1kHz input signal of 18 mV PP using an A-weighted filter. This represents a sound pressure level of 94 dB SPL. No input capacitor is connected for the measurement. SOUND PRESSURE LEVEL The volume of sound applied to a microphone is usually stated as a pressure level referred to the threshold of hear- ing of the human ear. The sound pressure level (SPL) in decibels is defined by: Sound pressure level (dB) = 20 log P m/PO Where, P m is the measured sound pressure P O is the threshold of hearing (20 µPa) In order to be able to calculate the resulting output voltage of the microphone for a given SPL, the sound pressure in dB 20058702 FIGURE 1. Built in Gain 20058709 FIGURE 2. A-Weighted Filter 20058710 FIGURE 3. Noise Measurement Setup www.national.com 10 |
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