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1、 February 2017 DocID025704 Rev 2 1/20 AN4426 Application note Tutorial for MEMS microphones Introduction This application note serves as a tutorial for MEMS microphones, providing general characteristics of these devices, both acoustic and mechanical, as well as summarizing the portfolio available f
2、rom ST. MEMS microphones target all audio applications where small size, high sound quality, reliability and affordability are key requirements. STs MEMS microphones are designed, developed and manufactured within ST, creating an industry- unique, vertically integrated supply chain. Both analog and
3、digital-input, top and bottom-port solutions are available. Our best-in-class AOP and SNR make STs MEMS microphones suitable for applications that require a very high dynamic range, improving the audio experience in any environment. Matching very tight sensitivity allows optimizing beamforming and n
4、oise cancelling algorithms for multi-microphone arrays. Low power consumption allows extending battery life. Contents AN4426 2/20 DocID025704 Rev 2 Contents 1 Mechanical specifications, construction details . 4 2 Acoustic parameters . 11 2.1 Sensitivity 11 2.2 Directionality 11 2.3 SNR 12 2.4 Dynami
5、c range and acoustic overload point . 12 2.5 Equivalent input noise . 13 2.6 Frequency response . 15 2.7 Total harmonic distortion . 16 2.8 PSRR and PSR . 16 3 MEMS microphone portfolio . 17 4 Revision history 19 AN4426 List of figures DocID025704 Rev 2 3/20 List of figures Figure 1: MEMS microphone
6、 inside package . 4 Figure 2: MEMS transducer mechanical specifications 4 Figure 3: Capacitance change principle . 5 Figure 4: 4 x 5 package 5 Figure 5: 3 x 4 metal cap package - bottom port 6 Figure 6: 3 x 4 package - top port . 6 Figure 7: 2 x 3 package - bottom port . 7 Figure 8: Faraday cage in
7、STs MEMS microphones . 7 Figure 9: RF immunity simulation . 8 Figure 10: EMC test setup 8 Figure 11: RF test disturbance signal with sinusoidal pattern 9 Figure 12: RF immunity test results - MP34DT04 9 Figure 13: RF test disturbance signal 217 Hz burst pattern 10 Figure 14: RF immunity of analog di
8、fferential microphones . 10 Figure 15: Omnidirectional microphone 11 Figure 16: A-weighted filter response . 12 Figure 17: Acoustic and electrical relationship - analog . 13 Figure 18: Acoustic and electrical relationship - digital . 14 Figure 19: MP45DT02-M frequency response . 15 Figure 20: MEMS m
9、icrophone portfolio 17 Figure 21: MEMS microphone notation 17 Mechanical specifications, construction details AN4426 4/20 DocID025704 Rev 2 1 Mechanical specifications, construction details A microphone is a dual-die device consisting of two components, the integrated circuit and the sensor, which a
10、re housed in a package using techniques that are proprietary to ST. Figure 1: MEMS microphone inside package The sensor uses MEMS technology (Micro-Electrical-Mechanical Systems) and it is basically a silicon capacitor. The capacitor consists of two silicon plates/surfaces. One plate is fixed while
11、the other one is movable (respectively, the green plate and the grey one shown in the following figure). The fixed surface is covered by an electrode to make it conductive and is full of acoustic holes which allow sound to pass through. The movable plate is able to move since it is bonded at only on
12、e side of its structure. A ventilation hole, allows the air compressed in the back chamber to flow out and consequently allows the membrane to move back. The chamber allows the membrane to move inside but also, in combination with the chamber created by the package will affect the acoustic performan
13、ce of the microphones in terms of frequency response and SNR. Figure 2: MEMS transducer mechanical specifications So basically the microphone MEMS sensor is a variable capacitor where the transduction principle is the coupled capacitance change between a fixed plate (back plate) and a movable plate
14、(membrane) caused by the incoming wave of the sound. MEMS sensor ASIC (Application specific integrated circuit) AN4426 Mechanical specifications, construction details DocID025704 Rev 2 5/20 Figure 3: Capacitance change principle The integrated circuit converts the change of the polarized MEMS capaci
15、tance into a digital (PDM modulated) or analog output according to the microphone type. Finally the MEMS microphone is housed in a package with the sound inlet placed in the top or in the bottom part of the package, hence the top-port or bottom-port nomenclature of the package. ST manufactures micro
16、phones using industry-wide techniques, but also has developed innovative packaging to achieve improved performance of the microphones. Packaging techniques will be discussed in further detail. The 4x5 package is widely used to house the digital microphone MP45DT02-M. It is a common packaging technique in a top-port configuration where the ASIC is placed under the sound inlet with glue on top (glob top) in order to protect the circuit from light and the MEMS sensor is placed beside the in
