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  • Differences between analog and digital microphones

        According to a special report MEMS IHS company , in recent years , due to the digital MEMS microphone with high immunity to electromagnetic interference and greater flexibility in the design phase , etc., are increasingly being use , although analog microphone in price cheaper , but digital microphone reflect the advantages in the design phase . If the analog signal processing , repeated each time a new design must adapt resistors, capacitors and speakers. For digital microphone design changes easier to implement , which can shorten the time to market.
        Analog and digital microphone output signals have different considerations in the design , in the structure also reflects the differences. So analog and digital MEMS microphone What ´s the difference?
        Analog Microphone
        The output impedance of the analog MEMS microphone is typically a few hundred ohms. This impedance is typically higher than the amplifier has a low output impedance , low impedance circuit behind the microphone is attenuated signal level. For example, some codecs before the ADC has a programmable gain amplifier (PGA). At high gain settings , PGA input impedance may be only a few thousand ohms. The output impedance of MEMS microphones behind with an input impedance of 200Ω to 2kΩ the PGA will make the signal level attenuation of nearly 10%.
        Analog output MEMS microphone is normally biased to a DC voltage between the ground and the supply voltage . The principle of selection bias voltage is the most significant output signal peak is not clamped by the power supply voltage and a ground potential limits. The presence of the DC bias voltage also means that the microphone is usually connected back through an amplifier or converter chip AC coupling . Series capacitor selection principle is that the high -pass filter circuit with a codec or amplifier input impedance together form does not make the low-frequency portion of the signal is located microphone natural low frequency roll-off above the roll-off.
        For the microphone to 100Hz frequency -3dB point having 10kΩ input impedance amplifiers or the codec ( both are common ), even a relatively small 1.0? F capacitor will be the angular frequency of the high pass filter is set to 16Hz, this value can affect the microphone is far beyond the scope of the response .
        Digital Microphone
        Digital microphone will shift from analog to digital conversion codec into the microphone , enabling the processor from the microphone to the all-digital audio capture channels. MEMS microphone in digital applications often susceptible to interference from the analog audio signal in use .
        For example, in the Tablet PC , the position of the microphone may not be close to ADC, the signal between these two points may pass through or close to Wi-Fi, Bluetooth or cellular antenna. These digital connection , they are not susceptible to radio interference and the noise or distortion in the audio signal . This pickup system noise harmful to improve the design of the microphone layout provides great flexibility.
        In only analog audio interface to connect an analog microphone digital microphone system is also useful . In only need to capture the audio playback system but do not like the surveillance camera , the use of digital output microphone after the need for a separate audio codecs or converters , the microphone can be directly connected to the digital processor.
        Of course , a good digital design experience must still be used in digital microphone clock and digital signals. 20Ω to 100Ω small values ​​of useful source termination resistor that ensures that at least several inches long traces with good integrity of the digital signal ( Figure 7 ) . When using shorter trace lengths , or running at a lower rate clock digital microphone , microphone pin can be connected directly to the codec or DSP, without any passive components.
        PDM is the most common digital microphone interface . This interface allows the two microphones share a common clock and data lines. Each microphone is configured to produce different respective edge of the clock signal output . The output can be maintained so that the two microphones are synchronized with each designer can ensure that data from each channel are simultaneously captured .
        In the worst case , from two microphones to capture the data signal may be separated by half a clock cycle time . Typical values ​​of such a clock frequency of about 3MHz, so the passage of time difference is only 0.16us, the listener can perceive much smaller than a threshold value. This same synchronization mechanism can be extended to more than two PDM system with microphones , just make sure that all microphones are connected to the same clock source , and the data signals are filtered and processed together . In the case of the analog microphone , this synchronization is achieved on the move ADC.
        Analog and digital MEMS microphone inside details of the differences
        For digital MEMS microphones are less sensitive to electromagnetic interference , and power supply rejection ratio (PSRR) improved to simplify the structure and improve the audio quality. MEMS ( micro- electro-mechanical systems ) technology to produce MEMS-based microphone is a microphone , simply means that a capacitor integrated on silicon micro- chip can be manufactured using surface mount technology , able to withstand high reflow temperatures, easy and CMOS process and other audio integrated circuits , and has improved noise elimination performance and good RF and EMI suppression can .
        MEMS microphone output is not directly from the MEMS transducer unit . The transducer is essentially a variable capacitor , and having a particularly high output impedance megohm .
        In microphone package, the transducer signal is first sent to the preamplifier, while the primary function of this amplifier is impedance transformation , when the audio signal chain to tap into the microphone output impedance is reduced to a more appropriate value. Microphone output circuit in the preamplifier circuit is also implemented .
        For analog MEMS microphone , which is basically a special circuit output impedance amplifier has . In digital MEMS microphone , the amplifier and the analog to digital converter (ADC) integrated in the pulse density modulation (PDM) or a digital output I2S format .
        MEMS microphone package rather unique in the semiconductor device , because there is a hole in the package for the arrival of acoustic energy transducer unit . In this package interior , MEMS microphone transducer and the analog or digital ASIC bound together and mounted on a common stack . And then at the top of the stack to bind a cover for closing the transducer and ASIC. Such a laminate is usually a small PCB, out of the IC for the microphone signals to pins outside of the package .
        Analog and digital MEMS microphone transducer and ASIC ( the epoxy resin below ) are mounted on the laminate. Additional binding digital microphone cable ASIC is connected to the electrical signal from the stack .
        Differences between analog and digital MEMS microphone package size
        Generally, analog MEMS microphone package size smaller than a digital microphone . This is because the package requires less analog microphone pin ( usually three , and digital microphone requires five or more ) , but also less than the analog preamplifier circuit numbers , so using the same fabrication process simulation before smaller than the figure put before release . In most space- constrained designs, many of such small mobile devices , analog microphone because of the small size and more popular.
        Analog microphone package size may be 2.5 × 3.35 × 0.88mm or less, and PDM microphone package size is typically 3 × 4 × 1mm, the volume of the package increased by 62 %.
        Analog and digital MEMS microphones in different applications can play to their strengths . To limit the choice of layout size and components of the integrated system , electrical connections and potential sources of noise and interference considerations.

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