Common Microphone Classifications
What is MEMS
Micro-Electro Mechanical System (MEMS) refers to a sensor device with a size of a few millimeters or even smaller, and its internal structure is generally on the order of microns or even nanometers. It is an independent intelligent system. Simply understand, MEMS is to miniaturize the mechanical parts of traditional sensors, fix the device on the silicon wafer (wafer) through three-dimensional stacking technology, such as three-dimensional through-silicon via TSV, and finally adopt special customized packaging according to different applications form, and finally cut and assembled silicon-based sensors. Benefiting from the cost advantage brought by the mass production of IC silicon wafer processing that cannot be achieved by ordinary sensors, MEMS also has the miniaturization and high integration that ordinary sensors cannot possess.
MEMS microphone
The MEMS element of a MEMS microphone sits on a printed circuit board (PCB) and is protected by a mechanical cover with a small hole cut into the housing to allow sound to enter the device. The location of the hole determines the type of microphone, if the hole is inside the top cover, the microphone is called a top hole microphone; if the hole is inside the PCB, it is called a bottom hole microphone. MEMS components typically have a mechanical diaphragm and a mounting structure mounted on a semiconductor chip.
The MEMS diaphragm forms a capacitor, and the sound pressure waves cause the diaphragm to move. Typically, MEMS microphones contain a second semiconductor die that acts as an audio preamplifier, which converts the varying capacitance of the MEMS into an electrical signal. Where an analog output signal is preferred, the output of the audio preamplifier may be provided to the user.
However, if a digital output signal is required, an analog-to-digital converter (ADC) is combined on the same chip as the audio preamplifier. Pulse Density Modulation (PDM) is the traditional format used for digital encoding in MEMS microphones and allows communication with only a single data line and clock. Additionally, decoding digital signals becomes easier at the receiver due to the single-bit encoding of the data.
ECM microphone
In an ECM, an electret diaphragm is a material with a fixed surface charge that is placed near a conductive plate and, like a MEMS microphone, creates a capacitor where the air gap forms the dielectric. The sound pressure wave moving the electret diaphragm causes a change in capacitance value, resulting in a change in voltage across the capacitor, ΔV=Q/ΔC (Q=fixed charge). These changes in capacitor voltage are amplified and buffered by a JFET inside the microphone housing. JFETs are usually configured in a common source configuration with DC blocking capacitors for external load resistors and external application circuits.
how to choose
There are a number of considerations when choosing between ECM and MEMS microphones. The market share of MEMS microphones continues to grow rapidly due to the many advantages offered by this new technology. For example, space-constrained applications will find the small footprint available for MEMS microphones attractive, while reducing PCB area and component cost due to the analog and digital circuitry contained within the MEMS microphone structure. The relatively low output impedance of analog MEMS microphones and the output of digital MEMS microphones are well suited for use in electrically noisy environments. In high-vibration environments, the use of MEMS microphone technology can reduce unwanted noise from mechanical vibrations. Additionally, the addition of semiconductor fabrication techniques and audio preamplifiers has made it possible to manufacture MEMS microphones with closely matched and temperature-stable performance characteristics. These tight performance characteristics are especially beneficial when using MEMS microphones in array applications. During product manufacturing, MEMS microphones can also be easily handled by pick-and-place machines and can withstand the reflow soldering temperature profile.
Although MEMS microphones are rapidly gaining popularity, in some applications electret condenser microphones may be the preferred choice. Many legacy designs have used ECM, so if the project is a simple upgrade to an existing design, it is best to continue using ECM. Options for interfacing the ECM to the application circuit include pins, wires, SMT, solder pads, and spring contacts, giving engineers additional design flexibility. If protection from dust and moisture is required, it is easy to find ECM products with a high degree of protection (IP) due to their large physical size. For projects requiring non-uniform spatial sensitivity, ECM products with inherent directivity (unidirectional or noise cancellation) are available, while the ECM's wide operating voltage range may be the preferred solution for products with loosely regulated voltage rails.
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