Microphones have become a standard device built into many electronic products. From wearable devices to home assistants, more and more devices are required to "hear" their environment and respond accordingly. This article will give a comprehensive overview of microphone types and basic principles, as well as the product features of CUI Devices’ MEMS Microphones.
Technical Differences Between ECM and MEMS Microphones
Along with the increase in microphone applications, the requirements for the sensitivity and volume of microphones is increasing as well. At present, the two most common technologies used to build microphones are MEMS and electret condenser. Let’s first introduce the basic knowledge of the MEMS and electret condenser microphone (ECM), compare the technical differences between them, and outline the advantages of each solution.
The MEMS microphone consists of MEMS components placed on a printed circuit board (PCB) and protected by a mechanical cover. It creates a small hole in the housing to allow sound to enter the microphone. If the hole is in the top cover, it is designated as top ported; if the hole is in the PCB, it is designated as bottom ported. MEMS components are usually designed with mechanical diaphragms and mounting structures created on semiconductor die.
The MEMS diaphragm forms a capacitor, and the sound pressure wave causes the movement of the diaphragm. The MEMS microphone usually contains a second semiconductor die, which is used as an audio preamplifier to convert the varying capacitance of the MEMS into electrical signals. If an analog output signal is required, the output of the audio preamplifier is provided to the user. If a digital output signal is required, the analog-to-digital converter (ADC) is included on the same die as the audio preamplifier. A common format for digital encoding in MEMS microphones is pulse density modulation (PDM), which allows communication with only a clock and a single data line. Due to the single bit encoding of data, the decoding of the digital signal at the receiver is simplified. Digital I²S output is the third option, including an internal decimation filter, which allows processing to be completed in the microphone itself. This means that the microphone can be directly connected to a digital signal processor (DSP) or microcontroller, thus eliminating the need for an ADC or codec in many applications.
The ECM contains an electret diaphragm (a material with a fixed surface charge) separated from but close to the conductive plate. Similar to the MEMS microphone, it forms a capacitor with an air gap as the dielectric. As the sound pressure wave moves the electret diaphragm, the voltage at both ends of the capacitor changes with the change of the capacitance value. The change of the capacitance voltage is amplified and buffered by the JFET inside the microphone housing. The JFET is usually configured as a common source configuration, while external load resistor and a DC blocking capacitor are used in external application circuits.
ECM and MEMS Microphones Each Have Their Own Advantages
Many factors need to be considered when choosing between ECM and MEMS microphones. Due to many advantages provided by MEMS, the market share of MEMS microphone continues to grow rapidly. For example, the small footprint of the MEMS microphone is very attractive for space constrained applications, and due to the analog and digital circuits contained in the MEMS microphone construction, the PCB area and component cost can be reduced. The output impedance of an analog MEMS microphone is relatively low, while the output of a digital MEMS microphone is very suitable for applications in an electrically noisy environments. In high vibration environments, the use of MEMS microphone technology can reduce the unwanted noise level introduced by mechanical vibration. Furthermore, with the addition of semiconductor manufacturing technology and an audio preamplifier, it is possible to manufacture a MEMS microphone with close matching and temperature stability. These stringent performance characteristics are particularly beneficial when MEMS microphones are used in array applications. In the process of product manufacturing, the MEMS microphone can also be easily handled by pick and place machines and tolerate reflow soldering temperature profiles.
Although MEMS microphones are rapidly gaining in popularity, there are still some applications that may prefer the ECM. Many legacy designs use the ECM, so if the project is a simple upgrade of the existing design, it is best to continue to use the ECM. The options for connecting the ECM to the application circuit include pins, wires, SMTs, solder pads and spring contacts, providing engineers with additional design flexibility. If dust and moisture are a problem, it is easy to find ECM products with high ingress protection (IP) level because of their large physical size. For projects requiring non-uniform spatial sensitivity, ECM products can provide unidirectional or noise canceling with internal directivity, and the wide operating voltage range of the ECM may be the preferred solution for products with loosely regulated voltage rails.
PDM and I²S Protocol Have Different Characteristics
In addition to a significantly reduced footprint, lower power requirements and greater electrical noise rejection, one of the main advantages of MEMS microphones is the addition of output options, providing greater flexibility for designers and engineers. Although the analog option is still available, the two popular output options are PDM and I²S digital protocol. Each of these interfaces have their own unique characteristics. The key factors to be considered include audio quality levels, power consumption levels, bill of materials costs, space constraints that the design must comply with, and the operating environment in which the hardware will be deployed.
PDM is used to convert an analog signal voltage into a unit pulse density modulated digital stream. PDM signals look more similar to longitudinal waves than stereotypical transverse waves related to audio, but they are digital representations of analog signals. The resulting signals have many advantages of digital signals and are still directly related to analog signals. Creating this PDM signal requires a higher sampling rate than usual (a rate higher than 3 MHz), because the occurrence frequency of digital pulses must be many times higher than the oscillation frequency of the analog signal they represent.
Due to the digital characteristics of the signal, PDM can better adapt to an electrically noisy environment than analog signals and has a higher error tolerance when the signal is degraded. High frequency signals do create distance limitations because an increase in capacitance on longer transmission lines may lead to unwanted attenuation and an accompanying degradation of audio quality. These signals also need to be further processed by an external DSP or microcontroller with an appropriate codec to run the PDM signal through a low-pass filter to extract or down sample it to a lower sampling rate so that it can be used for other devices.
Unlike PDM, I²S is a completely digital signal, which does not need to be encoded or decoded, and there is no generally required data transmission speed. However, the minimum speed depends on the data being transmitted and its accuracy. If the audio sampling rate is the industry standard of 44.1 kHz and the precision is 8 bits, the mono channel will require a clock speed of at least 352.8 kHz. Stereo applications will be twice at 705.6 kHz, and any change in accuracy will also change the minimum transmission bandwidth.
PDM provides better noise immunity and bit error tolerance, which can make it attractive for many applications that give priority to audio quality. In contrast, I²S supports ease of installation, reduced overall footprint and reduced number of components, which will have advantages when the product size or its price tag proves to be the main concern. It should also be noted that I²S interfaces will provide better signal integrity over a longer distance, so it is also suitable for implementations where the microphone and processing circuit cannot be close to each other on the circuit board.
Diversified Selection of MEMS Microphones
MEMS microphones have become commonplace in modern electronic design, and it is very important to have the best interface. There are many factors to consider when deciding which interface will be optimized for your specific application scenario. PDM can be an ideal choice in challenging application environments, thanks to its inherent resilience to noise ability. Instead, the use of I²S allows the input to be directly connected to its accompanying DSP or other processor/controller devices without any additional complexity.
CUI Devices have a wide range of MEMS microphone product portfolios, which can meet various audio system requirements. In addition to an analog interface unit, various microphones with PDM and I²S digital interface are also available. CUI Devices' MEMS microphones are packaged in compact, low profile packages as small as 2.75 x 1.85 x 0.90 mm, providing users with better audio quality and performance. With sensitivity levels ranging from -42 dB to -26 dB, signal-to-noise ratios ranging from 57 dBA to 65 dBA, and sensitivity tolerances as low as ±1 dB, these MEMS microphones are ideal for a range of portable consumer electronics applications. For easier prototyping and design testing, CUI Devices also provides a MEMS microphone development kit, which includes four independent microphone evaluation circuits.
CUI Devices' MEMS microphones have top and bottom port versions, analog and digital options, sensitivity levels of -42 DB to -26 dB, and current consumption as low as 80 µA. You can find the most suitable MEMS microphones for your needs at the following website: https://www.arrow.com/en/manufacturers/cui-devices/audio-components/microphones?promoGroupLevel=main&filters=Technology:MEMS.
Conclusion
The MEMS microphone has the advantages of small volume and strong noise resistance. It is the first choice for many consumer application products. CUI Devices provides product types with different sensitivities and interfaces and provides complete design resources to help customers to quickly develop corresponding products, which is worthy of further understanding and adoption.