Owners of smartphones and tablets are constantly seeking innovative ways to utilize their devices while expecting exceptional performance. The built-in audio function serves as a prime example. Users desire the ability to record social events, musical performances, and enjoy precise, realistic playback. They also want high-quality voice calls without background noise, even in noisy outdoor settings or inside vehicles. Additionally, there’s a growing need for microphones that can deliver excellent audio quality while simultaneously capturing other ambient sounds effectively. These evolving demands necessitate advanced microphones capable of delivering superior performance in compact sizes. Some phones now incorporate noise-canceling or 3D sound capabilities through the use of dual or multi-microphone setups. Furthermore, the rise of intelligent digital assistants responding to user voices is transforming how we interact with technology. This shift is pushing high-performance audio subsystems into a wider range of products, including wearables and future IoT devices. Consequently, there’s an increasing demand for microelectromechanical systems (MEMS) microphones that offer high performance, fidelity, and reliability in small packages for portable devices. Market research firm IHS Technology predicts that the MEMS microphone market could grow from 3.6 billion units in 2015 to over 6 billion units by 2019. Structure and Working Mode of MEMS Microphones MEMS microphones integrate a movable diaphragm and a static backplate onto a silicon wafer substrate using common fabrication techniques like deposition and selective etching. The backplate contains perforations that allow airflow without causing distortions. The diaphragm is engineered to respond to changes in air pressure due to sound waves. This movement alters the distance between the diaphragm and the backplate, resulting in a proportional change in capacitance. An accompanying integrated circuit (IC), packaged with the MEMS transducer, converts this capacitive change into an electrical signal, either in analog or digital form. On the market, you'll find MEMS microphones with both analog and digital outputs. Analog microphones typically consist of MEMS transducers paired with an analog amplifier IC, making them a popular choice for smaller handheld devices like feature phones and mid-range smartphones. Digitization Digital microphones, featuring integrated analog conditioning circuits and analog-to-digital converters (ADCs), are often favored in devices like PCs or high-end smartphones. Digital technology provides inherent resistance to radio frequency (RF) interference and electromagnetic interference (EMI), improving audio performance. As shown in Figure 1, digital microphones also simplify circuit design and make it easier to implement design changes without altering resistor and capacitor values. Most digital microphones come equipped with a clock and an L/R control input. The clock input manages the Δ∑ modulator, converting the sensor's analog signal into a digital pulse density modulation (PDM) signal. Typical clock frequencies range from 1MHz to 3.5MHz. During each clock cycle, the microphone drives its output to the appropriate level on the selected clock edge, then enters a high-impedance state during the second half of the cycle. This setup enables two digital microphone outputs to share a single data line (Figure 2). The L/R input determines the clock edge for valid data. Digital MEMS microphones offer significant benefits, including high noise immunity and simplified circuit design. These features make them ideal for multi-microphone arrays used in applications like noise cancellation, reverberation generation, and beamforming for directional sensitivity. In smartphones, a common method for noise cancellation involves placing one or more additional microphones away from the primary voice microphone, such as on the top or back of the device, to detect environmental noise and subtract it from the voice microphone's output, thereby improving call quality. Noise-canceling microphones are also frequently used in video recording modes. Beamforming also relies on arrays of two or more microphones. While most microphones exhibit omnidirectional sensitivity, certain applications might benefit from heightened sensitivity in specific directions or reduced sensitivity in others—for instance, enhancing audio quality during conference calls or driving conversations. Beamforming achieves this by applying a digital algorithm to the microphone outputs in the array, based on the phase differences of sound arriving from various directions. This helps determine the source direction of a particular sound. Application-Specific Integrated Circuit (ASIC) Design Microphone module manufacturers differentiate their products by selecting optimal MEMS microphone kits that combine carefully matched MEMS sensors with ASICs. ON Semiconductor focuses on developing highly integrated digital MEMS microphone ASICs compatible with various MEMS transducers produced by independent MEMS suppliers. For example, the LC706200 digital IC family integrates a feedforward delta-sigma ADC along with an integrated analog and low-pass filter, as depicted in Figure 4. It also includes a charge pump that supplies the operating voltage to the MEMS transducer. ON Semiconductor’s digital ASICs address critical performance benchmarks, helping contemporary MEMS microphone designers overcome challenges. High signal-to-noise ratios (SNRs) are essential for clear performance when microphones are used at longer distances, ensuring crisper audio capture. Advanced speech recognition algorithms depend on high SNRs for accurate transcription. Today, ASICs with SNRs exceeding 64 dB are required, complemented by MEMS engineers' efforts to optimize transducer characteristics. As users demand better devices, such as smartphones, microphones must operate without distortion in noisy environments to achieve high sound pressure levels (SPL). For instance, supporting social users to create high-quality recordings capturing their experiences at festivals. Digital MEMS Microphone for Future Independent Voice Commands Voice recognition engines and powerful assistants like Siri, "OK Google," and Amazon Echo have stringent requirements for voice command functionalities in IoT and portable devices. Current speech recognition systems typically consume substantial power to listen and recognize speech. Future voice command functions will likely be expected to activate independently upon voice activation. Low-power digital MEMS microphone technology will be well-suited for such future independent voice-triggering solutions—offering excellent performance with minimal power consumption and easy integration into existing designs. Algorithms like noise cancellation and beamforming, which analyze signals from multiple microphones, rely on consistent microphone sensitivities across the array, ideally within ±1 dB. While screening or grading presents a potential solution, microphone designers seek ASICs providing adjustable gain to account for variations in MEMS manufacturing. The LC706200 product line offers a high-performance solution. It features several attributes ensuring enhanced linear performance over a wide operating range, including low input reference noise (-106dBFS), peak compensation with an 8kHz low-pass filter, and low noise thanks to ON Semiconductor's Gigaohm resistor process, internal bias, and regulation circuits. The device also boasts a high power supply rejection ratio (PSRR), preventing undesirable noise from entering the signal chain and maintaining power management during voice command activations, including sleep mode and low-power modes. Conclusion: Changing user behaviors and expectations regarding computer and smart device usage are driving demand for reliable and high-performance MEMS microphones. The digital ASICs available today grant microphone developers unprecedented flexibility, enabling them to deliver top-tier products meeting these evolving needs. Solar Inverter,Efficiency Home Inverter,Solar System Inverter,Solar Pump Inverter GuangZhou HanFong New Energy Technology Co. , Ltd. , https://www.gzinverter.com
Figure 1. Demonstrating digital improvements in noise immunity
Figure 2. Digital microphone reduces the number of transmission lines
Figure 3. Microphone Expert Choosing the Right MEMS Microphone Kit
Figure 4. A feedforward delta-sigma ADC implements an integrated digital output small footprint microphone