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As the popularity of smartphones and tablets continues to rise, users are increasingly seeking innovative ways to utilize their devices while demanding exceptional performance. One prominent example is the onboard audio function. People want to be able to record social events and musical performances with accurate, realistic playback, or enjoy high-quality voice calls without background noise, even in challenging environments like outdoors or inside a moving vehicle. Additionally, achieving superior audio quality while simultaneously capturing the microphone's sound remains a key goal.
These evolving expectations have led to a growing demand for advanced microphones that deliver both high performance and reliability in a compact form factor, particularly in portable devices. Research from market analysis firm IHS Technology predicts that the MEMS microphone market will expand significantly, rising from 3.6 billion units in 2015 to over 6 billion units by 2019.
An Overview of MEMS Microphone Structure and Functionality
MEMS microphones are constructed using standard semiconductor fabrication techniques, such as deposition and selective etching, on a silicon wafer substrate. These microphones incorporate a movable diaphragm and a static backplate with perforations that allow airflow without causing distortions. The diaphragm is engineered to respond to variations in air pressure due to sound waves, causing it to move relative to the backplate and generate a corresponding change in capacitance. This change is then converted into an electrical signal by a companion integrated circuit (IC), which can be either analog or digital.
On the market, you’ll find both analog and digital MEMS microphones. Analog microphones typically combine a MEMS transducer with an accompanying analog amplifier IC, making them a popular choice for smaller handheld devices like feature phones and mid-range smartphones.
Digital microphones, on the other hand, integrate analog conditioning circuits along with an analog-to-digital converter (ADC) and are often favored in more advanced applications such as high-end smartphones and PCs. Digital technology provides inherent advantages like superior RF immunity and resistance to electromagnetic interference (EMI), as illustrated in Figure 1. Additionally, circuit design becomes simpler, and modifications can be made without altering resistor and capacitor values.
Figure 1: Demonstrating digital improvements in noise immunity
Most digital microphones also include a clock and L/R control input. The clock input manages the Δ∑ modulator, converting the analog signal from the sensor into a digital pulse density modulation (PDM) signal. Typical clock frequencies range from 1MHz to 3.5MHz. The microphone’s output is synchronized with the clock edge before transitioning to a high-impedance state during the opposite half of the clock cycle. This enables multiple digital microphone outputs to share a single data line (Figure 2).
Figure 2: Digital microphone reduces the number of transmission lines
Digital MEMS microphones excel in terms of noise immunity and circuit simplicity, making them ideal for multi-microphone setups used in noise cancellation, echo reduction, and beamforming for directional sensitivity. For instance, in a smartphone, placing one or more additional microphones away from the primary voice microphone—such as on the top or back of the device—can help detect ambient noise and enhance call quality through subtraction. Similarly, digital microphones are commonly used in video recording modes for noise reduction.
Beamforming also employs multiple microphone arrays. While most microphones exhibit omnidirectional sensitivity, certain applications may benefit from heightened sensitivity in a specific direction or reduced sensitivity elsewhere—for example, improving audio clarity during a conference call or a hands-free conversation while driving. Beamforming achieves this by applying a digital algorithm to the array’s microphone outputs, based on the phase differences of sounds arriving from various directions, thereby determining the source’s direction.
Designing Application-Specific Integrated Circuits (ASICs)
Manufacturers of microphone modules distinguish their products by selecting the optimal MEMS microphone kit, pairing a carefully chosen MEMS sensor with a tailored ASIC.
Figure 3: Microphone experts selecting the right MEMS microphone kit
ON Semiconductor is dedicated to creating highly integrated digital MEMS microphone ASICs that can be paired with MEMS transducers from independent suppliers. For instance, the LC706200 digital IC family includes a feedforward delta-sigma ADC, an integrated analog-to-digital converter, and a low-pass filter, as shown in Figure 4. Additionally, it features a charge pump to supply the necessary operating voltage to the MEMS transducer.
Figure 4: A feedforward delta-sigma ADC implements an integrated digital output small footprint microphone
ON Semiconductor’s digital ASICs meet critical performance benchmarks that assist MEMS microphone designers in overcoming current challenges. High signal-to-noise ratio (SNR) is essential for ensuring clear performance in larger distance applications, as well as for capturing cleaner audio. Advanced speech recognition algorithms depend on high SNR to achieve accurate transcription. Today’s ASICs now require SNRs greater than 64 dB, complemented by advancements made by MEMS engineers to optimize transducer characteristics.
As users increasingly demand better-performing devices like smartphones, microphones must operate without distortion in noisy environments to achieve high sound pressure levels (SPL). For example, supporting social users in capturing high-quality recordings of their experiences at festivals.
Digital MEMS Microphones for Future Voice Command Systems
Voice recognition engines and powerful voice assistants like Siri, “OK Google,†and Amazon Echo have stringent requirements for voice command functionality in IoT and portable devices. Current speech recognition systems typically consume significant power to listen and recognize speech. Future voice command systems will likely activate independently and open upon detecting a specific voice command. Low-power digital MEMS microphone technology is well-suited for such independent voice-triggering solutions, offering minimal power consumption and easy integration into existing designs.
Algorithms such as noise cancellation and beamforming, which analyze signals from multiple microphones, rely on uniform sensitivity across individual microphones in the array, ideally within ±1 dB. While screening or grading presents a potential solution, microphone designers seek ASICs that offer adjustable gain to compensate for process-related variations in MEMS manufacturing.
The LC706200 product line offers a high-performance solution. It boasts several features that ensure enhanced linear performance across 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. The device also features high power supply rejection ratio (PSRR), preventing undesirable noise from entering the signal chain and maintaining effective power management in response to voice commands, including sleep mode and low-power modes.
Conclusion:
The changing ways people interact with computers and smart devices are driving demand for dependable, high-performance MEMS microphones. Today’s digital ASICs give microphone developers maximum flexibility, enabling them to create top-tier products that meet these evolving needs.