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MEMS microphones are being used to replace electret condenser microphones (ECMs) in audio circuits. These two types of microphones perform the same function, but the connection between the microphone and the rest of the system is different for ECMs and MEMS microphones. This 2-page Application Note explains those differences and provides design details for a simple MEMS microphone based replacement circuit.
A microphone preamp circuit amplifies a microphone’s output signal to match the input level of the following device. Matching the peaks of the microphone’s signal level to the full-scale input voltage of an ADC makes maximum use of the ADC’s dynamic range and reduces the noise that subsequent processing may add to the signal. The MEMS microphone has a single-ended output, so a single op amp stage can be used as a preamp to add gain to the microphone signal or to buffer the output. This Application Note covers some of the key op amp specifications to consider for a preamp design, shows a few basic circuits, and provides a table of Analog Devices op amps that are appropriate for a preamp design. The ADMP504 MEMS analog microphone with 65 dB SNR is used as an example to describe different design choices.
Kalman Filter Estimates Orientation Angles Based on Sensor Data
The ADIS16480 MEMS inertial measurement unit (IMU) includes a 3-axis accelerometer, a 3-axis gyroscope, a 3-axis magnetometer, and a barometer. It also includes an extended Kalman filter (EKF) that computes dynamic orientation angles. The EKF estimates the orientation angles using a combination of gyroscope, accelerometer, and magnetometer contributions, with real-time filter weighting factors determined by covariance terms representing the level of uncertainty assigned to each sensor. Optimal performance comes from selecting appropriate covariance values for the sensors, given the application environment. This 8-page Application Note offers analytical tools for simplifying this process.
Mounting Tips for Inertial Measurement Units
The ADIS16334 is a low profile, fully calibrated MEMS inertial measurement units (IMU). Its package has four mounting holes with recessed mounting shelves that help manage the overall height of the attachment hardware. The mounting holes provide enough clearance for M2 × 0.4 mm or 2 to 56 machine screws. This 6-page Application Note provides hints for connector-up or connector-down mounting.
Beamforming Creates Directional Responses from Omnidirectional Microphones
All MEMS microphones have an omnidirectional pickup response, so they respond equally to sounds coming from any direction. Multiple microphones can be configured in an array to form a directional response, or a beam pattern. A beamforming microphone array can be designed to be more sensitive to sound coming from one or more specific directions than sound coming from other directions. Microphone beamforming is a rich, complex topic. This 8-page Application Note covers basic concepts; array configurations, including broadside summing arrays and differential endfire arrays; design considerations; spatial and frequency responses; and advantages and disadvantages of different array configurations.
This circuit incorporates a 3-axis ADXL362 digital accelerometer and an ADP195 high-side power switch to create an ultralow power, motion sensitive switch. The ADXL362 ultralow-power 3-axis accelerometer consumes less than 100 nA in wake-up mode. Unlike accelerometers that use power duty cycling to achieve low power consumption, it samples continuously at all data rates so it does not alias input signals by under sampling. The ADXL362 provides 12-bit output resolution and has three operating ranges, ±2 g, ±4 g, and ±8 g, with a resolution of 1 mg/LSB on the ±2 g range. For applications where a noise level lower than 480 μg/√Hz is desired, either of two lower noise modes (down to 120 μg/√Hz) can be selected at minimal increase in supply current. An on-chip, 12-bit temperature sensor is accurate to ±0.5°. The ADP195 high-side load switch, designed for operation between 1.1 V and 3.6 V, is protected against reverse current flow from output to input. The device contains a low on-resistance, P-channel MOSFET that supports over 1.1 A of continuous load current and minimizes power losses. This combination of devices offers an industry leading low power solution for a standalone motion switch controlling power to a load.
This circuit allows up to two digital MEMS microphones to be interfaced to a single data line. The ADMP441 consists of a MEMS microphone element and an I2S output, which allows stereo microphones to be used in an audio system without the need for a codec. MEMS microphones have high signal-to-noise ratio (SNR) and flat wideband frequency response, making them ideal for high-performance, low-power applications. Up to two ADMP441 microphones can be input to a single data line on the ADSP-BF527 Blackfin® processor. The ADSP-BF527 can have up to four serial data inputs, allowing up to eight microphones to connect to a single DSP.
This circuit interfaces an analog MEMS microphone to a microphone preamp. The ADMP504 consists of a MEMS microphone element and an output amplifier. Analog Devices’ MEMS microphones have a high signal-to-noise ratio (SNR) and a flat wideband frequency response, making them an excellent choice for high-performance, low-power applications. The SSM2167 low-voltage, low-noise mono microphone preamp is a good choice for use in low-power audio signal chains. This preamp includes built-in compression and noise gating, which gives it an advantage for this function over using just an op amp in the preamp circuit. Compressing the dynamic range of the microphone signal can reduce the peak signal levels and add additional gain to low level signals. Noise gating attenuates the level of signals below a certain threshold, so that only desired signals, such as speech, are amplified, and noise in the output signal is reduced. These features help to improve the intelligibility of the voice signal picked up by the microphone.
Jerad Lewis, Analog and digital MEMS microphone design considerations, EE Times, 2013-03-27
C. Goodall, S. Carmichael, and B. Scannell, The Battle Between MEMS and FOGs for Precision Guidance, EDN, 2013-01-21
Mark Looney, Stabilization systems and frequency response of inertial MEMS abilities, ECN, 2012-12-18
Jerad Lewis and Paul Schreier, Low self noise: The first step to high-performance MEMS microphone applications, EE Times, 2012-11-28
Bob Scannell, MEMS enable medical innovation, EE Times, 2012-10-12
Mark Looney, Analyzing Frequency Response of Inertial MEMS in Stabilization Systems, RF Globalnet, 2012-07-25
Bob Scannell, INS Face Off: MEMS vs. FOGs, InsideGNSS, 2012-07-15
Mark Looney, Analysing frequency response of inertial MEMS in stabilisation systems, New Electronics, 2012-07-10
Mark Looney, Analyzing Frequency Response of Inertial MEMS in Stabilization Systems, Analog Dialogue, 2012-07-02
High-Performance MEMS; What does that mean? – Learn the most common "high-performance metrics" that are associated with gyroscope/IMU applications, and gain insight into their characterization and performance impact in real-world applications. Topics covered in this webcast will also be useful for applications that use accelerometers.
Healthcare Webcast: MEMS Inertial Sensors in Healthcare Designs - In this webcast you will learn the basics on MEMS motion sensing including architectures and technology including accelerometers, gyroscopes, and IMUs. Applications explored include precise positioning/tracking of healthcare scanning equipment, surgical tool guidance, balance and control of prosthetics, and motion sensing.
Using MEMS Sensors for Industrial Platform Stabilization Systems - MEMS accelerometers and gyroscopes are ideal feedback sensing elements for many types of platform control and stabilization systems. This webcast will discuss typical performance requirements to consider when developing a MEMS-based stabilization system, along with insights on component selection and enabling quick, inexpensive, system-level integration of these functions.
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