Introduction to MEMS Microphone Technology—Analog vs Digital Microphones
MEMS silicon microphones are in everything from cellphones, hearing aids, smart speakers, computers, and vehicles. Learn the basics of how they work, their use, and what’s available.
Microelectromechanical systems, or MEMS, are etched and manufactured on silicon using techniques originally developed for integrated circuits (ICs). Micromachined ink jet nozzles may have been the first, however, since the 1990s, MEMS technology has created various sensors and other electromechanical devices, including microphones.
MEMS microphones are small, affordable, and readily available. The microphone element itself is under 1 millimeter, generally much smaller. Most come in surface-mount IC enclosures and include amplification circuitry with analog or digital outputs. As shown in Figure 1, the input port for the audio signal can be on either the top or bottom of the packaged MEMS IC.
Figure 1. MEMS microphones are available with sound ports on the top (left) or the bottom (right). Image [modified] used courtesy of STMicroelectronics
Most microphones are consumer-grade with good sound quality, although not equal to those used for professional audio.
How do MEMS Microphones Work? Analog vs Digital Microphone Outputs
All microphones begin with an analog audio signal and use a preamplifier (sometimes called a buffer) to boost the audio to a usable, but still low, level. Many use capacitive sensor technology, which will be covered in the next section. They include additional circuitry to convert the capacitance variation to an electrical signal.
MEMS Microphone Analog Output
Analog microphones send the boosted signal directly to the output. There are two output styles—single-ended and differential. Differential systems have two outputs that are 180 degrees out of phase with each other. Analog microphones have either three or four pins: power, common (ground), and one or two outputs, depending on whether the output is single-ended or differential.
Power is always supplied by a single, positive supply. This creates a DC offset on the output, which should be decoupled by a capacitor, as seen in Figure 2.
Figure 2. Analog output microphone.
Supply voltages are usually between 1.8 and 3.5 V, with typical DC offsets from 0.8 to 1.5 V.
MEMS Microphone Digital Output
MEMS microphones with a digital output perform an analog-to-digital (A/D) conversion to change the amplified analog audio signal to digital. Most use delta-sigma conversion to produce a PDM (pulse density modulated) output, as shown in Figure 3.
Figure 3. Pulse density modulation. The high pulses (blue) have a higher density when the audio signal is high. Image used courtesy of MyNewMicrophone.com
The pulse density (that is, the percentage of pulses that are logically high) is proportional to the voltage. This is not what you normally think of as digital because no digital words are created, just pulses. The pulse stream can be decoded by simply passing it through a low pass filter, although microprocessor programs or audio CODECs (coder/decoders) generally are used.
Most digital output MEMS microphones have five pins, as demonstrated in Figure 4:
- Common (ground)
- Clock input
- L/R (left/right) selection
Figure 4. Digital output microphones are used in a stereo system.
How does the L/R select work? If tied high (left), the A/D output is sent after the clock goes high. If low, data follows the low clock transition. In this way, left and right outputs can be sent over the same data line.
Some microphones use the I2S (Inter-IC Sound) standard, originally created by Philips Semiconductor (now NXP Semiconductors). Like PDM, it has a clock and L/R select inputs, but the output is digital words, not modulated pulses. Again, like PDM, it can be decoded by microprocessor software or an I2S CODEC. In addition, it cannot be decoded by a low pass filter.
MEMS Microphone Technology
The majority of MEMS microphones use capacitive sensor technology. A thin, plated membrane in the silicon structure vibrates with sound, creating a varying capacitance. The capacitor’s second plate is on a fixed surface in the silicon. A charge pump in the IC creates a high DC voltage for the capacitor. IC circuitry converts the capacitance changes to an electrical signal that is representative of the audio signal on the MEMS membrane.
More recently, some manufacturers have created microphones using piezoelectric sensing elements. The motion of the piezoelectric element produces the audio voltage. These companies claim some advantages over capacitive, but for most applications, it does not matter which technology you use.
You may also see the term “silicon microphone.” This is not a third technology, just a different way to describe silicon MEMS microphones.
MEMS Microphone Packaging
The microphone element and its circuit are not on the same silicon chip. Their manufacturing techniques are too different to manufacture them together. Instead, the microphone and a separate ASIC (application-specific integrated circuit) are combined in the same package, connected by wire bonding, as illustrated in Figure 5.
Figure 5. MEMS microphone packaging with top sound port. Image used courtesy of CUI Devices
MEMS microphones come in IC-like packages for surface mount assembly. They, of course, need ports to allow the sound to enter. As shown earlier in Figure 1, top and bottom ports are available. Figure 5 is an example of a top-port MEMS microphone. If using a microphone with a bottom port, you must put a hole in the circuit board below it, as demonstrated in Figure 6.
Figure 6. MEMS microphone packaging with bottom sound port. Image used courtesy of STMicroelectronics
PCB Mounting of MEMS Microphones
Standard reflow soldering techniques may be used for attaching MEMS microphones to PCBs. However, you must be careful, of course, to keep contaminants out of the sound ports. You may need to tape or seal the port during cleaning.
If you use vacuum pick-ups, do not let them contact the sound port. Also, do not blow air into the port or expose the microphone to a vacuum. Finally, in your design, place a supply-to-common bypass capacitor as close as possible to the microphone. A 0.1 uF ceramic capacitor is usually a good choice for power decoupling.
MEMS Microphone General Specifications
Most MEMS microphones have similar specifications. Here are some typical values. Sensitivity is confusing, so let’s try to deal with it first.
- Analog output sensitivity (typical): -38 dBV at 94 dB SPL, 1 kHz
- Digital output sensitivity (typical): -26 dB FS at 94 dB SPL, 1 kHz
Let’s take a more detailed look at what each of those specifications means.
- dBV means decibels referred to a 1 V reference. -38 dBV is equivalent to 12.6 mV.
- dB FS means decibels referred to the A/D converter’s full scale.
- SPL means sound pressure level. According to an online article, normal conversation 3 feet away is about 40 to 60 dB SPL. An 85 dB SPL at the ear can cause hearing damage. Thus, 94 dB SPL is a high value.
At normal voice levels, an analog microphone’s output would be low millivolts, and digital would be way below full scale. In a way, that’s a good thing because it leaves a lot of headroom for loud sounds.
- Frequency response: typically from 80 or 100 Hz at the low end, up to 10 or 15 kHz. Fine for voice, pretty good for most audio. Some go down to 20 Hz. At the high end, the response increases at higher frequencies, with significant ultrasonic resonance peaks around 30 to 40 kHz. Above that, the response drops.
- Operating temperature: most are -40 to +85 degrees C.
- Supply voltage: from 1.5 or 2 V to 3 or 3.6 V. Specs vary.
- Size: 3 x 4 mm or smaller and around 1 to 1.5 mm high. Most use the same surface mount pad pattern.
As always, check the specs of the microphone you plan to use.
MEMS technology has created amazing advances in sensors, including microphones. MEMS microphones, generally designed for circuit board mounting, include not only the microphone but also supporting circuitry with either analog or digital outputs. We’ve reviewed the basic technologies, the analog and digital outputs, the physical package, and typical specifications.
MEMS microphones are inexpensive and widely available from over a dozen manufacturers. You can find them at your favorite electronic distributor. Some are as low as a dollar or so in small quantities. For hobbyists, there also are breakout boards.