I2S has been a common digital interface for audio converters and processors for years, but it's just recently being integrated into the devices at the edges of the signal chain, such as a microphone. An I2S microphone has the same system design benefits as a PDM microphone, but instead of outputting a high-sample rate PDM output, its digital data is output at a decimated baseband audio sample rate. With a PDM microphone, this decimation happens in the codec or DSP, but with an I2S microphone this processing is done directly in the microphone, which in some systems can eliminate the need for an ADC or codec entirely.
An I2S microphone can connect directly to a DSP or microcontroller for processing with this standard interface (Figure 8). Like with PDM microphones, two I2S mics can be connected to a common data line, although the I2S format uses two clock signals, a word clock and a bit clock, instead of the one for PDM.
Figure 8: Stereo I2S microphone connection to a DSP
When Size Matters
Generally, analog MEMS microphones are available in smaller packages than digital microphones. This is because an analog microphone package needs fewer pins (typically three, vs. five or more for a digital microphone) and the analog preamp has less circuitry than a digital preamp. This makes the analog preamp smaller than a digital preamp manufactured in the same fab geometry. Consequently, in the most space-constrained designs, such as in many small mobile devices, analog microphones are preferred in part because of their small size.
An analog microphone can be in a package with dimensions 2.5 × 3.35 × 0.88 mm or smaller, while PDM microphones often come in a 3 × 4 × 1 mm package, an increase of 62% in package volume. Figure 9 shows a comparison of three bottom port microphone packages. The smallest is the ADMP504, an analog microphone in the 2.5 × 3.35 × 0.88 mm, the middle-sized microphone is the ADMP521, a PDM microphone in the 3 × 4 × 1 mm package, and the microphone in the largest package is the ADMP441, an I2S microphone in a 3.76 × 4.72 × 1.0 mm package.
Figure 9: Comparison of microphone package sizes
This last microphone is in this larger package to support its nine pins. Despite its larger size, a microphone like this is comparable in functionality to an analog microphone and an ADC together, so the savings in PCB area if a converter is otherwise not needed outweighs the slightly-larger microphone footprint.
Analog and digital MEMS microphones both have advantages in different applications. Considering the system's size and component placement constraints, electrical connections, and potential sources of noise and interference will lead to a well-informed decision on which type of microphone is best for your design.
About the author
Jerad Lewis is a MEMS microphone applications engineer at Analog Devices. He joined the company in 2001 after getting his BSEE from Penn State University. Since then, he's supported different audio ICs, such as SigmaDSPs, converters, and MEMS microphones. He is currently also pursuing a M.Eng. in Acoustics at Penn State University.