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Issues in analogue, digital MEMS microphone design

Posted: 08 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Microphones? MEMS? analogue-to-digital converter? pulse density modulated?

Microphones are transducers that convert acoustic pressure waves to electrical signals. Sensors have become more integrated with other components in the audio signal chain, and MEMS technology is enabling microphones to be smaller and available with either analogue or digital outputs.

Analogue and digital microphone output signals obviously have different factors to consider in a design. I will examine the differences and design considerations when integrating analogue and digital MEMS microphones into a system design.

Inside a MEMS microphone
The output of a MEMS microphone does not come directly from the MEMS transducer element. The transducer is essentially a variable capacitor with an extremely high output impedance in the gigaohm range.

Inside the microphone package, the transducer's signal is sent to a preamplifier, whose first function is an impedance converter to bring the output impedance down to something more usable when the microphone is connected in an audio signal chain. The microphone's output circuitry is also implemented in this preamp.

For an analogue MEMS microphone, this circuit whose block diagram is shown in figure 1 is basically an amplifier with a specific output impedance. In a digital MEMS microphone, that amplifier is integrated with an analogue-to-digital converter (ADC) to provide a digital output in either a pulse density modulated (PDM) or I2S format.

Figure 1: Typical analogue MEMS microphone block diagram.

Figure 2 shows a block diagram of a PDM-output MEMS microphone and figure 3 shows a typical I2S-output digital microphone. The I2S microphone contains all of the digital circuitry that a PDM microphone has, as well as a decimation filter and serial port.

A MEMS microphone package is unique among semiconductor devices, in that there is a hole in the package for the acoustic energy to reach the transducer element. Inside this package, the MEMS microphone transducer and the analogue or digital ASIC are bonded together and mounted on a common laminate. A lid is then bonded over the laminate to enclose the transducer and ASIC. This laminate is basically a small PCB that's used to route the signals from the ICs to the pins on the outside of the microphone package.

Figures 4 and 5 show the inside of analogue and digital MEMS microphones, respectively. In these pictures you can see the transducer on the left and ASIC (under the epoxy) on the right side, both mounted on the laminate. The digital microphone has additional bond wires to connect the electrical signals from the ASIC to the laminate.

Figure 2: Typical PDM MEMS microphone block diagram.

Figure 3: Typical I2S MEMS microphone block diagram.

Figure 4: Transducer and ASIC of an analogue MEMS microphone.

Analogue microphones
An analogue MEMS microphone's output impedance is typically a few hundred ohms. This is higher than the low output impedance that an op amp typically has, so you need to be aware of the impedance of the stage of the signal chain immediately following the microphone.

A low-impedance stage following the microphone will attenuate the signal level. For example, some codecs have a programmable gain amplifier (PGA) before the ADC. At high gain settings, the PGA's input impedance may be only a couple of kilo ohms. A PGA with a 2 k input impedance following a MEMS microphone with a 200 output impedance will attenuate the signal level by almost 10%.

Figure 5: Transducer and ASIC of a digital MEMS microphone.

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