A MEMS microphone IC is unique among Analog Devices, Inc., products in that its input is an acoustic pressure wave. For this reason, some specifications included in the data sheets for these parts may not be familiar, or familiar specifications may be applied in unfamiliar ways. This application note explains the specifications and terms found in MEMS microphone data sheets so that the microphone can be appropriately designed into a system.
The sensitivity of a microphone is the electrical response at its output to a given standard acoustic input. The standard reference input signal for microphone sensitivity measurements is a 1 kHz sine wave at 94 dB sound pressure level (SPL), or 1 pascal (Pa, a measurement of pressure). A microphone with a higher sensitivity value has a higher level output for a fixed acoustic input than a microphone with a lower sensitivity value. Microphone sensitivity in decibels (dB) is typically a negative number; therefore, a higher sensitivity is a smaller absolute value.
It is important to note the units presented with the sensitivity specifications of the microphone. It is incorrect to directly compare the sensitivity of two microphones if the sensitivity is not specified with the same unit. For analog microphones, the sensitivity is typically specified in units of dBV, that is, decibels with reference to 1.0 V rms. For digital microphones, the sensitivity is typically specified in dBFS, that is, decibels with reference to a full-scale digital output (dBFS). For digital microphones, a full-scale signal is defined as the highest signal level that can be output from the microphone; for Analog Devices MEMS microphones, this level is 120 dB SPL. See the Maximum Acoustic Input section for a fuller description of this signal level.
Sensitivity is the ratio of the input pressure to the electrical output (voltage or digital word). For analog microphones, sensitivity is typically measured in mV/Pa. This measurement is converted to a decibel value with the following equation:
where is the 1 V/Pa (1000 mV/Pa) reference output ratio.
For digital microphones, sensitivity is measured as a percentage of the full-scale output that is generated by a 94 dB SPL input. For a digital microphone, the conversion equation is
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