INMP521

For PDM data, the density of the pulses indicates the signal amplitude. A high density of high pulses indicates a signal near positive

full scale, and a high density of low pulses indicates a signal near negative full scale. A perfect zero (DC) audio signal

is indicated by an alternating pattern of high and low pulses.

The output PDM data signal has a small DC offset of approximately 5% of full scale. A high-pass filter in the codec that is connected

to the digital microphone typically removes this DC signal and does not affect the performance of the microphone.

PDM MICROPHONE SENSITIVITY

The sensitivity of a PDM output microphone is specified in units of dBFS (decibels relative to a full-scale digital output). A 0 dBFS sine

wave is defined as a signal whose peak just touches the full-scale code of the digital word (see Figure 10). This measurement

convention means that signals with a different crest factor may have an RMS level higher than 0dBFS. For example, a full-scale

square wave has an RMS level of 3dBFS.

Figure 10. 1 kHz, 0 dBFS Sine Wave

The definition of a 0 dBFS signal must be understood when measuring the sensitivity of the INMP521. An acoustic input signal of a

1 kHz sine wave at 94 dB SPL applied to the INMP521 results in an output signal with a −26 dBFS level. This means that the output

digital word peaks at −26 dB below the digital full-scale level. A common misunderstanding is that the output has an RMS level of

−29 dBFS; however, this is not the case because of the definition of a 0 dBFS sine wave.

There is no commonly accepted unit of measurement to express the instantaneous level of a digital signal output from the

microphone, as opposed to the RMS level of the signal. Some measurement systems express the instantaneous level of an individual

sample in units of D, where 1.0 D is digital full scale (see Figure 10). In this case, a −26 dBFS sine wave has peaks at 0.05 D.

For more information about digital microphone sensitivity, see the AN-1112 Application Note, Microphone Specifications Explained.

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Document Number: DS-INMP521-00

Revision: 1.0.