INMP621

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 shows an

alternating pattern of high and low pulses.

The output PDM data signal has a small DC offset of about 3% of full scale. A high-pass filter in the codec that is connected to the digital

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

PDM MICROPHONE SENSITIVITY

The sensitivity of a PDM output microphone is specified with the unit dBFS (decibels relative to digital full scale). A 0 dBFS sine wave

is defined as a signal whose peak just touches the full-scale code of the digital word (see Figure 12). This measurement convention also

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

RMS level of 3 dBFS.

This definition of a 0 dBFS signal must be understood when measuring the sensitivity of the INMP621. A 1 kHz sine wave at a

94 dB SPL acoustic input to the INMP621 results in an output signal with a −46 dBFS level. The output digital word peaks at

−46 dB below the digital full-scale level. A common misunderstanding is that the output has an RMS level of −49 dBFS; however, this is

not true because of the definition of the 0 dBFS sine wave.

Figure 12. 1 kHz, 0 dBFS Sine Wave

There is not a commonly accepted unit of measurement to express the instantaneous level, as opposed to the RMS level of the

signal, of a digital signal output from the microphone. Some measurement systems express the instantaneous level of an individual

sample in units of D, where 1.0 D is digital full scale. In this case, a −46 dBFS sine wave has peaks at 0.005 D.

1.0

–1.0

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

0

0.9

1.0

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

TIME (ms)

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Document Number: DS-INMP621-00

Revision: 1.0

Rev Date: 02/06/2014