INMP621

DYNAMIC RANGE CONSIDERATIONS

The full-scale digital output (0 dBFS) of the INMP621 is mapped to an acoustic input of 140 dB SPL. The microphone clips (THD =

10%) at 133 dB SPL (see Figure 7); however, it continues to output an increasingly distorted signal above that point. The peak output

level, which is controlled by the modulator, limits at about −3 dBFS (see Figure 8).

To fully use the 111 dB digital dynamic range of the output data of the INMP621 in a design, the digital signal processor (DSP), analog-

to-digital converter (ADC), or codec circuit following it must be chosen carefully. The decimation filter that inputs the PDM signal from

the INMP621 must have a dynamic range sufficiently better than the dynamic range of the microphone so that the overall noise

performance of the system is not degraded. If the decimation filter has a dynamic range of 10 dB better than the microphone (121

dB), the overall system noise only degrades by 0.4 dB.

CONNECTING PDM MICROPHONES

A PDM output microphone is typically connected to a codec with a dedicated PDM input. This codec separately decodes the left and right

channels and filters the high sample rate modulated data back to the audio frequency band. This codec also generates the clock for the

PDM microphones or is synchronous with the source that is generating the clock. Figure 13 and Figure 14 show mono and stereo

connections of the INMP621 to a codec. The mono connection shows an INMP621 set to output data on the right channel. To output on

the left channel, tie the L/R SELECT pin to VDD instead of tying it to GND.

Figure 13. Mono PDM Microphone (Right Channel) Connection to Codec

CLOCK OUTPUT

CODEC

0.1µF

1.8V TO 3.3V

GND

L/R SELECT

DATA

INMP621

CLK

VDD

DATA INPUT

Page 12 of 20

Document Number: DS-INMP621-00

Revision: 1.0

Rev Date: 02/06/2014