Data Sheet

ADMP621

DYNAMIC RANGE CONSIDERATIONS

The full-scale digital output (0 dBFS) of the ADMP621 is

mapped to an acoustic input of 140 dB SPL. The microphone

clips (THD = 10%) at 133 dB SPL (see Figure 9); 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 10).

To fully use the 111 dB digital dynamic range of the output data

of the ADMP621 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 ADMP621 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. See the Applications Information section

for additional details on connecting the ADMP621 to Analog

Devices, Inc., audio codecs with a PDM input. Figure 15 and

Figure 16 show mono and stereo connections of the ADMP621 to

a codec. The mono connection shows an ADMP621 set to output

data on the right channel. To output on the left channel, tie the

CLOCK OUTPUT

CODEC

0.1µF

1.8V TO 3.3V

GND

L/R SELECT

DATA

ADMP621

CLK

DATA INPUT

Figure 15. Mono PDM Microphone (Right Channel) Connection to Codec

CLOCK OUTPUT

CODEC

0.1µF

1.8V TO 3.3V

GND

L/R SELECT

DATA

ADMP621

CLK

DATA INPUT

0.1µF

1.8V TO 3.3V

GND

L/R SELECT

DATA

ADMP621

CLK

Figure 16. Stereo PDM Microphone Connection to Codec

board (PCB) layout allows.

Do not use a pull-up or pull-down resistor on the PDM data signal

line because it can pull the signal to an incorrect state during the

period that the signal line is tristated.

The DATA signal does not need to be buffered in normal use when

the ADMP621 microphone(s) is placed close to the codec on the

PCB. If the DATA signal must be driven over a long cable (>15 cm)

or other large capacitive load, a digital buffer may be required. Only

use a signal buffer on the DATA line when one microphone is in

use or after the point where two microphones are connected (see

Figure 17). The DATA output of each microphone in a stereo

configuration cannot be individually buffered because the two

buffer outputs cannot drive a single signal line. If a buffer is

used, take care to select one with low propagation delay so that

the timing of the data connected to the codec is not corrupted.

CLOCK OUTPUT

CODEC

DATA

ADMP621

ADMP621

CLK

DATA INPUT

DATA

CLK

Figure 17. Buffered Connection Between Stereo ADMP621s and a Codec

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