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INMP621ACEZ-R0

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21
INMP621
THEORY OF OPERATION
PDM DATA FORMAT
The output from the DATA pin of the INMP621 is in pulse density modulated (PDM) format. This data is the 1-bit output of a fourth-
order Σ-Δ modulator. The data is encoded so that the left channel is clocked on the falling edge of CLK, and the right channel is clocked
on the rising edge of CLK. After driving the DATA signal high or low in the appropriate half frame of the CLK signal, the DATA driver of
the microphone tristates. In this way, two microphones, one set to the left channel and the other to the right, can drive a single DATA
line. See Figure 1 for a timing diagram of the PDM data format; the DATA1 and DATA2 lines shown in this figure are two halves of the
single physical DATA signal. Figure 10 shows a diagram of the two stereo channels sharing a common DATA line.
Figure 10. Stereo PDM Format
If only one microphone is connected to the DATA signal, the output is only clocked on a single edge (Figure 11). For example, a left
channel microphone is never clocked on the rising edge of CLK. In a single microphone application, each bit of the DATA signal is
typically held for the full CLK period until the next transition because the leakage of the DATA line is not enough to discharge the line
while the driver is tristated.
Figure 11. Mono PDM Format
See Table 6 for the channel assignments according to the logic level on the L/R SELECT pin.
TABLE 6. INMP621 CHANNEL SETTING
DATA2 (L) DATA2 (L)DATA1 (R) DATA1 (R)
CLK
DATA
D
A
TA
1 (
R
) D
A
T
A
1
(
R
) D
A
T
A
1
(R
)
C
LK
D
A
TA
L/R SELECT Pin Setting
Channel
Low (tie to GND)
Right (DATA1)
High (tie to VDD)
Left (DATA2)
Page 10 of 21
Document Number: DS-INMP621-00
Revision: 1.1
Rev Date: 05/21/2014
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 1.00.80.70.60.50.40.30.20.1
DIGITAL AMPLITUDE (D)
TIME (ms)
Page 11 of 21
Document Number: DS-INMP621-00
Revision: 1.1
Rev Date: 05/21/2014
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 21
Document Number: DS-INMP621-00
Revision: 1.1
Rev Date: 05/21/2014
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21

INMP621ACEZ-R0

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MEMS Microphones Wide Dynamic Range Microphone with PDM Digital Output
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