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INMP522ACEZ-R0

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21
INMP522
THEORY OF OPERATION
PDM DATA FORMAT
The output from the DATA pin of the INMP522 is in 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 is
tristated. In this way, two microphones—one set to the left channel and the other to the right channel—can drive a single DATA line.
Figure 1 shows a timing diagram of the PDM data format; the DATA1 and DATA2 lines shown in Figure 1 are two halves of the single
physical DATA signal. Figure 8 shows a diagram of the two stereo channels sharing a common DATA line.
Figure 8. Stereo PDM Format
If only one microphone is connected to the DATA signal, the output is clocked on a single edge only (See Figure 9.)
Figure 9. Mono PDM Format
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 of the CLK signal because the leakage of the DATA line is
not sufficient to discharge the line while the driver is tristated.
The channel assignments are determined by the logic level on the L/R SELECT pin (see Table 6.)
TABLE 6. INMP522 CHANNEL SETTING
DATA2 (L) DATA2 (L)DATA1 (R) DATA1 (R)
CLK
DATA
D
A
TA
1 (
R
) D
A
TA
1 (
R
) D
AT
A
1 (
R)
CLK
D
ATA
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-INMP522-00
Revision: 1.1
Rev Date: 02/06/2014
INMP522
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 0 dBFS. For example, a full-scale
square wave has an RMS level of 3 dBFS.
Figure 10. 1 kHz, 0 dBFS Sine Wave
The definition of a 0 dBFS signal must be understood when measuring the sensitivity of the inMP522. An acoustic input signal of a
1 kHz sine wave at 94 dB SPL applied to the INMP522 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.
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-INMP522-00
Revision: 1.1
Rev Date: 02/06/2014
INMP522
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. The codec also generates the clock for
the PDM microphones or is synchronous with the source that generates the clock.
Figure 11 and Figure 12 show mono and stereo connections between the INMP522 and a codec. The mono connection shows an
INMP522 set to output data on the right channel. To output data on the left channel, tie the L/R SELECT pin to VDD instead of GND.
Figure 11. Mono PDM Microphone (Right Channel) Connection to Codec
CLOCK OUTPUT
CODEC
0.1µF
1.8V TO 3.3V
GND
L/R SELECT
DATA
INMP522
CLK
VDD
DATA INPUT
Page 12 of 21
Document Number: DS-INMP522-00
Revision: 1.1
Rev Date: 02/06/2014
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21

INMP522ACEZ-R0

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TDK InvenSense
Description:
MEMS Microphones Low-Noise, Low Sensitivity Tolerance, PDM Digital Microphone
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