ADMP621

Data Sheet

THEORY OF OPERATION

PDM DATA FORMAT

The output from the DATA pin of the ADMP621 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 3 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 12 shows a diagram of the two

stereo channels sharing a common DATA line.

DATA2 (L)

DATA2 (L)

DATA1 (R)

DATA1 (R)

CLK

DATA

Figure 12. Stereo PDM Format

If only one microphone is connected to the DATA signal, the

output is only clocked on a single edge (see Figure 13). 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.

DATA1 (R)

DATA1 (R)

DATA1 (R)

CLK

DATA

Figure 13. Mono PDM Format

See Table 6 for the channel assignments according to the logic

level on the L/R SELECT pin.

Table 6. Channel Setting

L/R SELECT Setting

Channel

Low (Tie to Ground)

DATA1 (right)

DATA2 (left)

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 14). 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 ADMP621. A 1 kHz sine wave

at a 94 dB SPL acoustic input to the ADMP621 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.

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)

Figure 14. 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.

Rev. 0 | Page 8 of 16