ADMP521

Data Sheet

Rev. A | Page 8 of 16

THEORY OF OPERATION

PDM DATA FORMAT

The output from the DATA pin of the ADMP521 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 tristates. In this way, two microphones,

one set to the left channel and the other to 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 10 shows a

diagram of the two stereo channels sharing a common DATA line.

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

output is only clocked on a single edge (see Figure 9). 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.

See Table 6 for the channel assignments according to the logic

level on the L/R SELECT pin.

Table 6. ADMP521 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 between 3% to

7% of full scale. This dc signal is typically removed by a high-pass

filter in the codec that is connected to the digital microphone.

DATA1 (R)

DATA1 (R)

DATA1 (R)

CLK

DATA

Figure 9. Mono PDM Format

DATA2 (L)

DATA2 (L)

DATA1 (R)

DATA1 (R)

CLK

DATA

Figure 10. Stereo PDM Format

PDM MICROPHONE SENSITIVITY

The acoustic input levels of the microphone in dB SPL are rms

measurements; however, the sensitivity and output level of a

digital microphone is given as a peak level. This is because its

output is referenced to the full-scale digital word, which is a

peak value. This convention is different from the output levels

of analog microphones, which are given as an rms voltage. The

ADMP521 has a sensitivity of −26 dBFS. A 94 dB SPL (rms) input

signal gives a −26 dBFS peak output level; therefore, the rms

level of this digital output is −29 dBFS.

This convention of using peak levels to specify the output of

digital microphones must be kept in mind when configuring

downstream signal processing that may rely on precise signal

levels. For example, dynamic range processors (compressors,

limiters, noise gates) typically set thresholds based on rms

signal levels; therefore, adjust the signals of the microphone

from peak to rms by lowering the dBFS value by 3 dB.