T3903
Page 11 of 21
Document Number: DS-000358
Revision: 1.0
THEORY OF OPERATION
PDM DATA FORMAT
The output from the DATA pin of the T3903 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 9 shows a diagram of the two stereo channels sharing a common DATA line.
Figure 9. Stereo PDM Format
If only one microphone is connected to the DATA signal, the output is only clocked on a single edge (Figure 10). 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 10. Mono PDM Format
See Table 9 for the channel assignments according to the logic level on the SELECT pin. The setting on the SELECT pin is sampled at
power-up and should not be changed during operation.
TABLE 9. T3903 CHANNEL SETTING
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 dB FS (decibels relative to digital full scale). A 0 dB FS sine
wave is defined as a signal whose peak just touches the full-scale code of the digital word (see 10). This measurement convention also
means that signals with a different crest factor may have an RMS level higher than 0 dB FS. For example, a full-scale square wave has an
RMS level of 3 dB FS.
DATA2 (L)
DATA2 (L)
DATA1 (R)
DATA1 (R)
CLK
DATA
DATA1 (R)
DATA1 (R)
DATA1 (R)
CLK
DATA
SELECT Pin Setting
Channel
Low (tie to GND)
Right (DATA1)
High (tie to VDD)
Left (DATA2)
