THEORY OF OPERATION PDM DATA FORMAT The output from the DATA pin of the T3902 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 8 shows a diagram of the two stereo channels sharing a common DATA line. CLKDATADATA2 (L)DATA1 (R)DATA2 (L)DATA1 (R)
Figure 8. Stereo PDM Format If only one microphone is connected to the DATA signal, the output is only clocked on a single edge (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. CLKDATADATA1 (R)DATA1 (R)DATA1 (R)
Figure 9. Mono PDM Format See Table 10 for the channel assignments according to the logic level on the SELECT pin. TABLE 10. T3902 CHANNEL SETTING SELECT Pin Setting Channel Low (tie to GND) Right (DATA1) High (tie to VDD) Left (DATA2) 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 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 Figure 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. This definition of a 0 dB FS signal must be understood when measuring the sensitivity of the T3902. A 1 kHz sine wave at a 94 dB SPL acoustic input to the T3902 results in an output signal with a −26 dB FS level in Standard Mode and −32 dB FS in High Performance Mode. In the Standard Mode example, the output digital word peaks at −26 dB below the digital full‐scale level. A common Page 12 of 22 Document Number: DS‐000357 Revision: 1.0
