Nexys4 DDR™ FPGA Board Reference Manual
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15.1 Pulse Density Modulation (PDM)
PDM data connections are becoming more and more popular in portable audio applications, such as cellphones
and tablets. With PDM, two channels can be transmitted with only two wires. The frequency of a PDM signal
usually falls in the range of 1 MHz to 3 MHz. In a PDM bitstream, a 1 corresponds to a positive pulse and a 0
corresponds to a negative pulse. A run consisting of all ‘1’s would correspond to the maximum positive value and a
run of ‘0’s would correspond to the minimum amplitude value. Figure 25 shows how a sine wave is represented in
PDM signal.
0101101111111111111101101010010000000000000100010
Sine Wave
PDM Signal
Figure 25. PDM representation of a sine wave.
A PDM signal is generated from an analog signal through a process called delta-sigma modulation. A simple
idealized circuit of delta-sigma modulator is shown in Figure 26.
Integral
-
+
PDM
Analog
clk
Flip-Flop
Figure 26. Simple delta-sigma modulator circuit.
Sum
Integrator Out
Flip-flop Output
0.4-0=0.4
0+0.4=0.4
0
0.4-0=0.4
0.4+0.4=0.8
1
0.4-1=-0.6
0.8-0.6=0.2
0
0.4-0=0.4
0.2+0.4=0.6
1
0.4-1=-0.6
0.6-0.6=0
0
0.4-0=0.4
0+0.4=0.4
0
0.4-0=0.4
0.4+0.4=0.8
1
0.4-1=-0.6
0.8-0.6=0.2
0
Table 6. Sigma Delta Modulator with a 0.4Vdd input.
To keep things simple, assume that the analog input and digital output have the same voltage range 0~Vdd. The
input of the flip-flop acts like a comparator (any signal above Vdd/2 is considered as ‘1’ and any input bellow Vdd/2
is considered ‘0’). The input of the integral circuit is the difference of the input analog signal and the PDM signal of
the previous clock cycle. The integral circuit then integrates both of these inputs, and the output of the integral
circuit is sampled by a D-Flip-flop. Table 6 shows the function of the delta-sigma modulator with an input of
0.4Vdd.
Note that the average of the flip-flop output equals the value of the input analog signal. So in order to get the
value of analog input, all that is needed is a counter that counts the ‘1’s for a certain period of time.
