In low-end audio systems, digital potentiometers can be used as audio attenuators or amplifiers (Fig. 1). Unfortunately, a large change in volume at an arbitrary time can cause an abrupt discontinuity in the audio signal. The result is audible clicking, or zipper noise, which precludes the use of this simple design in mid-level or high-end audio systems.
But designers can reduce zipper noise by converting the random change in volume versus time into a defined pattern where volume changes occur only at zero crossings. Consequently, no abrupt volume changes will transpire. This technique works by inserting a zero-crossing window detector in the Chip Select (/CS) line of the digital potentiometer, delaying the device update until the audio signal reaches the zero-crossing window (Fig. 2).
Because the input signal can operate at any dc level, the signal is ac-coupled by C1. The dc offset created by the AD8541 low-power, single-supply, rail-torail op amp (U6) determines the zerocrossing reference point. To ensure logic compatibility, the input signal is attenuated by R4 and R5 before being fed to the window comparator formed by the two ADCMP371 (U2, U3) general-purpose comparators and the AND gate (U4b). Resistor string R1, R2, and R3 set the window (5 mV) so that the output goes high whenever the input falls between 2.5025 and 2.4975 V.
The comparator output is ANDed with the Chip Select signal, causing the digital potentiometer to update whenever the signal crosses the window. Audio signals have large bipolar voltage swings. Therefore, the digital potentiometer must have low distortion and high-voltage capability.
Figure 3 depicts the end result. The lower trace shows the volume level changing from a quarter-scale to fullscale when the volume change occurs near the zero-crossing window. Note that the volume changes level smoothly.