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Data Sheet

AD5293

Rev. E | Page 23 of 24

AUDIO VOLUME CONTROL

The excellent THD performance and high voltage capability

of the AD5293 make it ideal for digital volume control. The

AD5293 is used as an audio attenuator; it can be connected

directly to a gain amplifier. A large step change in the volume

level at any arbitrary time can lead to an abrupt discontinuity of

the audio signal, causing an audible zipper noise. To prevent this,

a zero-crossing window detector can be inserted to the CS line to

delay the device update until the audio signal crosses the window.

Because the input signal can operate on top of any dc level,

rather than absolute 0 V level, zero crossing in this case means

the signal is ac-coupled, and the dc offset level is the signal zero

reference point.

The configuration to reduce zipper noise is shown in Figure 56,

and the results of using this configuration are shown in Figure 55.

The input is ac-coupled by C1 and attenuated down before

feeding into the window comparator formed by U2, U3, and U4B.

U6 is used to establish the signal as zero reference. The upper

limit of the comparator is set above its offset and, therefore, the

output pulses high whenever the input falls between 2.502 V

and 2.497 V (or 0.005 V window) in this example. This output is

AND’ed with the chip select signal such that the AD5293 updates

whenever the signal crosses the window. To avoid a constant

update of the device, program the chip select signal as two pulses

instead of one.

In Figure 55, the lower trace shows that the volume level changes

from a quarter-scale to full-scale when a signal change occurs

near the zero-crossing window.

CHANNEL 1

FREQ = 20.25kHz

1.03V p-p

Figure 55. Zipper Noise Detector

100kΩ

200Ω

VIN

V+

V–

AD8541

100kΩ

90kΩ

10kΩ

1µF

VDD

VSS

SCLK

SDIN

V+

V–

AD5293

20kΩ

+15V

–15V

0.1µF

GND

SDIN

SCLK

VCC

GND

VCC

GND

ADCMP371

+15V

–15V

+5V

VOUT

U4A

U4B

7408

SYNC

Figure 56. Audio Volume Control with Zipper Noise Reduction.

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