AD5293

Rev. 0 | Page 19 of 20

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 SYNC line

to delay the device update until the audio signal crosses the win-

dow. 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 42,

and the results of using this configuration are shown in Figure 43.

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 a 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, the chip select signal should be

programmed as two pulses, rather than as one.

In Figure 43, 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.

100kΩ

200Ω

5V

V+

V–

AD8541

5V

U6

100kΩ

90kΩ

10kΩ

C1

1µF

SCLK

DIN

V+

V–

AD5293

20kΩ

+15V

–15V

C3

0.1µF

C2

0.1µF

A

B

W

GND

DIN

SCLK

U1

GND

GND

ADCMP371

ADCMP371

+15V

–15V

+5V

+5V

U3

U2

U5

U4A

U4B

1

6

2

4

5

7408

7408

SYNC

SYNC

Figure 42. Audio Volume Control with Zipper Noise Reduction

CHANNEL 1

FREQ = 20.25kHz

1.03V p-p

1

2

Figure 43. Zipper Noise Detector