AD5291 datasheet(29/30 Pages) AD

29 / 30 page

Data Sheet

AD5291/AD5292

Rev. E | Page 29 of 30

AUDIO VOLUME CONTROL

The excellent THD performance and high voltage capability

make the AD5291 and AD5292 ideal for a digital volume

control as an audio attenuator or gain amplifier. A typical

problem in these systems is that 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 window. Because the input signal can

operate on top of any dc level rather than absolute zero volt

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 74,

and the results of using this configuration is shown in Figure 75.

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 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 SYNC signal such that the AD5291 and AD5292

updates whenever the signal crosses the window. To avoid a

constant update of the device, the SYNC signal should be

programmed as two pulses, rather than as one.

In Figure 75, 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

VIN

V+

V–

AD8541

100kΩ

90kΩ

10kΩ

1µF

VDD

VSS

SCLK

SDIN

V+

V–

AD5292

20kΩ

+15V

–15V

0.1µF

GND

SDIN

SCLK

VCC

GND

VCC

GND

ADCMP371

+15V

–15V

+5V

VOUT

U4A

U4B

7408

SYNC

Figure 74. Audio Volume Control with Zipper Noise Reduction

CH1 500mV BW CH2 500mV

W M20µs

A CH2

210mV

T 50.0%

CHANNEL 1

FREQ = 20.25kHz

1.03V p-p

Figure 75. Zipper Noise Detector

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