Datasheet
AD5293
Rev. D | 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, the chip select signal should be
programmed as two pulses, rather than as 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
1
2
0
7675-158
Figure 55. Zipper Noise Detector
R
1
100k
R
2
200
5V
V
IN
V+
V–
AD8541
5V
U6
R
3
100k
R
4
90k
R
5
10k
C1
1µF
V
DD
V
SS
SCLK
SDIN
V+
V–
AD5293
20k
+15V
–15V
C3
0.1µF
C2
0.1µF
A
B
W
GND
SDIN
SCLK
U1
V
CC
GND
V
CC
GND
ADCMP371
ADCMP371
+15V
–15V
+5V
+5V
U3
U2
V
OUT
U5
U4A
U4B
16
2
4
5
7408
7408
SYNC
SYNC
07675-157
Figure 56. Audio Volume Control with Zipper Noise Reduction.