Datasheet
AD5291/AD5292
Rev. D | Page 29 of 32
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.
07674-157
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–
AD5292
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
OU
T
U5
U4A
U4B
16
2
4
5
7408
7408
SYNC
SYNC
Figure 74. Audio Volume Control with Zipper Noise Reduction
0
7674-158
CHANNEL 1
FREQ = 20.25kHz
1.03V p-p
1
2
Figure 75. Zipper Noise Detector