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AN-1209
APPLICATION NOTE
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Logarithmic Audio Volume Control with Glitch Reduction
Using the AD5292 Digital Potentiometer
Rev. B | Page 1 of 4
CIRCUIT FUNCTION AND BENEFITS
This circuit provides a logarithmic audio volume control with
glitch reduction using the AD5292 digital potentiometer in
conjunction with the dual AD8676 and single AD8541 op amps,
ADCMP371 comparators, and 7408 AND gates.
This circuit provides low total harmonic distortion (THD), a
maximum signal attenuation of 46 dB, and a shutdown function
that attenuates up to 130 dB, as shown in Figure 3. The AD5292
can be placed in shutdown mode by executing a software
shutdown command. This feature places the RDAC in a special
state in which terminal A is open-circuited and wiper W is
connected to terminal B.
This circuit offers a logarithmic gain control function over an
output voltage range of ±14 V (10 V rms) and is capable of
delivering up to ±20 mA output current. The AD5292 is
programmable over an SPI-compatible serial interface.
In addition, the AD5292 has an internal 20-times programmable
memory that allows a customized volume setting at power-up.
This circuit provides low noise, low THD, high signal
attenuation, low tempco, and high voltage capatibility.
It is well suited for many audio applications.
CIRCUIT DESCRIPTION
Table 1. Devices Connected/Referenced
Product Description
AD5292
Digital potentiometer, 10 bits, 1% resistor
tolerance
AD8676 Ultraprecision, dual, rail-to-rail output op amp
AD8541
General-purpose CMOS, single, rail-to-rail
amplifier
ADCMP371 Comparator with push-pull output stage
This circuit employs the AD5292 digital potentiometer in
conjunction with the dual AD8676 and single AD8541 op amps,
ADCMP371 comparators, and 7408 AND gates, providing a
low distortion logarithmic audio volume control with glitch
reduction. The logarithmic taper is achieved by adding resistor
R8 between the wiper connection and ground. This method is
described in detail in the article “Tack a Log Taper onto a
Digital Potentiometer” by Hank Zumbahlen, EDN, 1/20/00.
The circuit provides an input/output buffer, minimizing the
load effects with other external circuits; the AD8676 dual
op amp ensures low noise and precision rail-to-rail output
voltage. Figure 2 shows the logarithmic output voltage,
V
OUT
, normalized by V
IN
.
08499-001
R
1
100kΩ
±0.1%
R
2
806Ω
±0.1%
V
CC
+3.3V
V
IN
V+
V–
AD8541
+3.3V
U2
U1A
R
3
100kΩ
±0.1%
R
7
33.2kΩ
±0.1%
R
6
27.4kΩ
±0.1%
R
4
90.9kΩ ±0.1%
0V ±14V
R
5
9.09kΩ ±0.1%
C1
100nF
V
DD
+15V
V
SS
–15V
AD5292
R
AB
20kΩ
A
B
W
U6
U1B
2/2
V
CC
GND
V
CC
GND
ADCMP371
ADCMP371
+3.3V
+3.3V
+3.3V
U4
U3
V
OUT
U4B
U5A
7408
7408
SYNC
SYNC
AD8676
V+
+15V
V–
–15V
100kΩ
R
8
20kΩ
+1.645V
±1.4V
+1.81V
+1.657V
0.0133V
+1.643V
SERIAL
INTERFACE
AD8676
1/2
2/2
1/2
Figure 1. Logarithmic Audio Volume Control with Glitch Reduction (Simplified Schematic: Decoupling and All Connections Not Shown)
