AD8591/AD8592/AD8594
When headphones are plugged into the jack, the normalizing
contacts disconnect from the audio contacts. This allows the
voltage to the AD8592 shutdown pins to be pulled to 5 V,
activating the amplifiers. With no plug in the output jack, the
shutdown voltage is pulled to 100 mV through the R1 and R3 + R5
voltage divider. This powers the AD8592 down when it is not
needed, saving current from the power supply or battery.
If gain is required from the output amplifier, add four additional
resistors, as shown in Figure 39. The gain of the AD8592 can
be set as
A
V
=
R
7
R
6
+5V
AV
DD1 25
AV
DD2 38
LINE_OUT_L
35
2
U1-A
4
3
5
VREF
27
6
R6
10kΩ
7
C2
100µF
U1-B
9
R3
2kΩ
R5
20Ω
R7
10kΩ
+5V
10
1
C1
100µF
+5V
R4
20Ω
R2 NC
2kΩ
A COMBINED MICROPHONE AND SPEAKER
AMPLIFIER FOR CELLPHONE AND PORTABLE
HEADSETS
The dual amplifiers in the AD8592 make an efficient design for
interfacing with a headset containing a microphone and speaker.
interface to a codec.
+5V
R1
2.2kΩ
C1
0.1µF
R2
10kΩ
R3
100kΩ
+5V
2
U1-A
4
3
10
1
TO
CODEC
(5)
R7
1kΩ
MICROPHONE
AND SPEAKER
JACK
NC
+5V
R8
100kΩ
5
V
REF
FROM CODEC
6
U1-B
7
R4
10kΩ
FROM CODEC
MONO OUT
(OR LEFT OUT)
01106-041
C2
10µF
9
R6
10kΩ
U1 = AD8592
R5
10kΩ
8
R1
100kΩ
(RIGHT OUT)
R6
10kΩ
(OPTIONAL)
Figure 40. Speaker/Microphone Headset Amplifier Circuit
AD1881A*
(AC’97)
LINE_OUT_R
36
AV
SS1 26
8
R7
10kΩ
A
V
=
R7
R6
U1 = AD8592
U1-A is used as a microphone preamplifier, where the gain of
the preamplifier is set as R3/R2. R1 is used to bias an electret
microphone, and C1 blocks any dc voltages from the amplifier.
U1-B is the speaker amplifier, and its gain is set at R5/R4. To
sum a stereo output, add R6, equal in value to R4.
Using the same principle described in the PC98-Compliant
on the microphone/speaker jack can be used to put the AD8592
into shutdown when the headset is not plugged in. The AD8592
shutdown inputs can also be controlled with TTL- or CMOS-
compatible logic, allowing microphone or speaker muting, if
desired.
= 6dB WITH VALUES SHOWN
01106-040
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 39. PC98-Compliant Headphone/Line Out Amplifier with Gain
Input coupling capacitors are not required for either circuit
because the reference voltage is supplied from the
R4 and R5 help protect the AD8592 output in case the output
jack or headphone wires accidentally are shorted to ground. The
output coupling capacitors, C1 and C2, block dc current from the
headphones and create a high-pass filter with a corner frequency of
AN INEXPENSIVE SAMPLE-AND-HOLD CIRCUIT
The independent shutdown control of each amplifier in the
AD8592 allows a degree of flexibility in circuit design. One
particular application for which this feature is useful is in
designing a sample-and-hold circuit for data acquisition. Figure 41
shows a schematic of a simple, yet extremely effective, sample-
and-hold circuit using a single AD8592 and one capacitor.
+5V
2
10
8
f
−
3
dB
=
1
2
π
C
1
(
R
4
+
R
L
)
(6)
where
R
L
is the resistance of the headphones.
U1-B
4
1
C1
1nF
9
+5V
U1-A
V
IN
3
5
SAMPLE
CLOCK
SAMPLE
AND HOLD
OUTPUT
7
6
Figure 41. An Efficient Sample-and-Hold Circuit
Rev. B | Page 13 of 16
01106-042
U1 = AD8592
AD [ ANALOG DEVICES ]
