Datasheet

AD8591/AD8592/AD8594
Rev. B | Page 13 of 16
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
6
7
R
R
A
V
=
(5)
A
V
= = 6dB WITH VALUES SHOWN
R7
R6
U1-A
4
C1
100µF
+5V
1
10
2
3
5
+5
V
AV
DD1
AV
DD2
LINE_OUT_L
AD1881A*
(AC’97)
LINE_OUT_
R
AV
SS1
R4
20Ω
+5V
R1
100kΩ
7
8
6
9
R5
20Ω
R6
10kΩ
R7
10kΩ
R7
10kΩ
R6
10kΩ
C2
100µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
U1-B
U1 = AD8592
NC
R2
2kΩ
R3
2kΩ
25
38
35
36
VREF
27
26
01106-040
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 AD1881A.
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
()
L
dB
RRC
f
+π
=
−
412
1
3
(6)
where R
L
is the resistance of the headphones.
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.
Figure 40 demonstrates a simple method for constructing an
interface to a codec.
U1-A
4
+5V
1
10
2
3
5
C2
10µF
U1 = AD8592
7
8
6
9
U1-B
FROM CODEC
MONO OUT
(OR LEFT OUT)
TO
CODEC
V
REF
FROM CODEC
MICROPHONE
A
ND SPEAKER
JACK
R1
2.2kΩ
+5V
+5V
C1
0.1µF
NC
(RIGHT OUT)
R2
10kΩ
R3
100kΩ
R8
100kΩ
R5
10kΩ
R6
10kΩ
(OPTIONAL)
R4
10kΩ
R7
1kΩ
01106-041
Figure 40. Speaker/Microphone Headset Amplifier Circuit
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
Headphone/Speaker Amplifier section, the normalizing contact
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.
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.
V
IN
U1-A
C1
1nF
U1-B
SAMPLE
AND HOLD
OUTPUT
+5V
1
2
3
5
9
8
7
6
SAMPLE
CLOCK
U1 = AD8592
+5V
4
10
0
1106-042
Figure 41. An Efficient Sample-and-Hold Circuit