AD8594 Analog Devices, AD8594 Datasheet - Page 13

• Manufacturer Part Number: AD8594
• Manufacturer: Analog Devices

Specifications of AD8594

• -3db Bandwidth: 2.2MHz
• Slew Rate: 3.5V/µs
• Vos: 25mV
• Ib: 5pA
• # Opamps Per Pkg: 4
• Input Noise (nv/rthz): 30nV/rtHz
• Vcc-vee: 2.5V to 6V
• Isy Per Amplifier: 1.25mA
• Packages: SOIC,SOP

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
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
where R
LINE_OUT_R
LINE_OUT_L
*ADDITIONAL PINS OMITTED FOR CLARITY.
AD1881A*
(AC’97)
Figure 39. PC98-Compliant Headphone/Line Out Amplifier with Gain
A
AV
AV
f
AV
VREF
V
−
DD1
DD2
SS1
L
3
=
dB
is the resistance of the headphones.
R
R
25
38
35
27
36
26
=
7
6
10kΩ
10kΩ
2
+5V
R6
R6
π
C
A
1
2
3
7
8
V
(
R
=
5
1
6
U1-A
U1-B
4
R7
R6
10kΩ
10kΩ
R7
R7
+5V
+
= 6dB WITH VALUES SHOWN
10
R
4
L
100kΩ
)
9
U1 = AD8592
1
R1
+5V
100µF
100µF
C1
C2
R2
2kΩ
R3
2kΩ
20Ω
20Ω
R5
R4
NC
Rev. B | Page 13 of 16
(5)
(6)
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.
AND SPEAKER
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.
MICROPHONE
V
IN
JACK
2
3
U1 = AD8592
Figure 40. Speaker/Microphone Headset Amplifier Circuit
SAMPLE
1kΩ
CLOCK
U1-A
R7
2.2kΩ
+5V
R1
Figure 41. An Efficient Sample-and-Hold Circuit
10
5
+5V
NC
4
1
0.1µF
C1
100kΩ
10µF
C2
R8
AD8591/AD8592/AD8594
10kΩ
+5V
R2
C1
1nF
9
8
7
2
3
U1 = AD8592
10kΩ
U1-B
5
100kΩ
U1-A
R5
U1-B
R3
6
+5V
10
6
4
7
8
+5V
1
(OPTIONAL)
9
10kΩ
10kΩ
R4
R6
TO
CODEC
V
FROM CODEC
FROM CODEC
MONO OUT
(OR LEFT OUT)
(RIGHT OUT)
REF
SAMPLE
AND HOLD
OUTPUT