Datasheet

AD8614/AD8644
Rev. B | Page 12 of 16
If gain is required from the output amplifier, four additional
resistors should be added as shown in
Figure 31.
U1-A
4
5V
5
V
1
10
2
3
5
AV
DD1
AV
DD2
LINE_OUT_L
AD1881A
(AC'97)
LINE_OUT_R
AV
SS1
7
8
6
9
U1-B
U1 = AD8644
VREF
38
35
27
36
26
25
+
+
A
V
= +6dB WITH VALUES SHOWN
R6
R5
=
NOTES
1. ADDITIONAL PINS OMITTED FOR CLARITY.
R6
20kΩ
R5
10kΩ
C1
100µF
R3
20Ω
R1
2kΩ
R4
20Ω
R2
2kΩ
C2
100µF
R5
10kΩ
R6
20kΩ
06485-031
Figure 31. A PC-99-Compliant Headphone/Speaker Amplifier with Gain
The gain of the AD8644 can be set as
5
6
R
R
A
V
=
Input coupling capacitors are not required for either circuit as
the reference voltage is supplied from the
AD1881A.
The resistors R4 and R5 help protect the AD8644 output in case
the output jack or headphone wires are accidentally 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
RR4C1
f
+π
=
−
2
1
dB3
where R
L
is the resistance of the headphones.
The remaining two amplifiers can be used as low voltage
microphone preamplifiers. A single AD8614 can be used as a
standalone microphone preamplifier.
Figure 32 shows this
implementation.
10kΩ
21
MIC1
1kΩ
2.2kΩ
5V
5V
MIC 1
MIC 2
22
MIC2
27
+
+
VREF
AD1881A
(AC'97)
A
V
= 20dB
A
V
= 20dB
2.2kΩ
10kΩ
1µF
1kΩ
1µF
06485-032
Figure 32. Microphone Preamplifier