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

5V

1

10

2

3

5

LINE_OUT_L

AD1881A

(AC'97)

LINE_OUT_R

7

8

6

9

U1-B

U1 = AD8644

VREF

38

35

27

36

26

25

+

+

= +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

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

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

R

R4

C1

f

+

π

=

−

2

1

dB

3

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)

2.2kΩ

10kΩ

1µF

1kΩ

1µF

Figure 32. Microphone Preamplifier