AD8591/AD8592/AD8594

–10–

REV. A

If gain is required from the output amplifier, four additional

resistors should be added as shown in Figure 35. The gain of

the AD8592 can be set as:

A

R

R

V

=

7

6

(5)

U1-A

R2

2k

4

C1

100 F

+5V

1

10

2

3

5

+5V

AD1881

(AC97)

R4

20

+5V

R1

100k

7

8

6

9

R5

20

C2

100 F

NOTE: ADDITIONAL PINS

OMITTED FOR CLARITY

U1-B

U1 = AD8592

R3

2k

R7

20k

R7

20k

R6

10k

R6

10k

= +6dB WITH VALUES SHOWN

R7

R6

NC

38

35

27

36

Figure 35. A PC-98 Compliant Headphone/Line Out

Amplifier With Gain

Input coupling capacitors are not required for either circuit as

the reference voltage is supplied from the AD1881.

R4 and R5 help protect the AD8592 output in case the output

jack or headphone wires accidentally get 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:

f

CR

R

dB

L

–3

1

21

4

=

+

()

π

(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 36 demonstrates a simple method for constructing an

interface to a codec.

U1-A

4

+5V

1

10

2

3

5

C2

10 F

U1 = AD8592

7

8

6

9

R5

10k

U1-B

R6

10k

(OPTIONAL)

R4

10k

FROM CODEC

MONO OUT

(OR LEFT OUT)

TO

CODEC

R3

100k

FROM CODEC

MIC + SPEAKER

JACK

R1

2.2k

+5V

R2

10k

R8

100k

+5V

C1

0.1 F

NC

(RIGHT OUT)

R7

1k

Figure 36. A Speaker/Mic 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, R6 should be added, equal in value to R4.

Using the same principle as described in the previous 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 con-

trolled with TTL or CMOS compatible logic, allowing micro-

phone 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 par-

ticular application for which this feature is useful is in designing a

sample-and-hold circuit for data acquisition. Figure 37 shows a

schematic of a simple, yet extremely effective sample-and-hold

circuit using a single AD8592 and one capacitor.

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

Figure 37. An Efficient Sample-and-Hold Circuit