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

(5)

= 6dB WITH VALUES SHOWN

R7

R6

U1-A

4

C1

100µF

+5V

1

10

2

3

5

+5V

LINE_OUT_L

AD1881A*

(AC’97)

LINE_OUT_R

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

26

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

R

R

C

f

+

π

=

−

4

1

2

1

3

(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.

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

FROM CODEC

MICROPHONE

AND 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

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.

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 41. An Efficient Sample-and-Hold Circuit