AD8614/AD8644

–7–

REV. 0

The optimum values for the snubber network should be determined

empirically based on the size of the capacitive load. Table I shows a

few sample snubber network values for a given load capacitance.

Table I. Snubber Networks for Large Capacitive Loads

Load Capacitance

Snubber Network

0.47 nF

300

Ω, 0.1 µF

4.7 nF

30

Ω, 1 µF

47 nF

5

Ω, 1 µF

Direct Access Arrangement

Figure 26 shows a schematic for a 5 V single supply transmit/receive

telephone line interface for 600

Ω transmission systems. It allows

full duplex transmission of signals on a transformer-coupled 600

Ω

line. Amplifier A1 provides gain that can be adjusted to meet the

modem output drive requirements. Both A1 and A2 are configured

to apply the largest possible differential signal to the transformer.

The largest signal available on a single 5 V supply is approximately

4.0 V p-p into a 600

Ω transmission system. Amplifier A3 is config-

ured as a difference amplifier to extract the receive information from

the transmission line for amplification by A4. A3 also prevents the

transmit signal from interfering with the receive signal. The gain of

A4 can be adjusted in the same manner as A1’s to meet the modem’s

input signal requirements. Standard resistor values permit the use of

SIP (Single In-Line Package) format resistor arrays. Couple this with

the AD8644 14-lead SOIC or TSSOP package and this circuit can

offer a compact solution.

6.2V

6.2V

TRANSMIT

TxA

RECEIVE

RxA

C1

0.1 F

R1

10k

R2

9.09k

2k

P1

Tx GAIN

ADJUST

A1

A2

A3

A4

A1, A2 = 1/2 AD8644

A3, A4 = 1/2 AD8644

R3

360

1:1

T1

TO TELEPHONE

LINE

1

2

3

7

6

5

2

3

1

6

5

7

10 F

R7

10k

R8

10k

R5

10k

R6

10k

R9

10k

R14

14.3k

R10

10k

R11

10k

R12

10k

R13

10k

C2

0.1 F

P2

Rx GAIN

ADJUST

2k

600

5V DC

MIDCOM

671-8005

Figure 26. A Single-Supply Direct Access Arrangement for

Modems

A One-Chip Headphone/Microphone Preamplifier Solution

Because of its high output current performance, the AD8644

makes an excellent amplifier for driving an audio output jack in

a computer application. Figure 27 shows how the AD8644 can

be interfaced with an ac codec to drive headphones or speakers

U1-A

R1

2k

4

C1

100 F

5V

1

10

2

3

5

5V

AD1881

(AC'97)

R3

20

7

8

6

9

R4

20

C2

100 F

NOTE: ADDITIONAL PINS

OMITTED FOR CLARITY

U1-B

U1 = AD8644

R2

2k

28

35

36

Figure 27. A PC-99 Compliant Headphone/Line Out Amplifier

If gain is required from the output amplifier, four additional

resistors should be added as shown in Figure 28. The gain of

the AD8644 can be set as:

A

R

R

6

5

U1-A

R1

2k

4

C1

100 F

5V

1

10

2

3

5

5V

AD1881

(AC97)

R3

20

7

8

6

9

R4

20

C2

100 F

NOTE: ADDITIONAL PINS

OMITTED FOR CLARITY

U1-B

U1 = AD8644

R2

2k

R6

20k

R6

20k

R5

10k

R5

10k

= +6dB WITH VALUES SHOWN

R6

R5

38

35

27

36

Figure 28. A PC-99-Compliant Headphone/Speaker

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 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:

f

CR

R

dB

L

−

=

+

()

3

1

21

4

π