AD8531/AD8532/AD8534

REV. D

–13–

Direct Access Arrangement for Telephone Line Interface

Figure 42 illustrates a 5 V only transmit/receive telephone line

interface for 600

W transmission systems. It allows full duplex

transmission of signals on a transformer coupled 600

W line in a

differential manner. 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 signal on

a single supply to the transformer. Because of the high output

current drive and low dropout voltage of the AD8531/AD8532/

AD8534s, the largest signal available on a single 5 V supply is

approximately 4.5 V p-p into a 600

W transmission system.

Amplifier A3 is configured as a difference amplifier for two

reasons: (1) It prevents the transmit signal from interfering with

the receive signal and (2) it extracts the receive signal from the

transmission line for amplification by A4. A4’s gain 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.

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 AD8532

A3, A4 = 1/2 AD8532

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 42. A Single-Supply Direct Access Arrangement

for Modems