AD8591/AD8592/AD8594

Rev. B | Page 14 of 16

The U1-A amplifier is configured as a unity-gain buffer driving

a 1 nF capacitor. The input signal is connected to the noninverting

input, and the sample clock controls the shutdown for that

amplifier. When the sample clock is high, the U1-A amplifier is

low, U1-A shuts down with the output of the amplifier going to

a high impedance state, holding the voltage on the C1 capacitor.

SINGLE-SUPPLY DIFFERENTIAL LINE DRIVER

Figure 43 shows a single-supply differential line driver circuit

that can drive a 600 Ω load with less than 0.7% distortion from

20 Hz to 15 kHz with an input signal of 4 V p-p and a single 5 V

supply. The design uses an AD8594 to mimic the performance

of a fully balanced transformer-based solution. However, this

design occupies much less board space, while maintaining low

distortion, and can operate down to dc. Like the transformer-based

design, either output can be shorted to ground for unbalanced

line driver applications without changing the circuit gain of 1.

The U1-B amplifier is used as a unity-gain buffer to prevent

loading on C1. Because of the low input bias current of the U1-B

CMOS input stage and the high impedance state of the U1-A

output in shutdown, there is little voltage droop from C1 during

the hold period. This circuit can be used with sample frequencies as

high as 500 kHz and as low as 1 Hz. By increasing the C1 value,

lower voltage droop is achieved for very low sample rates.

600Ω

C1

22µF

A2

9

8

7

3

1

2

A1

+5V

R1

10kΩ

R2

10kΩ

R11

10kΩ

R7

10kΩ

8

7

A1

+5V

+5V

R8

100kΩ

R9

100kΩ

C2

1µF

R12

10kΩ

R14

50Ω

A2

1

2

3

R3

10kΩ

R6

10kΩ

R13

10kΩ

C3

47µF

C4

47µF

A1, A2 = 1/2 AD8592

GAIN = R3

R2

SET: R7, R10, R11 = R2

SET: R6, R12, R13 = R3

R10

10kΩ

R5

50Ω

10

4

10

4

9

DIRECT ACCESS ARRANGEMENT FOR PCMCIA

MODEMS (TELEPHONE LINE INTERFACE)

Figure 42 illustrates a 5 V transmit/receive telephone line

interface for 600 Ω systems. It allows full duplex transmission

of signals on a transformer-coupled 600 Ω 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 voltages of the AD8594, the largest signal

available on a single 5 V supply is approximately 4.5 V p-p into

a 600 Ω transmission system. Amplifier A3 is configured as a

difference amplifier for two reasons. It prevents the transmit

signal from interfering with the receive signal, and it extracts

the receive signal from the transmission line for amplification

by A4. The gain of A4 can be adjusted in the same manner as

the gain of A1 to meet the input signal requirements of the

modem. Standard resistor values permit the use of single

inline package (SIP) format resistor arrays. Couple this with

the 16-lead TSSOP or SOIC footprint of the AD8594, and this

circuit offers a compact, cost-effective solution.

Figure 43. Low Noise, Single-Supply Differential Line Driver

R8 and R9 set up the common-mode output voltage equal to

half of the supply voltage. C1 is used to couple the input signal

and can be omitted if the dc voltage of the input is equal to half

of the supply voltage.

The circuit can also be configured to provide additional gain, if

desired. The gain of the circuit is

2

3

R

R

V

V

A

IN

OUT

V

=

=

(7)

R7

10kΩ

R8

10kΩ

+5V

6.2V

6.2V

TRANSMIT

TxA

RECEIVE

RxA

C1

0.1µF

R1

10kΩ

R2

9.09kΩ

2kΩ

P1

Tx GAIN

ADJUST

A1

A2

A3

A4

A1, A2 = 1/4 AD8594

A3, A4 = 1/4 AD8594

R3

360Ω

1:1

T1

TO TELEPHONE

LINE

1

2

3

7

6

5

11

12

10

15

14

16

R5

10kΩ

R6

10kΩ

R9

10kΩ

R14

14.3kΩ

R10

10kΩ

R11

10kΩ

R12

10kΩ

R13

10kΩ

C2

0.1µF

P2

Rx GAIN

ADJUST

2kΩ

600Ω

MIDCOM

671-8005

SHUTDOWN

9

9

9

9

10µF

where:

R2 = R7 = R10 = R11

R3 = R6 = R12 = R1

Figure 42. Single-Supply Direct Access Arrangement for PCMCIA Modems