OP292/OP492

Rev. C | Page 14 of 20

TYPICAL APPLICATIONS

DIRECT ACCESS ARRANGEMENT FOR TELEPHONE

LINE INTERFACE

Figure 36 shows a 5 V single-supply transmit/receive telephone line

interface for a modem circuit. It allows full duplex transmission

of modem signals on a transformer-coupled 600 V line in a

differential manner. The transmit section gain can be set for the

specific modem device output. Similarly, the receive amplifier

gain can be appropriately selected based on the modem device

input requirements. The circuit operates on a single 5 V supply.

The standard value resistors allow the use of a SIP-packaged

resistor array; coupled with a quad op amp in a single package,

this offers a compact, low part count solution.

5V DC

6.2V

6.2V

T1

1:1

TO

TELEPHONE

LINE

0.1µF

50kΩ

10µF

MODEM

TX GAIN ADJUST

RX GAIN ADJUST

0.1µF

300kΩ

20kΩ

20kΩ

20kΩ

20kΩ

20kΩ

20kΩ

0.1µF

20kΩ

100pF 5kΩ

5kΩ

1/4

OP492

1/4

OP492

1/4

OP492

5V

TRANSMIT

TXA

RECEIVE

RXA

300kΩ

20kΩ

50kΩ

Figure 36. Universal Direct Access Arrangement for Telephone Line Interface

SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER

A low cost, single-supply instrumentation amplifier can be built

as shown in Figure 37. The circuit uses two op amps to form a

high input impedance differential amplifier. Gain can be set by

voltage is a function of the gain times the differential input

0 V to 4 V, as long as the input common-mode voltage of the

amplifier is not exceeded.

8

5V

7

4

1

5

40kΩ

20kΩ

5kΩ

20kΩ

5kΩ

1/2

OP292

1/2

OP292

Figure 37. Single-Supply Instrumentation Amplifier

In this configuration, the output can swing to near 0 V;

however, be careful because the common-mode voltage range of

the input cannot operate to 0 V. This is because of the limitation

of the circuit configuration where the first amplifier must be able to

swing below ground to attain a 0 V common-mode voltage,

which it cannot do. Depending on the gain of the instrumentation

amplifier, the input common-mode extends to within about 0.3 V

of zero. The worst-case common-mode limit for a given gain

can be easily calculated.

DAC OUTPUT AMPLIFIER

The OP292/OP492 are ideal for buffering the output of single-

supply DACs. Figure 38 shows a typical amplifier used to buffer

the output of a CMOS DAC that is connected for single-supply

operation. To do that, the normally current output 12-bit CMOS

DAC (R-2R ladder type) is connected backward to produce a

voltage output. This operating configuration necessitates a low

voltage reference. In this case, a 1.235 V low power reference is

used. The relatively high output impedance (10 kΩ) is buffered

by the OP292, and at the same time, gained up to a much more

usable level. The potentiometer provides an accurate gain trim

for a 4.095 V full-scale, allowing 1 mV increment per LSB of

control resolution.

The DAC8043 device comes in an 8-lead PDIP package, providing

a cost-effective, compact solution to a 12-bit analog channel.

VDD

Clk

Sri

1

2

3

4

8

7

6

5

DAC8043

5V

5V

5V

7.5kΩ

1.235V

AD589

NC

DIGITAL

CONTROL

500kΩ

8.45kΩ

20kΩ

1/2

OP292

1mV/LSB

0V – 4.095V

FS

GND

LD

LD

SRI

SRI

CLK

CLK

Figure 38. 12-Bit Single-Supply DAC with Serial Bus Control