REV. B

OP292/OP492

–10–

100dV/DIV

MKR: 16.9 V/ Hz

25 kHz

BW: 150 Hz

0Hz

MKR: 1000 Hz

CH A: 800dV FS

TPC 31. Voltage Noise Density

APPLICATION INFORMATION

Phase Reversal

The OP492 has built-in protection against phase reversal when

the input voltage goes to either supply rail. In fact, it is safe for

the input to exceed either supply rail by up to 0.6 V with no risk

of phase reversal. However, the input should not go beyond the

positive supply rail by more than 0.9 V, otherwise the output

will reverse phase. If this condition can occur, the problem can

be fixed by adding a 5 k

current limiting resistor in series with

the input pin. With this addition, the input can go to more than

5 V beyond the positive rail without phase reversal.

An input voltage that is as much as 5 V below the negative rail

will not result in phase reversal.

OP492

2K

5V

OV

11.8V p-p

10

90

100

0%

1V

5µS

Figure 1. Output Phase Reverse If Input Exceeds the Posi-

tive Supply (V+) by More Than 0.9 V

OP492

2k

5V

OV

10V p-p

10

90

100

0%

1V

5µS

10

90

100

0%

1V

5µS

Figure 2. No Negative Rail Phase Reversal, Even with Input

Signal at 5 V Below Ground

Power Supply Considerations

The OP292/OP492 are designed to operate equally well at single 5 V

or

15 V supplies. The lowest supply voltage recommended is 4.5 V.

It is a good design practice to bypass the supply pins with a 0.1

mF

ceramic capacitor. It helps improve filtering of high frequency noise.

For dual supply operation, the negative supply (V–) must be applied

at the same time, or before V . If V

is applied before V–, or in

the case of a loss of V– supply, while either input is connected to

ground or other low impedance source, excessive input current

may result. Potentially damaging levels of input current can de-

stroy the amplifier. If this condition can exist, simply add a l k

or larger resistor in series with the input to eliminate the problem.

TYPICAL APPLICATIONS

Direct Access Arrangement for Telephone Line Interface

Figure 3 shows a 5 V- only transmit/receive telephone line inter-

face for a modem circuit. It allows full duplex transmission of modem

signals on a transformer-coupled 600 V line in a differential man-

ner. The transmit section gain can be set for the specific modem

device output. Similarly the receive amplifier gain can be appro-

priately 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.

50k

5V dc

6.2V

6.2V

T1

1:1

To

Telephone

Line

0.1 F

50k

10 F

MODEM

TX GAIN ADJUST

RX GAIN ADJUST

0.1 F

300k

20k

20k

20k

20k

20k

20k

20k

0.1 F

20k

100pF 5k

5k

1/4

OP492

1/4

OP492

1/4

OP492

5V

TRANSMIT

TXA

RECEIVE

RXA

300k

Figure 3. A Universal Direct Access Arrangement for

Telephone Line Interface

A Single-Supply Instrumentation Amplifier

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

as shown in Figure 4. The circuit utilizes two op amps to form a

high-input impedance differential amplifier. Gain can be set by

output voltage is a function of the gain times the differential input