8/21/97 4:00 PM

OP297

–7–

REV. D

Figure 24. Open Loop Output

Impedance vs Frequency

APPLICATIONS INFORMATION

Extremely low bias current over the full military temperature

range makes the OP297 attractive for use in sample-and-hold

amplifiers, peak detectors, and log amplifiers that must operate

over a wide temperature range. Balancing input resistances is

degraded only minimally by high source resistance, even when

unbalanced.

The input pins of the OP297 are protected against large differ-

ential voltage by back-to-back diodes and current-limiting resis-

tors. Common-mode voltages at the inputs are not restricted,

and may vary over the full range of the supply voltages used.

The OP297 requires very little operating headroom about the

supply rails, and is specified for operation with supplies as low

as +2 V. Typically, the common-mode range extends to within

one volt of either rail. The output typically swings to within one

volt of the rails when using a 10 k

Ω load.

AC PERFORMANCE

The OP297’S AC characteristics are highly stable over its full

operating temperature range. Unity-gain small-signal response is

shown in Figure 25. Extremely tolerant of capacitive loading on

the output, the OP297 displays excellent response even with

1000 pF loads (Figure 26).

10

0%

100

90

5µs

20mV

Figure 25. Small-Signal Transient Response

10

0%

100

90

5µs

20mV

Figure 26. Small-Signal Transient Response

10

0%

100

90

5µs

20mV

Figure 27. Large-Signal Transient Response

GUARDING AND SHIELDING

To maintain the extremely high input impedances of the

OP297, care must be taken in circuit board layout and manu-

facturing. Board surfaces must be kept scrupulously clean and

free of moisture. Conformal coating is recommended to provide

GAIN

PHASE

FREQUENCY (Hz)

100

80

60

40

20

0

–20

–40

100

1k

10k

100

1M

10M

90

135

180

225

+EDGE

–EDGE

70

60

50

40

30

20

10

0

LOAD CAPACITANCE (pF)

10

100

1000

10000

FREQUENCY (Hz)

1000

100

10

1

0.1

0.01

0.001

10

100

1k

10k

100k

1M

Figure 22. Open Loop Gain,

Phase vs. Frequency

Figure 23. Small Signal Overshoot

vs. Capacitance Load