REV. D

OP497

–7–

APPLICATIONS INFORMATION

Extremely low bias current over the full military temperature range

makes the OP497 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 not necessary

minimally by high source resistance, even when unbalanced.

The input pins of the OP497 are protected against large differen-

tial voltage by back-to-back diodes and current-limiting resistors.

Common-mode voltages at the inputs are not restricted, and may

vary over the full range of the supply voltages used.

The OP497 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

1 V of either rail. The output typically swings to within 1 V of

the rails when using a 10 k

Ω load.

AC PERFORMANCE

The OP497’s ac characteristics are highly stable over its full

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

shown in Figure 1. Extremely tolerant of capacitive loading on

the output, the OP497 displays excellent response even with

1000 pF loads (Figure 2).

10

90

100

0%

20mV

5 s

Figure 1. Small-Signal Transient Response

10

90

100

0%

20MV

5 s

Figure 2. Small-Signal Transient Response

10

90

100

0%

2V

50 s

GUARDING AND SHIELDING

To maintain the extremely high input impedances of the OP497,

care must be taken in circuit board layout and manufacturing.

Board surfaces must be kept scrupulously clean and free of mois-

ture. Conformal coating is recommended to provide a humidity

barrier. Even a clean PC board can have 100 pA of leakage currents

between adjacent traces, so guard rings should be used around

the inputs. Guard traces are operated at a voltage close to that

on the inputs, as shown in Figure 4, so that leakage currents

become minimal. In noninverting applications, the guard ring

should be connected to the common-mode voltage at the invert-

ing input. In inverting applications, both inputs remain at ground,

so the guard trace should be grounded. Guard traces should be

on both sides of the circuit board.

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OP497

UNITY GAIN FOLLOWER

NONINVERTING AMPLIFIER

INVERTING AMPLIFIER

B

8

A

1

MINI-DIP

BOTTOM VIEW

–

+

–

+

–

+

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OP497

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OP497

Figure 4. Guard Ring Layout and Connections