LT1012ACS8 datasheet(11/20 Pages) LINER

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LT1012A/LT1012

The LT1012 may be inserted directly into OP-07, LM11,

108A or 101A sockets with or without removal of external

frequency compensation or nulling components. The

LT1012 can also be used in 741, LF411, LF156 or OP-15

applications provided that the nulling circuitry is

removed.

Although the OP-97 is a copy of the LT1012, the LT1012

directly replaces and upgrades OP-97 applications. The

LT1012C and D have lower offset voltage and drift than the

OP-97F. The LT1012A has lower supply current than the

OP-97A/E. In addition, all LT1012 grades guarantee

operation at

±1.2V supplies.

Achieving Picoampere/Microvolt Performance

In order to realize the picoampere/microvolt level

accuracy of the LT1012, proper care must be exercised.

For example, leakage currents in circuitry external to

the op amp can significantly degrade performance. High

quality insulation should be used (e.g. Teflon, Kel-F);

cleaning of all insulating surfaces to remove fluxes and

other residues will probably be required. Surface coating

may be necessary to provide a moisture barrier in high

humidity environments.

Board leakage can be minimized by encircling the input

circuitry with a guard ring operated at a potential close

to that of the inputs: in inverting configurations the

guard ring should be tied to ground, in non-inverting

connections to the inverting input at Pin 2. Guarding both

sides of the printed circuit board is required. Bulk leakage

reduction depends on the guard ring width. Nanoampere

level leakage into the offset trim terminals can affect offset

voltage and drift with temperature.

Microvolt level error voltages can also be generated in

the external circuitry. Thermocouple effects caused by

temperature gradients across dissimilar metals at the

contacts to the input terminals can exceed the inherent

drift of the amplifier. Air currents over device leads should

be minimized, package leads should be short, and the two

input leads should be as close together as possible and

maintained at the same temperature.

Noise Testing

For application information on noise testing and calcula-

tions, please see the LT1008 data sheet.

Frequency Compensation

The LT1012 can be overcompensated to improve

capacitive load handling capability or to narrow noise

bandwidth. In many applications, the feedback loop around

the amplifier has gain (e.g. Iogarithmic amplifiers);

overcompensation can stabilize these circuits with a single

capacitor.

The availability of the compensation terminal permits

the use of feedforward frequency compensation to

enhance slew rate. The voltage follower feedforward

scheme bypasses the amplifier’s gain stages and slews at

nearly 10V/

µs.

The inputs of the LT1012 are protected with back-to-back

diodes. Current limiting resistors are not used, because

the leakage of these resistors would prevent the realization

of picoampere level bias currents at elevated tempera-

tures. In the voltage follower configuration, when the input

is driven by a fast, large signal pulse (>1V), the input

protection diodes effectively short the output to the input

during slewing, and a current, limited only by the output

short-circuit protection will flow through the diodes.

The use of a feedback resistor, as shown in the voltage

follower feedforward diagram, is recommended because

this resistor keeps the current below the short-circuit

limit, resulting in faster recovery and settling of the output.

OUTPUT

OVER COMP

OFFSET TRIM

GUARD

V –

LT1012A * AI01

APPLICATIO S I FOR ATIO

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