8

LT1024

1024fa

Achieving Picoampere/Microvolt Performance

In order to realize the picoampere/microvolt level

accuracy of the LT1024, proper care must be exercised.

For example, leakage currents in circuitry external to the

op amp can significantly degrade performance. High qual-

ity insulation should be used (e.g., Teflon™, Kel-F); clean-

ing 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 noninverting connec-

tions, to the inverting input. Guarding both sides of the

printed circuit board is required. Bulk leakage reduction

depends on the guard ring width. Nanoampere level leak-

age 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 tem-

perature gradients across dissimilar metals at the con-

tacts 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.

Test Circuit for Offset Voltage and its Drift with Temperature

R1

50k*

R3

50k*

R2

100Ω*

15V

–15V

–

+

3

(10)

4

(11)

LT1024

(7)

14

12 (5)

13

(6)

RESISTORS MUST HAVE LOW

THERMOELECTRIC POTENTIAL

THIS CIRCUIT IS ALSO USED AS THE BURN-IN

CONFIGURATION FOR THE LT1024. WITH SUPPLY

VOLTAGES INCREASED TO ±20V, R1 = R3 = 20k,

R2 = 200Ω, AV = 100

LT1024 • AI02

*

**

Teflon is a trademark of Dupont.

APPLICATIO S I FOR ATIO