6

LTC1050

1050fb

S

APPLICATI

IFOR ATIO

ACHIEVING PICOAMPERE/MICROVOLT

PERFORMANCE

Picoamperes

In order to realize the picoampere level of accuracy of the

LTC1050, proper care must be exercised. Leakage currents

in circuitry external to the amplifier 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 necessary—

particularly for high temperature performance. Surface

coating may be necessary to provide a moisture barrier in

high humidity environments.

Board leakage can be minimized by encircling the input

connections 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 connections

to the inverting input (see Figure 1). Guarding both sides

of the printed circuit board is required. Bulk leakage reduc-

tion depends on the guard ring width.

Microvolts

Thermocouple effect must be considered if the LTC1050’s

ultralow drift is to be fully utilized. Any connection of dis-

similar metals forms a thermoelectric junction producing

an electric potential which varies with temperature (Seebeck

effect). As temperature sensors, thermocouples exploit this

phenomenon to produce useful information. In low drift

amplifier circuits the effect is a primary source of error.

Connectors, switches, relay contacts, sockets, resistors,

solder and even copper wire are all candidates for thermal

TEST CIRCUITS

–

+

LTC1050

OUTPUT

1050 TC01

1k

1M

6

7

2

3

4

–

+

LTC1050

–

+

LT

1050 TC02

10Ω

100k

158k

316k

475k

475k

0.015µF

FOR 1Hz NOISE BW, INCREASE ALL

THE CAPACITORS BY A FACTOR OF10

0.015µF

0.015µF

TO X-Y

RECORDER

Electrical Characteristics Test Circuit

DC-10Hz Noise Test Circuit

EMF generation. Junctions of copper wire from different

manufacturers can generate thermal EMFs of 200nV/°C—

4 times the maximum drift specification of the LTC1050.

The copper/kovar junction, formed when wire or printed

circuit traces contact a package lead, has a thermal EMF of

approximately 35µV/°C—700 times the maximum drift

specification of the LTC1050.

Minimizing thermal EMF-induced errors is possible if ju-

dicious attention is given to circuit board layout and

component selection. It is good practice to minimize the

number of junctions in the amplifier’s input signal path.

Avoid connectors, sockets, switches and relays where

possible. In instances where this is not possible, attempt

to balance the number and type of junctions so that differ-

ential cancellation occurs. Doing this may involve

deliberately introducing junctions to offset unavoidable

junctions.

8

1

7

6

OUTPUT

1050 F01

5

OPTIONAL

EXTERNAL

CLOCK

4

GUARD

2

3

Figure 1