7

LTC1151

1151fa

S

APPLICATI

I FOR ATIO

ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE

Picoamperes

In order to realize the picoampere level of accuracy of the

LTC1151 proper care must be exercised. Leakage currents

in circuitry external to the amplifier can significantly de-

grade performance. High quality insulation should be used

(e.g., Teflon); 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. Guarding both sides of the printed

circuit board is required. Bulk leakage reduction depends

on the guard ring width.

Microvolts

Thermocouple effects must be considered if the LTC1151’s

ultra low drift is to be fully utilized. Any connection of

dissimilar metals forms a thermoelectric junction produc-

ing 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

EMF generation. Junctions of copper wire from different

manufacturers can generate thermal EMFs of 200nV/

°C;

four times the maximum drift specification of the LTC1151.

Minimizing thermal EMF-induced errors is possible if

judicious 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

differential cancellation occurs. Doing this may involve

deliberately introducing junctions to offset unavoidable

junctions.

Figure 1 is an example of the introduction of an unneces-

sary resistor to promote differential thermal balance.

Maintaining compensating junctions in close physical

proximity will keep them at the same temperature and

reduce thermal EMF errors.

When connectors, switches, relays and/or sockets are

necessary they should be selected for low thermal EMF

activity. The same techniques of thermally balancing and

coupling the matching junctions are effective in reducing

the thermal EMF errors of these components.