LTC1049

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ACHIEVING PICOAMPERE/MICROVOLT

PERFORMANCE

Picoamperes

In order to realize the picoampere level of accuracy of the

LTC1049, proper care must be exercised. Leakage

currents in circuitry external to the amplifier can signifi-

cantly 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 perfor-

mance. 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 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.

Microvolts

Thermocouple effects must be considered if the LTC1049’s

ultralow 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—

twice the maximum drift specification of the LTC1049. The

copper/kovar junction, formed when wire or printed circuit

traces contact a package lead, has a thermal EMF of

approximately 35

µV/°C—300 times the maximum drift

specification of the LTC1049.

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.

PACKAGE-INDUCED OFFSET VOLTAGE

Package-induced thermal EMF effects are another impor-

tant source of errors. It arises at the copper/kovar

junctions formed when wire or printed circuit traces

contact a package lead. Like all the previously mentioned

thermal EMF effects, it is outside the LTC1049’s offset

nulling loop and cannot be cancelled. The input offset

voltage specification of the LTC1049 is actually set by the

package-induced warm-up drift rather than by the circuit

itself. The thermal time constant ranges from 0.5 to 3

minutes, depending on package type.

LOW SUPPLY OPERATION

The minimum supply for proper operation of the LTC1049

is typically below 4.0V (

±2.0V). In single supply applica-

tions, PSRR is guaranteed down to 4.7V (

±2.35V) to

ensure proper operation down to the minimum TTL

specified voltage of 4.75V.

PIN COMPATIBILITY

The LTC1049 is pin compatible with the 8-pin versions of

7650, 7652 and other chopper-stabilized amplifiers. The

7650 and 7652 require the use of two external capacitors

connected to Pins 1 and 8 which are not needed for the

LTC1049. Pins 1, 5, and 8 of the LTC1049 are not con-

nected internally; thus, the LTC1049 can be a direct plug-

in for the 7650 and 7652, even if the two capacitors are left

on the circuit board.

APPLICATIO S I FOR ATIO