LTC1150MJ8(RevB) Просмотр технического описания (PDF) - Linear Technology

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LTC1150MJ8
(Rev.:RevB)

±15V Zero-Drift Operational Amplifier with Internal Capacitors

Linear Technology 

LTC1150

APPLICATIO S I FOR ATIO

ACHIEVING PICOAMPERE/MICROVOLT

PERFORMANCE

Picoamperes

In order to realize the picoampere level of accuracy of the

LTC1150, proper care must be exercised. Leakage cur-

rents 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 sur-

faces 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

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 LTC1150’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—four times the maximum drift specification

of the LTC1150. 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 LTC1150.

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.

NOMINALLY UNNECESSARY

RESISTOR USED TO

THERMALLY BALANCE

OTHER INPUT RESISTOR

RESISTOR LEAD, SOLDER,

COPPER TRACE JUNCTION

LEAD WIRE/SOLDER

COPPER TRACE JUNCTION

LTC1150

OUTPUT

LTC1150 •AI01

Figure 1. Extra Resistors Cancel Thermal EMF

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.

Resistors are another source of thermal EMF errors.

Table 1 shows the thermal EMF generated for different

resistors. The temperature gradient across the resistor is

important, not the ambient temperature. There are two

junctions formed at each end of the resistor and if these

junctions are at the same temperature, their thermal EMFs

will cancel each other. The thermal EMF numbers are

approximate and vary with resistor value. High values give

higher thermal EMF.

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