LTC1049 Просмотр технического описания (PDF) - Linear Technology
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LTC1049 Datasheet PDF : 12 Pages
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LTC1049
Applications Information
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 deg rade 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 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 LTC1049’s
ultralow drift is to be fully utilized. Any connection of dis-
similar metals forms a thermoelectric junction produci ng 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 35µV/°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
important 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 specifica-
tion 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 connected
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.
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