Datasheet

LTC1049
6
1049fb
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 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 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
WUUU