Datasheet LTC1049 (Analog Devices) - 6

LTC1049
applicaTions inFormaTion ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE
Minimizing thermal EMF-induced errors is possible if judicious attention is given to circuit board layout and
Picoamperes
component selection. It is good practice to minimize the In order to realize the picoampere level of accuracy of number of junctions in the amplifier’s input signal path. the LTC1049, proper care must be exercised. Leakage Avoid connectors, sockets, switches, and relays where currents in circuitry external to the amplifier can signifi- possible. In instances where this is not possible, attempt to cantly de grade performance. High quality insulation should balance the number and type of junctions so that differential be used (e.g., Teflon™, Kel-F); cleaning of all insulating cancellation occurs. Doing this may involve deliberately surfaces to remove fluxes and other residues will probably introducing junctions to offset unavoidable junctions. be necessary—particularly for high temperature perfor- mance. Surface coating may be necessary to provide a
PACKAGE-INDUCED OFFSET VOLTAGE
moisture barrier in high humidity environments. Package-induced thermal EMF effects are another Board leakage can be minimized by encircling the input impor tant source of errors. It arises at the copper/kovar connections with a guard ring operated at a potential close junctions formed when wire or printed circuit traces contact to that of the inputs: in inverting configurations, the guard a pack age lead. Like all the previously mentioned thermal ring should be tied to ground; in noninverting connections, EMF effects, it is outside the LTC1049’s offset nulling loop to the inverting input. Guarding both sides of the printed and cannot be cancelled. The input offset voltage specifica- circuit board is required. Bulk leakage reduction depends tion of the LTC1049 is actually set by the package-induced on the guard ring width. warm-up drift rather than by the circuit itself. The thermal time constant ranges from 0.5 to 3 minutes, depending
Microvolts
on package type. Thermocouple effects must be considered if the LTC1049’s ultralow drift is to be fully utilized. Any connection of dis-
LOW SUPPLY OPERATION
similar metals forms a thermoelectric junction produc ing an The minimum supply for proper operation of the LTC1049 electric potential which varies with temperature (Seebeck is typically below 4.0V (±2.0V). In single supply applica- effect). As temperature sensors, thermocouples exploit this tions, PSRR is guaranteed down to 4.7V (±2.35V) to ensure phenomenon to produce useful information. In low drift proper operation down to the minimum TTL specified amplifier circuits the effect is a primary source of error. voltage of 4.75V. Connectors, switches, relay contacts, sockets, resistors, solder, and even copper wire are all candidates for thermal
PIN COMPATIBILITY
EMF generation. Junctions of copper wire from different manufacturers can generate thermal EMFs of 200nV/°C — The LTC1049 is pin compatible with the 8-pin versions of twice the maximum drift specification of the LTC1049. 7650, 7652 and other chopper-stabilized amplifiers. The The copper/kovar junction, formed when wire or printed 7650 and 7652 require the use of two external capacitors circuit traces contact a package lead, has a thermal EMF connected to Pins 1 and 8 which are not needed for the of approximately 35µV/°C—300 times the maximum drift LTC1049. Pins 1, 5, and 8 of the LTC1049 are not connected specification of the LTC1049. 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. 1049fb 6 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Package/order information Electrical Characteristics Typical Performance Characteristics Test Circuits Applications Information Typical Applications Package Description Typical Application