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LTC1150CJ8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1150CJ8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC1150 9 1150fb 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. LTC1150 •AI01 OUTPUT NOMINALLY UNNECESSARY RESISTOR USED TO THERMALLY BALANCE OTHER INPUT RESISTOR RESISTOR LEAD, SOLDER, COPPER TRACE JUNCTION LEAD WIRE/SOLDER COPPER TRACE JUNCTION LTC1150 + – 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. APPLICATIO S I FOR ATIO |
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