Datasheet LTC1150 (Analog Devices) - 9

LTC1150
U U W U APPLICATIO S I FOR ATIO ACHIEVING PICOAMPERE/MICROVOLT
number of junctions in the amplifier’s input signal path.
PERFORMANCE
Avoid connectors, sockets, switches, and relays where possible. In instances where this is not possible, attempt
Picoamperes
to balance the number and type of junctions so that In order to realize the picoampere level of accuracy of the differential cancellation occurs. Doing this may involve LTC1150, proper care must be exercised. Leakage cur- deliberately introducing junctions to offset unavoidable rents in circuitry external to the amplifier can significantly junctions. degrade performance. High quality insulation should be Figure 1 is an example of the introduction of an unneces- used (e.g., Teflon, Kel-F); cleaning of all insulating sur- sary resistor to promote differential thermal balance. faces to remove fluxes and other residues will probably Maintaining compensating junctions in close physical be necessary–particularly for high temperature perfor- proximity will keep them at the same temperature and mance. Surface coating may be necessary to provide a reduce thermal EMF errors. moisture barrier in high humidity environments. Board leakage can be minimized by encircling the input NOMINALLY UNNECESSARY RESISTOR USED TO LEAD WIRE/SOLDER connections with a guard ring operated at a potential THERMALLY BALANCE COPPER TRACE JUNCTION close to that of the inputs: in inverting configurations the OTHER INPUT RESISTOR + guard ring should be tied to ground; in noninverting LTC1150 OUTPUT connections to the inverting input. Guarding both sides – of the printed circuit board is required. Bulk leakage RESISTOR LEAD, SOLDER, COPPER TRACE JUNCTION 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- LTC1150 •AI01 ing an electric potential which varies with temperature (Seebeck effect). As temperature sensors, thermocouples
Figure 1. Extra Resistors Cancel Thermal EMF
exploit this phenomenon to produce useful information. In low drift amplifier circuits the effect is a primary source When connectors, switches, relays and/or sockets are of error. necessary, they should be selected for low thermal EMF activity. The same techniques of thermally-balancing and Connectors, switches, relay contacts, sockets, resistors, coupling the matching junctions are effective in reducing solder, and even copper wire are all candidates for the thermal EMF errors of these components. thermal EMF generation. Junctions of copper wire from different manufacturers can generate thermal EMFs of Resistors are another source of thermal EMF errors. 200nV/°C—four times the maximum drift specification Table 1 shows the thermal EMF generated for different of the LTC1150. The copper/kovar junction, formed when resistors. The temperature gradient across the resistor is wire or printed circuit traces contact a package lead, has important, not the ambient temperature. There are two a thermal EMF of approximately 35µV/°C—700 times the junctions formed at each end of the resistor and if these maximum drift specification of the LTC1150. junctions are at the same temperature, their thermal EMFs will cancel each other. The thermal EMF numbers are Minimizing thermal EMF-induced errors is possible if approximate and vary with resistor value. High values give judicious attention is given to circuit board layout and higher thermal EMF. component selection. It is good practice to minimize the 1150fb 9