Datasheet LTC1151 (Analog Devices) - 7

LTC1151
O U U W U APPLICATI S I FOR ATIO ACHIEVING PICOAMPERE/MICROVOLT PERFORMANCE
Connectors, switches, relay contacts, sockets, resistors, solder, and even copper wire are all candidates for thermal
Picoamperes
EMF generation. Junctions of copper wire from different In order to realize the picoampere level of accuracy of the manufacturers can generate thermal EMFs of 200nV/°C; LTC1151 proper care must be exercised. Leakage currents four times the maximum drift specification of the LTC1151. in circuitry external to the amplifier can significantly de- Minimizing thermal EMF-induced errors is possible if grade performance. High quality insulation should be used judicious attention is given to circuit board layout and (e.g., Teflon); cleaning of all insulating surfaces to remove component selection. It is good practice to minimize the fluxes and other residues will probably be necessary, number of junctions in the amplifier’s input signal path. particularly for high temperature performance. Surface Avoid connectors, sockets, switches and relays where coating may be necessary to provide a moisture barrier in possible. In instances where this is not possible, attempt high humidity environments. to balance the number and type of junctions so that Board leakage can be minimized by encircling the input differential cancellation occurs. Doing this may involve connections with a guard ring operated at a potential close deliberately introducing junctions to offset unavoidable to that of the inputs: in inverting configurations the guard junctions. ring should be tied to ground; in noninverting connections Figure 1 is an example of the introduction of an unneces- to the inverting input. Guarding both sides of the printed sary resistor to promote differential thermal balance. circuit board is required. Bulk leakage reduction depends Maintaining compensating junctions in close physical on the guard ring width. proximity will keep them at the same temperature and reduce thermal EMF errors.
Microvolts
When connectors, switches, relays and/or sockets are Thermocouple effects must be considered if the LTC1151’s necessary they should be selected for low thermal EMF ultra low drift is to be fully utilized. Any connection of activity. The same techniques of thermally balancing and dissimilar metals forms a thermoelectric junction produc- coupling the matching junctions are effective in reducing ing an electric potential which varies with temperature the thermal EMF errors of these components. (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. 1151fa 7