Datasheet LTC1051, LTC1053 (Analog Devices) - 6

LTC1051/LTC1053
U U W U APPLICATIO S I FOR ATIO ACHIEVING PICOAMPERE/MICROVOLT 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 LTC1051/LTC1053, proper care must be exercised. Leak- deliberately introducing junctions to offset unavoidable age currents in circuitry external to the amplifier can junctions. significantly degrade performance. High quality insulation When connectors, switches, relays and/or sockets are should be used (e.g., Teflon, Kel-F); cleaning of all insulat- necessary, they should be selected for low thermal EMF ing surfaces to remove fluxes and other residues will activity. The same techniques of thermally balancing and probably be necessary —particularly for high temperature coupling the matching junctions are effective in reducing performance. Surface coating may be necessary to provide the thermal EMF errors of these components. a moisture barrier in high humidity environments. Resistors are another source of thermal EMF errors. Board leakage can be minimized by encircling the input Table 1 shows the thermal EMF generated for different connections with a guard ring operated at a potential close resistors. The temperature gradient across the resistor is to that of the inputs: in inverting configurations, the guard important, not the ambient temperature. There are two ring should be tied to ground; in noninverting connections, junctions formed at each end of the resistor and if these to the inverting input. Guarding both sides of the printed junctions are at the same temperature, their thermal EMFs circuit board is required. Bulk leakage reduction depends will cancel each other. The thermal EMF numbers are on the guard ring width. approximate and vary with resistor value. High values give higher thermal EMF.
Microvolts Table 1. Resistor Thermal EMF
Thermocouple effects must be considered if the LTC1051/ LTC1053’s ultra low drift op amps are to be fully utilized.
RESISTOR TYPE THERMAL EMF/
°
C GRADIENT
Any connection of dissimilar metals forms a thermoelec- Tin Oxide ~mV/°C tric junction producing an electric potential which varies Carbon Composition ~450µV/°C with temperature (Seebeck effect.) As temperature sen- Metal Film ~20µV/°C sors, thermocouples exploit this phenomenon to produce Wire Wound Evenohm ~2 useful information. In low drift amplifier circuits, this effect µV/°C Manganin ~2µV/°C is a primary source of error. Connectors, switches, relay contacts, sockets, resistors,
Input Bias Current, Clock Feedthrough
solder, and even copper wire are all candidates for thermal EMF generation. Junctions of copper wire from different At ambient temperatures below 60°C, the input bias cur- manufacturers can generate thermal EMFs of 200nV/°C— rent of the LTC1051/LTC1053 op amps’ is dominated by 4 times the maximum drift specification of the LTC1051/ the small amount of charge injection occurring during the LTC1053. The copper/kovar junction, formed when wire or sampling and holding of the op amps’ input offset voltage. printed circuit traces contact a package lead, has a thermal The average value of the resulting current pulses is 10pA EMF of approximately 35µV/°C—700 times the maximum to 15pA with sign convention shown in Figure 1. drift specification of the LTC1051/LTC1053. I + + B I T B A < 60°C TA > 85°C + + Minimizing thermal EMF-induced errors is possible if 1/2 1/2 I – – B LTC1051 IB LTC1051 judicious attention is given to circuit board layout and – – component selection. It is good practice to minimize the
(a) (b)
number of junctions in the amplifier’s input signal path. 1051/53 F01
Figure 1. LTC1051 Bias Current
10513fa 6