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LTC1050 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1050 Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LTC1050 1050fb Figure 2 is an example of the introduction of an unneces- sary resistor to promote differential thermal balance. Maintaining compensating junctions in close physical prox- imity will keep them at the same temperature and reduce thermal EMF errors. S APPLICATI IFOR ATIO PACKAGE-INDUCED OFFSET VOLTAGE Package-induced thermal EMF effects are another impor- tant source of errors. It arises at the copper/kovar junctions formed when wire or printed circuit traces contact a package lead. Like all the previously mentioned thermal EMF effects, it is outside the LTC1050’s offset nulling loop and cannot be cancelled. The input offset voltage specifi- cation of the LTC1050 is actually set by the package-induced warm-up drift rather than by the circuit itself. The thermal time constant ranges from 0.5 to 3 minutes, depending upon package type. OPTIONAL EXTERNAL CLOCK An external clock is not required for the LTC1050 to operate. The internal clock circuit of the LTC1050 sets the nominal sampling frequency at around 2.5kHz. This fre- quency is chosen such that it is high enough to remove the amplifier 1/f noise, yet still low enough to allow internal circuits to settle.The oscillator of the internal clock circuit has a frequency 4 times the sampling frequency and its output is brought out to Pin 5 through a 2k resistor. When the LTC1050 operates without using an external clock, Pin 5 should be left floating and capacitive loading on this pin should be avoided. If the oscillator signal on Pin 5 is used to drive other external circuits, a buffer with low input capacitance is required to minimize loading on this pin. Figure 3 illustrates the internal sampling frequency versus capacitive loading at Pin 5. – + LTC1050 OUTPUT 1050 F02 NOMINALLY UNNECESSARY RESISTOR USED TO THERMALLY BALANCE OTHER INPUT RESISTOR RESISTOR LEAD, SOLDER COPPER TRACE JUNCTION LEAD WIRE/SOLDER/COPPER TRACE JUNCTION Figure 2 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. Table 1. Resistor Thermal EMF RESISTOR TYPE THERMAL EMF/°C GRADIENT Tin Oxide ~mV/°C Carbon Composition ~450µV/°C Metal Film ~20µV/°C Wire Wound Evenohm ~2µV/°C Manganin ~2µV/°C CAPACITANCE LOADING (pF) 1 1 2 10 5 100 1050 F03 3 VS = ± 5V Figure 3. Sampling Frequency vs Capacitance Loading at Pin 5 |
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