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EX10xxA Theory of Operation 281
FIGURE 7-2: TYPICAL CMRR AT 60 HZ AND 1 kHz
COLD JUNCTION COMPENSATION
Cold junction compensation is accomplished via twelve precision thermistors that monitor
isothermal interface on the four input connectors. A thermistor is an element whose resistance
represents its applied temperature. Each thermistor resistance is converted to a representative
voltage that is measured at the beginning of each scan. Each voltage is then transformed into a
temperature value through the application of the thermistor’s Steinhart-Hart coefficients. This
temperature value is then used to generate a thermocouple type-specific compensating voltage that
is mathematically added to the measured input voltage of the appropriate channels.
CALIBRATION
Calibration in the EX10xxA takes two forms: full calibration and self-calibration. In both cases, the
input signal conditioning paths are disconnected from the input jacks and connected instead to a
calibration bus that is driven by an internal calibration source. Through measurement of the
conditioning paths at multiple calibration source points, the voltage gain and offset of each path is
calculated. Full calibration involves the additional steps of measuring the calibration source with a
precision voltmeter. Thus, self-calibration is a subset of full calibration. Because of the internal input
disconnection mechanism, the user does not have to remove the actual input connections to perform
calibration.
THERMOCOUPLE CALCULATIONS
There are two thermocouple type-specific calculations that are performed in the EX10xxA. The first
calculation transforms the CJC temperature into a compensating voltage that is mathematically
added to the measured input voltage. The second calculation transforms this total voltage into its
final thermocouple temperature. For maximum accuracy, both of these calculations are performed
using the full-order polynomial equations and coefficients from the NIST ITS-90 Thermocouple
Database, not from lookup tables or piecewise linear approximations.
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