VTI Instruments EX1401, User Manual

Page: 101 / 114

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EX1401 Theory of Operation 101
done to find the thermistor resistance. Calculation of the junction temperature is then done digitally
in the FPGA based on the coefficients of the thermistor used. The BIST ADC cycles through all
measurements continuously which reduces the CJC measurement rate to 10 SPS to 20 SPS
(depending upon customer selected sample rate.)
The BIST ADC also measures the temperature of each channel’s active electronics. This is done
separately from the CJC measurement; it is not using the thermistor. The temperature of the active
electronics is on the other side of the thermal barrier from the CJC thermistor. The temperature is
reported for every channel, as part of the BIST report.
The BIST ADC also measures various power supplies on each analog channel and these are also
part of the BIST report.
I
SOLATION
B
ARRIER
Each channel has its own, isolated power supply. The isolated power supply takes the bulk 5V output
power from the PoE+ power supply and develops the isolated power supplies (+13V, -13V, 3.3V)
needed by the PGA, ADC and BIST circuits. The isolator for both power and digital
communications gives galvanic isolation between the two sides. Maximum continuous working
voltage across the isolation barrier is 500Vpeak. The isolation barrier is designed to conform with
IEC61010-1 (3
rd
edition) for: pollution degree 2, altitude < 5000m, overvoltage category II.
T
HERMOCOUPLE
C
ALCULATIONS
There are two thermocouple type-specific calculations that are performed in the EX1401. 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.