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MAX31855 Datasheet PDF : 13 Pages
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MAX31855
Cold-Junction Compensated
Thermocouple-to-Digital Converter
Detailed Description
The MAX31855 is a sophisticated thermocouple-to-
digital converter with a built-in 14-bit analog-to-digital
converter (ADC). The device also contains cold-junction
compensation sensing and correction, a digital control-
ler, an SPI-compatible interface, and associated control
logic. The device is designed to work in conjunction with
an external microcontroller (FC) in thermostatic, process-
control, or monitoring applications. The device is avail-
able in several versions, each optimized and trimmed for
a specific thermocouple type (K, J, N, T, or E; contact the
factory for S and R types). The thermocouple type is indi-
cated in the suffix of the part number (e.g., MAX31855K).
See the Ordering Information table for all options.
Temperature Conversion
The device includes signal-conditioning hardware to
convert the thermocouple’s signal into a voltage com-
patible with the input channels of the ADC. The T+ and
T- inputs connect to internal circuitry that reduces the
introduction of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into equiv-
alent temperature values, it is necessary to compensate
for the difference between the thermocouple cold-
junction side (device ambient temperature) and a 0NC
virtual reference. For a K type thermocouple, the volt-
age changes by about 41FV/NC, which approximates
the thermocouple characteristic with the following linear
equation:
VOUT = (41.276FV/NC) x (TR - TAMB)
where VOUT is the thermocouple output voltage (FV), TR
is the temperature of the remote thermocouple junction
(NC), and TAMB is the temperature of the device (NC).
Other thermocouple types use a similar straight-line
approximation but with different gain terms. Note that the
MAX31855 assumes a linear relationship between tem-
perature and voltage. Because all thermocouples exhibit
some level of nonlinearity, apply appropriate correction
to the device’s output data.
Cold-Junction Compensation
The function of the thermocouple is to sense a difference
in temperature between two ends of the thermocouple
wires. The thermocouple’s “hot” junction can be read
across the operating temperature range (Table 1). The
reference junction, or “cold” end (which should be at
Table 1. Thermocouple Wire Connections and Nominal Sensitivities
TYPE
T- WIRE
K
Alumel
J
Constantan
N
Nisil
S*
Platinum/
Rhodium
T
Constantan
E
Constantan
R*
Platinum/
Rhodium
*Contact factory.
T+ WIRE
Chromel
Iron
Nicrosil
Platinum
Copper
Chromel
Platinum
TEMP RANGE (°C)
-200 to +1350
-40 to +750
-200 to + 1300
+50 to +1600
-250 to +400
-40 to +900
-50 to +1770
SENSITIVITY (µV/°C)
41.276
(0NC to +1000NC)
57.953
(0NC to +750NC)
36.256
(0NC to +1000NC)
9.587
(0NC to +1000NC)
52.18
(0NC to +400NC)
76.373
(0NC to +1000NC)
10.506
(0NC to +1000NC)
COLD-JUNCTION
SENSITIVITY (µV/°C)
(0NC TO +70NC)
40.73
52.136
27.171
6.181
41.56
44.123
6.158
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