Datasheet

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 compensa-
tion sensing and correction, a digital controller, an SPI-
compatible interface, and associated control logic. The
device is designed to work in conjunction with an external
microcontroller (µC) in thermostatic, process-control, or
monitoring applications. The device is available in several
versions, each optimized and trimmed for a specific thermo-
couple type (K, J, N, T, S, R, or E.). The thermocouple type is
indicated 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 con-
vert the thermocouple’s signal into a voltage compatible
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 equiva-
lent temperature values, it is necessary to compensate
for the difference between the thermocouple coldjunction
side (device ambient temperature) and a 0°C virtual ref-
erence. For a K-type thermocouple, the voltage changes
by about 41µV/°C, which approximates the thermocouple
characteristic with the following linear equation:
V
OUT
= (41.276µV/°C) x (T
R
- T
AMB
)
where V
OUT
is the thermocouple output voltage (µV), T
R
is the temperature of the remote thermocouple junction
(°C), and T
AMB
is the temperature of the device (°C).
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 the
Table 1. Thermocouple Wire Connections and Nominal Sensitivities
TYPE T- WIRE T+ WIRE TEMP RANGE (°C) SENSITIVITY (µV/°C)
COLD-JUNCTION
SENSITIVITY (µV/°C)
(0°C TO +70°C)
K Alumel Chromel -270 to +1372
41.276
(0°C to +1000°C)
40.73
J Constantan Iron -210 to +1200
57.953
(0°C to +750°C)
52.136
N Nisil Nicrosil -270 to + 1300
36.256
(0°C to +1000°C)
27.171
S Platinum Platinum/Rhodium -50 to +1768
9.587
(0°C to +1000°C)
6.181
T Constantan Copper -270 to +400
52.18
(0°C to +400°C)
41.56
E Constantan Chromel -270 to +1000
76.373
(0°C to +1000°C)
44.123
R Platinum Platinum/Rhodium -50 to +1768
10.506
(0°C to +1000°C)
6.158
MAX31855 Cold-Junction Compensated
Thermocouple-to-Digital Converter
www.maximintegrated.com
Maxim Integrated │ 8