Datasheet

LTC2484
38
2484fd
APPLICATIONS INFORMATION
Complete Thermocouple Measurement System with
Cold Junction Compensation
The LTC2484 is ideal for direct digitization of thermocouples
and other low voltage output sensors. The input has a
typical offset error of 500nV (2.5μV max) offset drift of
10nV/°C and a noise level of 600nV
RMS
.
Figure 44 (last page of this data sheet) is a complete type
K thermocouple meter. The only signal conditioning is a
simple surge protection network. In any thermocouple
meter, the cold junction temperature sensor must be
at the same temperature as the junction between the
thermocouple materials and the copper printed circuit
board traces. The tiny LTC2484 can be tucked neatly
underneath an Omega MPJ-K-F thermocouple socket
ensuring close thermal coupling.
The LTC2484’s 1.4mV/°C PTAT circuit measures the
cold junction temperature. Once the thermocouple volt-
age and cold junction temperature are known, there
are many ways of calculating the thermocouple temp-
erature including a straight-line approximation, lookup
tables or a polynomial curve fi t. Calibration is per-
formed by applying an accurate 500mV to the ADC input
derived from an LT
®
1236 reference and measuring the
local temperature with an accurate thermometer as
shown in Figure 43. In calibration mode, the up and
down buttons are used to adjust the local tempera-
ture reading until it matches an accurate thermometer.
Both the voltage and temperature calibration are easily
automated.
The complete microcontroller code for this application is
available on the LTC2484 product Web page at:
http://www.linear.com
It can be used as a template for may different instruments
and it illustrates how to generate calibration coeffi cients
for the onboard temperature sensor. Extensive comments
detail the operation of the program. The read_LTC2484()
function controls the operation of the LTC2484 and is
listed below for reference.
CS
SCK
SDO
SDI
f
O
6
9
7
1
10
V
CC
5V
LTC2484
REF
GND
IN
–
IN
+
3
ISOTHERMAL
2
C7
0.1μF
C8
1μF
4
R2
2k
R7
8k
62
5
4
R8
1k
5
2484 F43
26.3C
TYPE K
THERMOCOUPLE
JACK
(OMEGA MPJ-K-F)
118
GND
IN OUT
G1
NC1M4V0
TRIM
GND
LT1236
+
Figure 43. Calibration Setup