Datasheet

VREFP1
VIN4
VIN3
VA
+
1 PF
0.1 PF
+
1 PF
0.1 PF
VIO
LMP90080
XOUT
XIN/CLK
SCLK
CSB
SDO
SDI
D6 = DRDYB
Microcontroller
5V 2.7V
LM94022
IC Temp
Sensor
Thot
Tcold
+
TC [ VIN4 ± VIN3]
-
Tcold
VIN5
VREFP1
Thermocouple
2k
2k
2.2 PF
10 nF
10 nF
LM4140-4.1
+
0.1 PF
0.1 PF
+
1 PF
0.1 PF
5V
5V
VIN7
+
LM [ VIN5]
-
GND
1 PF
LMP90077, LMP90078, LMP90079, LMP90080
www.ti.com SNAS521F –JULY 2011–REVISED MARCH 2013
Thermocouple and IC Analog Temperature
Figure 67. Thermocouple with CJC
The LMP900xx is also ideal for thermocouple temperature applications. Thermocouples have several advantages
that make them popular in many industrial and medical applications. Compare to RTDs, thermistors, and IC
sensors, thermocouples are the most rugged, least expensive, and can operate over the largest temperature
range.
A thermocouple is a sensor whose junction generates a differential voltage, VIN, that is relative to the
temperature difference (T
hot
– T
cold
). T
hot
is also known as the measuring junction or “hot” junction, which is
placed at the measured environment. T
cold
is also known as the reference or “cold” junction, which is placed at
the measuring system environment.
Because a thermocouple can only measure a temperature difference, it does not have the ability to measure
absolute temperature. To determine the absolute temperature of the measured environment (T
hot
), a technique
known as cold junction compensation (CJC) must be used.
In a CJC technique, the “cold” junction temperature, T
cold
, is sensed by using an IC temperature sensor, such as
the LM94022. The temperature sensor should be placed within close proximity of the reference junction and
should have an isothermal connection to the board to minimize any potential temperature gradients.
Once T
cold
is obtained, use a standard thermocouple look-up-table to find its equivalent voltage. Next, measure
the differential thermocouple voltage and add the equivalent cold junction voltage. Lastly, convert the resulting
voltage to temperature using a standard thermocouple look-up-table.
For example, assume T
cold
= 20°C. The equivalent voltage from a type K thermocouple look-up-table is 0.798
mV. Next, add the measured differential thermocouple voltage to the T
cold
equivalent voltage. For example, if the
thermocouple voltage is 4.096 mV, the total would be 0.798 mV + 4.096 mV = 4.894 mV. Referring to the type K
thermocouple table gives a temperature of 119.37°C for 4.894 mV.
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