LMP90100MHESLASHNOPB datasheet(52/69 Pages) TI1

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VREFN1

VREFP1

VIN4

VIN3

1 PF

0.1 PF

1 PF

0.1 PF

VIO

LMP90100

XOUT

XIN/CLK

SCLK

CSB

SDO

SDI

D6 = DRDYB

Microcontroller

2.7V

LM94022

IC Temp

Sensor

Thot

Tcold

TC [ VIN4 ± VIN3]

Tcold

VIN5

VREFP1

Thermocouple

2.2 PF

10 nF

LM4140-4.1

0.1 PF

1 PF

0.1 PF

VIN7

LM [ VIN5]

GND

1 PF

LMP90100

SNAS510P – JANUARY 2011 – REVISED MARCH 2013

www.ti.com

Thermocouple and IC Analog Temperature

Figure 72. Thermocouple with CJC

The LMP90xxx 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 (Thot – Tcold). Thot is also known as the measuring junction or “hot” junction, which is

placed at the measured environment. Tcold 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 (Thot), a technique

known as cold junction compensation (CJC) must be used.

In a CJC technique, the “cold” junction temperature, Tcold, 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 Tcold 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 Tcold = 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 Tcold 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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