Datasheet

TEMPERATURE (°C)

THERMOCOUPLE VOLTAGE

(mV)

-10

-200 0 200 400 600 800 1000 1200

LMV841, LMV842, LMV844

SNOSAT1G –OCTOBER 2006–REVISED FEBRUARY 2013

www.ti.com

The K-type thermocouple has good sensitivity when compared to many other types; the sensitivity is 41 uV/°C.

Lower sensitivity requires more gain and makes the application more sensitive to noise.

In addition, a K-type thermocouple is not expensive, many other thermocouples consist of more expensive

materials or are more difficult to produce.

Figure 44. K-Type Thermocouple Response

Thermocouple Example

For this example suppose the range of interest is from 0°C to 500°C, and the resolution needed is 0.5°C. The

power supply for both the LMV841, LMV842, or LMV844 and the ADC is 3.3V.

The temperature range of 0°C to 500°C results in a voltage range from 0mV to 20.6mV produced by the

thermocouple. This is shown in Figure 44.

To obtain the best accuracy the full ADC range of 0 to 3.3V is used and the gain needed for this full range can

be calculated as follows:

= 3.3V / 0.0206V = 160.

If R

is 2kΩ, then the value for R

can be calculated with this gain of 160. Since A

= R

/ R

, R

can be

calculated as follows:

= A

* R

= 160 x 2kΩ = 320kΩ

To get a resolution of 0.5°C a step smaller then the minimum resolution is needed. This means that at least 1000

steps are necessary (500°C/0.5°C). A 10-bit ADC would be sufficient as this will give 1024 steps. A 10-bit ADC

such as the two channel 10-bit ADC102S021 would be a good choice.

Unwanted Thermocouple Effect

At the point where the thermocouple wires are connected to the circuit, usually copper wires or traces, an

unwanted thermocouple effect will occur.

At this connection, this could be the connector on a PCB, the thermocouple wiring forms a second thermocouple

with the connector. This second thermocouple disturbs the measurements from the intended thermocouple.

Using an isothermal block as a reference will compensate for this additional thermocouple effect . An isothermal

block is a good heat conductor. This means that the two thermocouple connections both have the same

temperature. The temperature of the isothermal block can be measured, and thereby the temperature of the

thermocouple connections. This is usually called the cold junction reference temperature.

In the example, an LM35 is used to measure this temperature. This semiconductor temperature sensor can

accurately measure temperatures from −55°C to 150°C.

Product Folder Links: LMV841 LMV842 LMV844