Datasheet

TEMPERATURE (°C)

THERMOCOUPLE VOLTAGE

(mV)

-10

-200 0 200 400 600 800 1000 1200

LMV851, LMV852, LMV854

SNOSAW1A –OCTOBER 2007–REVISED MARCH 2013

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Figure 56. K-Type Thermocouple Response

Thermocouple Example

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

supply for both the LMV851/LMV852, or LMV854 and the ADC is 3.3V.

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

thermocouple. This is indicated in Figure 56 by the dotted lines.

To obtain the highest resolution, the full ADC range of 0 to 3.3V is used. The gain needed for the full range can

be calculated as follows:

= 3.3V / 0.0206V = 160 (3)

If R

is 2 kΩ, then the value for R

can be calculated for a gain of 160. Since A

= R

/ R

, RF can be calculated

as follows:

= A

x R

= 160 x 2 kΩ = 320 kΩ (4)

To get a resolution of 0.5°C, the LSB of the ADC should be smaller then 0.5°C / 500°C = 1/1000. A 10-bit ADC

would be sufficient as this gives 1024 steps. A 10-bit ADC such as the two channel 10-bit ADC102S021 can be

used.

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 enables correction for this unwanted 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.

The two channel ADC in this example also converts the signal from the LM35 to a digital signal. Now the

microprocessor can compensate the amplified thermocouple signal, for the unwanted thermocouple effect.

Product Folder Links: LMV851 LMV852 LMV854