LMV841MGX NSC [National Semiconductor], LMV841MGX Datasheet - Page 15

• Manufacturer Part Number: LMV841MGX
• Description: CMOS Input, RRIO, Wide Supply Range Operational Amplifiers
• Manufacturer: NSC [National Semiconductor]

Thermocouple Example
Suppose the range we are interested in for this example is
from 0°C to 500°C, and the resolution needed is 0.5°C. The
power supply for both the LMV841 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 shown in Figure 7
To obtain the best accuracy the full ADC range of 0 to 3.3V is
used.
We can calculate the gain we need for the full input range of
the ADC : A
FIGURE 7. K-Type Thermocouple Response
V
= 3.3V / 0.0206V = 160.
FIGURE 8. Thermocouple Amplifier
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When we use 2 kΩ for R
with this gain of 160. We can use A
the gain, so we can calculate R
x 2 kΩ = 320 kΩ.
To get a resolution of 0.5°C we need a step smaller then the
minimum resolution, this means we need at least 1000 steps
(500°C / 0.5°C). A 10-bit ADC would be sufficient as this will
give us 1024 steps. This could be a 10 bit ADC like the two
channel 10-bit ADC102S021.
Unwanted Thermocouple Effect
At the point where the thermocouple wires are connected to
the circuit, usually copper wires or traces, an unwanted ther-
mocouple 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 measure-
ments from the intended thermocouple.
We can compensate for this thermocouple effect by using an
isothermal block as a reference. An isothermal block is a good
heat conductor. This means that the two thermocouple con-
nections both have the same temperature. We can now mea-
sure the temperature of the isothermal block, 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 mea-
sure temperatures from −55°C to 150°C.
The ADC in this example also coverts the signal from the
LM35 to a digital signal. Now the microprocessor can com-
pensate the amplified thermocouple signal, for the unwanted
thermocouple effect.
G
, we can calculate the value for R
F
by using R
20168353
V
= R
F
F
/ R
= A
G
V
www.national.com
to calculate
x R
G
= 160
F