參數(shù)資料
型號(hào)： LMV844MT
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 運(yùn)動(dòng)控制電子
英文描述： CMOS Input, RRIO, Wide Supply Range Operational Amplifiers
中文描述： QUAD OP-AMP, 800 uV OFFSET-MAX, 4.5 MHz BAND WIDTH, PDSO14
封裝： TSSOP-14
文件頁(yè)數(shù)： 15/18頁(yè)
文件大小： 419K
代理商： LMV844MT
20168370
FIGURE 7. K-Type Thermocouple Response
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
V
= 3.3V / 0.0206V = 160.
When we use 2 k
for R
, we can calculate the value for R
F
with this gain of 160. We can use A
= R
F
/ R
G
to calculate
the gain, so we can calculate R
F
by using R
F
= A
V
x R
G
= 160
x 2 k
= 320 k
.
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.
20168353
FIGURE 8. Thermocouple Amplifier
15
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