LMV852EVAL National Semiconductor, LMV852EVAL Datasheet - Page 17

• Manufacturer Part Number: LMV852EVAL
• Description: BOARD EVALUATION LMV852 8MHZ
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMV852EVAL

• Channels Per Ic: 2 - Dual
• Amplifier Type: General Purpose
• Output Type: Single-Ended, Rail-to-Rail
• Slew Rate: 4.5 V/µs
• Current - Output / Channel: 65mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 820µA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 5.5 V, ±1.35 V ~ 2.75 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMV852
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• -3db Bandwidth: -

THERMOCOUPLE AMPLIFIER
The following circuit is a typical example for a thermocouple
amplifier application using an LMV851/LMV852, or LMV854.
A thermocouple converts a temperature into a voltage. This
signal is then amplified by the LMV851/LMV852, or LMV854.
An ADC can convert the amplified signal to a digital signal.
For further processing the digital signal can be processed by
a microprocessor and used to display or log the temperature.
The temperature data can for instance be used in a fabrication
process.
Characteristics of a Thermocouple
A thermocouple is a junction of two different metals. These
metals produce a small voltage that increases with tempera-
ture.
The thermocouple used in this application is a K-type ther-
mocouple. A K-type thermocouple is a junction between Nick-
el-Chromium and Nickel-Aluminum. This is one of the most
commonly used thermocouples. There are several reasons
for using the K-type thermocouple, these include: tempera-
ture range, the linearity, the sensitivity, and the cost.
A K-type thermocouple has a wide temperature range. The
range of this thermocouple is from approximately −200°C to
approximately 1200°C, as can be seen in Figure 8. This cov-
ers the generally used temperature ranges.
Over the main part of the temperature range the output volt-
age depends linearly on the temperature. This is important for
easily converting the measured signal levels to a temperature
reading.
The K-type thermocouple has good sensitivity when com-
pared to many other types; the sensitivity is about 41 uV/°C.
Lower sensitivity requires more gain and makes the applica-
tion 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 7. Pressure Sensor Application
17
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 8 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 cal-
culated as follows:
If R
of 160. Since A
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.
G
is 2 kΩ, then the value for R
FIGURE 8. K-Type Thermocouple Response
R
F
= A
V
A
= R
V
V
= 3.3V / 0.0206V = 160
x R
F
/ R
G
G
= 160 x 2 kΩ = 320 kΩ
, RF can be calculated as follows:
F
can be calculated for a gain
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