LM741AH/883 National Semiconductor, LM741AH/883 Datasheet - Page 2

• Manufacturer Part Number: LM741AH/883
• Manufacturer: National Semiconductor

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
(30µA) in the thermistor. It allows the thermistor to operate at
a power level not exceeding its maximum power rating while
maintaining specified accuracy.
Lowering the value of R will increase the temperature range
over which the thermistor’s transfer function is linear. With a
4.7k bias resistor, the slope at higher temperatures in-
creases, providing more resolution, at the cost of greater
power consumption (600µA)
Figure 4 compares the overall system accuracy when using
a thermistor with an 8-bit ADC and R=100k, a thermistor with
a 10-bit ADC and R=33k, or an LM20 with an 8-bit ADC. The
quantization error and ADC Total Unadjusted Error (a com-
bination of offset, gain and linearity errors) was considered
to determine the overall system accuracy. The black lines
show the min/max system performance of the LM20 super-
imposed over the blue min/max performance of the ther-
mistor. Overall system accuracy for the LM20 remains con-
stant over temperature. At temperatures above 60˚C the
LM20 wins big when compared to the performance of the
thermistor and 8-bit ADC!
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systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
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FIGURE 3. Thermistor output voltage vs.
temperature for different values of R
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2. A critical component is any component of a life
Increasing the ADC resolution and decreasing the value of R
in the thermistor circuit decreases the overall system error.
Improving the accuracy of the voltage reference will bring the
LM20 system accuracy closer to that of the specifications
found on the LM20 data sheet of
−55˚C, and
LM20 is negative, the gain error introduced by the reference
voltage plays less of a role in the overall accuracy as the
temperature increases, thus the slight negative slope of the
LM20 performance.
Designers have numerous options for sensing temperature.
Most common are thermistors, RTDs, thermocouples, and
active silicon sensors. IC sensors have major advantages
when the temperatures to be measured fall within the normal
operating temperature range of silicon ICs. Among these
advantages are low system cost, small size, and fast design
time (because external signal conditioning circuitry is either
minimal or not required). ICs can include extensive addi-
tional functions, such as built-in trip-point comparators or
digital I/O. Since they include on-chip linearity correction
when needed, there is no need for lookup tables to correct
linearity errors.
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
FIGURE 4. Comparison of Thermistor to LM20
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±
1.5˚C at +30˚C. Since the output slope of the
System Performance
±
2.5˚C at +130˚C and
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