LM57B10EB/NOPB National Semiconductor, LM57B10EB/NOPB Datasheet - Page 4

• Manufacturer Part Number: LM57B10EB/NOPB
• Description: Temperature Switch/Sensor Eval. Board
• Manufacturer: National Semiconductor

Specifications of LM57B10EB/NOPB

• Silicon Manufacturer: National
• Application Sub Type: Temperature Sensor
• Kit Application Type: Sensing - Temperature
• Silicon Core Number: LM57
• Kit Contents: Board And Literature

www.national.com
Quantization Noise
Obtaining maximum precision in a sensor measurement re-
quires attention to quantization noise error, which is the error
introduced by the conversion from the analog signal to binary
data. As the analog signal is digitized, it is assigned a digital
value that is the closest to the actual analog value being mea-
sured. The smallest increment of digital measurement, called
a least-significant bit (LSB), is voltage which is equal to that
of the ADC reference divided by the number of codes that the
ADC can count. For example, a 2.56V reference used on an
8-bit ADC would produce an LSB size of 2.56V ÷ 2
Any difference between the analog value being measured
and the digitized value will be an error in the conversion; this
is called quantization noise or quantization error. For exam-
ple, if you are trying to sample a 1.384V signal and it gets
digitized to the nearest 10 mV value, say 1.380V, then a
quantization noise value of 4mV results on the sampled value.
For a more thorough discussion of quantization noise, see
“The ABCs of ADCs” on the national.com website (http://
www.national.com/appinfo/adc/files/ABCs_of_ADCs.pdf).
So what does this noise mean in terms of temperature error?
The answer depends on the gain of the sensor output. A high-
er gain magnitude from the sensor is less affected by noise
than a lower gain — the higher the sensor gain, the less error
FIGURE 4. Comparison of Typical Supply Current Between the
LM57 and Two NTC Thermistor-Based Discrete Circuits
8
= 10 mV.
4
from quantization noise. Figure 5 compares the temperature
measurement error that results from quantization noise be-
tween the LM57, a 100 kΩ NTC thermistor circuit, and a 10
kΩ NTC thermistor circuit. The analog output of the LM57 is
very linear with a typical gain of −12.92 mV/°C when the trip
temperature is set to 125°C. (Actually the LM57 has four pos-
sible gains, depending on the trip point value selected; 125°
C is used in this example.) This means that for every millivolt
of noise, the effect on the temperature is 1/12.92 = 0.077 de-
grees of temperature error.
The effect of noise is significantly worse in the 10 kΩ NTC
circuit below −35°C and above 75°C. At the cold operating
temperature limit for the thermistor of −40°C, the temperature
error is 0.102°C for every mV of noise. At the hot operating
temperature limit of 125°C, the error is 0.265°C/mV. The 100
kΩ NTC circuit is even more affected by noise. It has more
error per mV of noise than the LM57 below −5°C and above
100°C. At the cold operating limit, it shows 0.436°C/mV of
error and at the hot limit it is 0.155°C/mV.
When the analog output of the LM57 is set to its highest gain
and is sampled by an ADC, the quantization error is much less
in the cold and hot regions of the operating temperature than
it is when sampling the discrete thermistor circuits.
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