NTC
Thermistor Theory
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Technical
Note from Analog Devices
Thermistors
are electrical circuit elements
formed with semiconducting
mterials that are characterized
by a high negative temperature
coefficient (NTC) or positive
temperature coefficient (PTC).
An NTC thermistor acts like
resistor with temperature
coefficients of typically-3
to –5%/C. Thermistors
offer the benefits of high
stability, precision,size
and compatibility at a competitive
price in many applications.
They also offer fast response
times and are among the highest
sensitivity temperature trnsducers
available. Thermistors can
be excited by using either
voltage or current methods.
Thermistors are used as thermal
sensors or thermal probes
in communications, instrumentation,
automotive, medical, aerospace
and consumer market segments.
In medical
applications for example thermistors
are used in skin sensors,renal
dialysis, blood and urine
analysers, incubators and
respirators and also clinical
and domestic themometers.
In communication app;ications
they are used for temperature
monitoring and compensation
in mobile phones, base stations
nd laser drives. They are
also used extensively in the
protection of mobile battery
packs against overcharging.
In data acquisition
applications, high
resolution analog to digital
converters are required to
digitize the signal produced
from the measurement circuit
incorporating the thermistor
. The AD7711.
A signal conditioning analog – to – digital
converter from Analog
Devices is an ideal choice
in temperature measurement
applications using thermistors
and RTDs. The AD7711 as
shown in Figure 1
is a complete analog front
end for low ferquency measurement
applications. The device accepts
low level signals directly
from a transducer and outputs
a serial digital word. It
employs a sigma-delta conversion
tefhnique to realize up to
24 bits of no missing codes
performance. The input signal
is applied to a proprietary
programmable gain
front end based around an
analog modulator. Te AD7711 on-chip
programmable gain
amplifer (PGA) with gains
from 1 to 128 is used in applications
to amplify the signal
from the front end transducer
in order to use the full dynamic
range of the ADC.
With a 2.5V reference and
a gain range of 1 to 128 the
AD7711 can
accept unipolar signal between
0 20mV and 0 to 2.5V and bipolar
signal rnges from +/- 20mV
to +/-2.5V. The modulator
output is processed by an
on-chip digital filter. The
first notch of this digital
filter can be programed via
the on-chip control register
allowing adjustment of the
filter cutoff and setting
time. The part features one
differential analog input
and one single ended analog
input as well as a different
reference input. Normally,
one of the input channels
will be used as the main channel
with the second channel used
as an auxiliary input to periodically
measure a second voltage.
It can be operated from a
single supply (by tying the
VSS pin to AGND) provided
that the input signals on
the analog inputs are more
positive than –30 mV.
By taking the VSS pin negative,
the part can convert signals
down to –VREF on its
inputs. The part
provides two current sources
that can be used to provide
excitation in three-wire and
four wire RTD configurations.
A full data sheet on the AD7711
(Figure 1) can be found
on the Analog Devices web
site at http://www.analog.com.
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