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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