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Technical Note
INTRODUCTION
Thermistors are electrical circuit elements formed with
semiconducting materials that are characterized by a high
negative temperature coefficient (NTC) or positive
temperature coefficient (PTC). An NTC thermistor acts
like a 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 transducers
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
thermometers. In communication applications they are
used for temperature monitoring and compensation in
mobile phones, base stations and laser drives. They are also
used extensively in the protection of mobile phone battery
packs against overcharging.
In data acquisition applications, high resolution analog to
digital converters are required to digitize the signal pro-
duced 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 frequency measurement
applications. The device accepts low level signals directly
from a transducer and outputs a serial digital word. It
employs a sigma-delta conversion technique 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. The AD7711 on-chip
programmable gain amplifier (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 signals between 0 to
20mV and 0 to 2.5V and bipolar signal ranges 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 programmed via the on-chip control register allowing
adjustment of the filter cutoff and settling time. The part
features one differential analog input and one single ended
analog input as well as a differential 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
a
Temperature Measurement using a Thermistor
and the AD7711 Sigma Delta ADC
by Albert O'Grady
Figure 1. AD7711 Signal Conditioning Analog-To-Digital
Converter.
can be used to provide excitation in three-wire and four-
wire RTD configurations. A full data sheet on the AD7711
can be found on the Analog Devices web site at :
http://www-corp.analog.com.
C LO C K
G E N E R A T IO N
AU TO-ZERO ED

MO DU LATOR
C HAR GE BALAN CING A /D
CO NVERTER
D IG IT A L
F ILT E R
S E R IA L IN T E R F A C E
M C L K IN
M C L K O U T
M O D E S D A T A
A 0
AD77 11
A = 1- 12 8
P G A
D G N D
A G N D
R E F O U T
V B IA S
A IN 1 (+ )
A IN 1 (- )
R E F IN (+ )
S C LK
D R D Y
A VD D
D V DD
M U X
R T D 1
O U T P U T
R E G IS T E R
C O N T R O L
R E G IS T E R
R F S
T F S
S Y N C
R E F IN (- )
20 0
µ
A
2.5V
R EF E R E N C E
VS S
20 0
µ
A
R T D 2
A IN 2
A VD D - 2 -
THERMISTOR APPLICATIONS USING CURRENT
EXCITATION
Figure 2. shows a circuit using the AD7711 to digitize the
output voltage generated across a thermistor using the
AD7711s on-chip 200uA current source.
to 128 is used in this application to amplify the signal from
the thermistor in order to maximize the signal-to-noise
ratio (SNR) of the system. With a 2V reference and a gain
range of 1 to 128 the AD7711 can accept unipolar signals
between 0 to 15mV and 0 to 2V. For example if the
operating range is from 25°C and to 100°C, the maximum
output voltage generated across the thermistor is 60mV,
this allows a gain of 32 to be used in covering this entire
range.
THERMISTOR APPLICATIONS USING VOLTAGE
EXCITATION
Figure 3 shows a circuit where the thermistor is excited
using a voltage source which is obtained from the AD7711.
The thermistor shown is the 10K3A1 thermistor from
The thermistor chosen in this example is the 0.3K1A1
from Betatherm*. Appendix 1 shows a table of the R-T
characteristics of this device. This device has a nominal
resistance of 300 ohms at 25°C. In this example the same
200uA current source used to excite the thermistor is also
used to generate the reference voltage for the AD7711. As
a result variations in the excitation current do not affect
performance. The most common wiring arrangement in
these applications is a 4-wire force/sense configuration in
order to reduce the effects of lead resistances on system
performance. Lead resistance of the drive wires only shift
the common mode voltage and do not degrade the perfor-
mance of the circuit. Lead resistance of the sense wires is
immaterial as no current flow in these wires due to the high
input impedance af the AD7711 analog inputs. However,
the the reference setting resistor must have a low tempera-
ture coefficient to avoid errors in the reference voltage over
temperature. The operating temperature range of this
circuit is from -35°C to + 100°C, the limitation on the
lower end is due to voltage headroom in the circuit
associated with the impedance of the thermistor increasing
as the the temperature decreases and the output compli-
ance of the current source. The output voltage from the
thermistor seen by the analog inputs of the AD7711 varies
from 7mV at 100°C to 0.75V at -35°C. The AD7711 on-
chip programmable gain amplifier (PGA) with gains from 1
Figure 2. Temperature Measurement Application Using
Current Excitation of the Thermistor
AD7711*
DGN D
AGND
VS S
R EF O UT
V B IAS
AIN 1(+)
A IN 1(-)
R E F IN (+)
RT
AVDD
DVDD
+5V
THE R MIST OR
R TD 1
0.3K 1A 1
R EF IN (-)
200
µ
A
R1
10K * D ig ita l pins om mitted for clarity
Figure 3. Temperature Measurement Application Using
Voltage Excitation of the Thermistor and Linearizing the
Output in the Analog Domain.
Betatherm which has a nominal impedance of 10k at
25°C. Appendix 2 shows a table of the R-T characteristics
of this device.
The circuit shown uses two resistors RS and R1 in series
with the thermistor. Resistor RS is used to limit the
thermistor power dissipation - see section on eliminating
self heating effects. Resistor R1 is used to linearize the
output of the thermistor. The output voltage across R1
varies from 33mV at -50°C to +2.329V at + 100°C. The
AD7711 operating at a gain of 1 in unipolar operating
*Betatherm Corporation
910 Turnpike Road, Shrewsbury, MA 01545 .
Tel No. 508 842 0516.
AD7711*
DGND
AGN D
VSS
R E F IN (-)
R EF O UT
V B IAS
AIN1 (+ )
AIN 1(-)
RE F IN (+)
RT
AV DD
DV DD
+5V
TH ER MIS TOR
10 K3A1
2.5V
R EFER EN CE
R1
9.1 K * D ig ital pins o m m itted fo r cla rity
RS
6 K * Betatherm Ireland Limited, Ballybrit Industrial Estate,
Galway, Ireland. Tel No. 353-91-753238.
Web Site: http://www.betatherm.com - 3 -
mode accomodates an input signal range of 0V to 2.5V. If
the application requires the full dynamic range of the ADC
to be used, then the system calibration feature on the ADC
can be used to calibrate out the 33mV at the low end.
Therefore, when the ADC sees 33mV on its input its
output digital code would be all zeroes. Likewise at the top
end a system fullscale can be used so that an input voltage
of 2.329V outputs all ones from the AD7711. The AD7711
on-chip programmable gain amplifier (PGA) with gains
from 1 to 128 can be used in this application to amplify the
signal generated across R1 in order to maximize the signal-
to-noise ratio (SNR) of the system.
LINEARIZING A THERMISTORS OUTPUT
Thermistors are non-linear devices and require linearization
techniques to obtain accurate measurements. In general
applications, linearization techniques can be implemented
in one of two ways. Linearization can be performed in the
digital domain using a look up table containing the
manufacturer's device characteristics to linearize the
thermistor output. Linearization can also be performed in
the analog domain with the addition of series or parallel
resistors which forces the voltage or the resistance of a
simple fixed-resistor-thermistor to have zero error along a
linear temperature scale at three equidistant points.
Figure 3 uses a resistor R1 in series with the thermistor to
linearize the output of the thermistor . This approach forces
the voltage across R1 to have zero error along a linear
temperature scale at three points across the required
temperature range. The maximum error is determined by
the temperature range of the applicat