AN38

Application Note 38
Basic introduction to the use of the ZNI1000
Nickel Temperature Sensor
Howard Mason, September 2003
This application note describes the ZNI1000
temperature sensor, which uses a nickel
element connected between 2 pins of a SOT23
package to measure temperature accurately.
Nickel has a very precisely known resistance
versus temperature characteristic and this is
expressed as a polynomial. The element is
designed to be 1000⍀ at 0°C. By measuring the
resistance, temperature can be measured to
within ± 2°C down to -55°C and to within ± 1.5°C
up to +150°C.
The element is connected between pins 1 and 2
of a standard SOT23 package. Pin 3 is not
connected electrically, but has very good
thermal contact with the element. This means
it can be soldered to the object whose
temperature is being measured, thus following
changing temperatures quickly and accurately.
The resistance can be expressed as follows:
RT = R0 * (1 + A*T + B*T2 + C*T4 + D*T6)
where
RT is the resistance at temperature T
R0 is the resistance at 0°C (1000⍀)
A = 5.485 * 10-3
B = 6.650 * 10-6
C = 2.805 * 10-11
D = -2.000 * 10-17
Clearly, for most applications, the sixth order
term can be omitted. For example, even at
+150°C, it only decreases the resistance, which
is almost 2000⍀ at that temperature, by 0.2⍀.
The curve of resistance versus temperature is
shown in Figure 1.
Figure 1. Resistance of ZNI1000 versus temperature in degrees Celsius
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In addition to the polynomial, there is a
specification of the deviation in degrees.
For temperatures from -55°C to 0°C, the
deviation in °C is:
Error ° = ± (0.4 + (0.028 * |T|) °C.
Note the modulus signs around the T, so that
the two error terms still ADD despite T being
negative - they do NOT cancel.
For temperatures from 0°C to +150°C, the
deviation in °C is:
Error ° = ± (0.4 + (0.007 * T) °C Again
the two error terms ADD.
At first sight enabling a readout of temperature
versus resistance measured appears to require
a look-up table, but a technique can be used
which "linearises" the output and makes
readout much simpler.
If the element is put into a bridge circuit which
is driven by a constant current then, as the
bridge resistance increases, the applied bridge
voltage increases and this applies a non-linear
term to the Vout versus Resistance equation
which causes the higher order terms to be
partially canceled out.
The bridge circuit is shown in Figure 3.
This produces a Deviation graph as shown in
Figure 2.
Figure 2. ZNI1000 Error in Degrees Celsius versus Operating Temperature
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The constant current chosen is shown as 6mA
in the circuit, but is strictly 6.0623mA to give
exactly 1 volt output at +100°C. The 500⍀
trimpot is adjusted so that the output is exactly
1 volt at +100°C and the DVM will then read
with ± 2°C over the range -55°C to +150°C.
The error introduced by this circuit by
assuming that Vout is linearly proportional to
temperature is only -0.8°C at -55°C and only
+0.2°C at +150°C, obviously being zero by
definition at 0°C and zero by calibration at
+100°C. The worst case error between 0°C and
+100°C is 0.033°C in addition to the error due to
the deviation formula above, which gives 1.1°C
at 100°C, so within the 0°C to +100°C range all
the errors only add up to ± 1.2°C.
It should be noted from the spec that the
maximum continuous current is specified as
4mA. This is because it is necessary to make the
nickel element very thin to obtain 1000⍀
resistance as nickel has a very low resistivity. In
practice the user should keep the current as low
as possible because of the thermal resistance of
a SOT23 package. This will not be appreciable if
the unit is soldered down to a metal tab, as this
will always ensure that the nickel element is at
the correct temperature. However, if the unit is
used in free air, the thermal resistance will be
about 200°C/Watt, so the current should really
be kept down to 2mA to keep the errors due to
self-heating to below 1°C.
Figure 3. ZnI1000 and ZMR500 used with a DVM as a Thermometer
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