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by a nominal temperature coefficient a = (385k2)10-5 "C'.
With an excitation current of 373 pA and a total
differential subtraction gain of 68.2 V/V, a sensitivity of
+)(cc
10 pV/mK has been obtained at the measurement circuit
output (voltage Vhem in Figg. 1 and 2). The output
voltage from the measurement circuit of Fig. 2 is sent to
an A/D converter (Harris, ICL7129) which then drives a
liquid crystal display (Varitronix, BD503DP) with 4%
digits. An analog voltage is also available at a BNC
output connector on the back of the thermometer case.
Due to the kinds of A/D converter and display that have
been adopted, the theoretical resolution of the digital
measurements turns out to be 1 mK or 10 mK in the
temperature intervals of 320 "C or +200 "C, respectively.
The desired measurement range can be selected by a
manual switch acting on the A/D converter and on the
dynamic of the display (SO0 mV or +2 V, respectively).
Calibration of the instrument offset and gain is made
possible by means of the Rwl and Rw2 potentiometers,
respectively. The power supply consists of two 9-V
batteries which are connected in series. Owing to the low-
power consumption of the adopted electronic circuit, the
power supply allows for a continuous operation of the
thermometer for a period longer than 100 hours.
A picture of the entire instrument and sensor is shown
in Fig. 3. This thermometer has been developed mostly
for near-room-temperature operation and it is specifically
designed for thermoelectronic control loops aimed at the
temperature stabilization of small optical or mechanical
parts even out side of the laboratory environment.
Henceforth, the device is portable and battery operated.
loop topology is now becoming widely used for
temperature measurements using resistive sensors. When
compared to a standard bridge circuit, the Anderson's
loop also presents the clear advantage of generating an
output voltage which varies; linearly with the sensor
resistance. If a perfectly linear and calibrated electronic
circuit is developed, the remaining measurement errors
then arise from the sensor non-linearity and from the
aging of both the sensor and the electronic circuit.
In this paper the design and performance of a compact
and accurate digital thermometer, with a 4% digit display,
is described. The thermometer uses a four-lead Pt-100
commercial temperature transducer sensed by a doubly
differential subtraction circlcit in an Anderson's loop
configuration. The combination of a low-noise electronics
and a careful instrument calibration allows for accurate
temperature measurements in a e00 OC interval withm
the operating range of the sensor.
2. Digital Thermometer Design
The thermometer design is based on a current loop
excitation acting on a resistive thermal sensor whereas
active doubly differential subtraction is used to read the