Temperature sensing can be one of the more challenging
physical entities to measure if a variety of ranges and levels
of accuracy are required by one electrical system. The circuit
in Figure 1 uses a low power, single-supply analog-to-digital
converter, ADS7816, configured with a thermocouple and
CJC sub-circuit to achieve the desired range of responsiveness
as well as high accuracy. The thermocouple is interfaced
with a 12-bit A/D converter that allows for an adjustable
input range. This range is adjusted with the voltage
reference input. The range of the voltage reference input of
the ADS7816 A/D converter is from the supply voltage
(+5V typically) all the way down to 0.1V. The voltage
reference range mimics a gain stage for the 12-bit converter.
With this converter a suto-gain stage provides 17.6-bit
virtual resolution against a 5V scale. When this circuit is
properly calibrated for the desired temperature range, the
effective resolution can be increased by 50x over a system
with a standard 5V full-scale range.