Suitable thermocouple types:
Type K (Chromel - Alumel) and Type T (Copper - Constantan) thermocouples offer
the best performance at very low temperatures.
The table on page 3 of the AD595 datasheet shows that the AD595 is calibrated
for use down to -200degC. The -55 to +125 temperature range given in the
specification on page 2 applies to the ambient temperature around the IC, not
to the temperature which can be measured by the thermocouple.
One point about the construction of the circuit to mention is the importance of
careful choice of connecting materials and temperature gradients in connecting
the AD595 to the thermocouple. Thermocouple voltages will exist wherever two
dissimilar metals are in contact. We try to get the effect in each lead to
cancel the other out, so it is important that the input is symmetrical with
respect to any temperature gradients, so that parasitic thermocouple effects
are minimised. This becomes more difficult to ensure, when more complex EMI
filtering is located at the input.
The attached document is an example of the extreme input filtering sometimes
required. This customer was unlucky in that his system was particularly
susceptible to radiated EMI being picked up by the thermocouple leads.
Please see our Sensor Signal Conditioning seminar on the web.
Chapter 7 deals with temperature sensing.