Example Issue:
When the MAX31856 is powered on without a thermocouple connected, the MAX31856 does not consistently identify an open circuit fault. Temperature measurement conversions result in a temperature of exaclty 0C. If we then connect a thermocouple to the connector and disconnect it, the MAX31856 correctly identifies the open circuit.
Solution:
It is likely this is actually due to an Over-Voltage/Under-Voltage (OVUV) error. A conversion result of exactly 0C indicates conversions are not occuring. In auto-conversion mode, conversions happen every 100ms and open-circuit detections in this mode occur every 16 conversions. If there are no conversions happening (due to this OVUV error), then there would be no automatic open-circuit detection. With the TC pins floating, it is possible that the potentials detected on T+ and T- may have floated above VCC or below GND. Placing large (Mohms) resistors from the T terminals to GND provides a very weak reference path to GND to prevent those nodes from floating out of range.