The MAX31855 uses its own die temperature as the cold junction temperature when compensating for the thermocouple value. The actual cold junction is located where the thermocouple cables contact the circuit board (or other connector). The farther away this contact is from the MAX31855, the more likely there is a difference in temperature between the actual cold junction and the MAX31855 die temperature. This temperature difference will lead to errors in the reported thermocouple temperature value since the MAX31855 automatically uses its die temperature as the cold junction compensation value. The remedy is to make sure to keep the actual cold junction as close as possible to the MAX31855 so that they have as little difference in temperature as possible.
In addition, the long wires used by thermocouples act like an antenna and may couple ambient RF energy. A 10nF ceramic capacitor (maximum value) should be placed as close to the MAX31855 package as possible between the T+ and T- pins. Additional 10nF decoupling capacitors (maximum value) should be placed from T+ to GND and from T- to GND as implemented in the MAX31855 Evaluation Kit. With a small ferrite bead or series resistor (100MHz), the MAX31855’s ADC will produce large offset readings.