The recommended PCB layout for the MAX31855A involves keeping the thermocouple traces as short as possible, using a dedicated ground plane, and minimizing noise coupling. A 4-layer PCB with a dedicated ground plane is recommended. The thermocouple traces should be routed on the top layer, and the ground plane should be on the inner layer.
The MAX31855A has an internal cold junction compensation (CJC) circuit that measures the temperature of the IC itself. The CJC voltage is available on the CJCOUT pin, which can be connected to an ADC to measure the temperature. The CJC voltage can then be used to compensate for the cold junction temperature in the thermocouple measurement.
The accuracy of the MAX31855A's internal temperature measurement is ±2°C (typical) and ±4°C (maximum) over the operating temperature range of -20°C to +150°C. This accuracy is sufficient for most thermocouple applications, but if higher accuracy is required, an external temperature sensor can be used.
The MAX31855A is designed to work with standard thermocouple types such as K, J, N, T, E, B, R, and S. However, it can be used with non-standard thermocouple types by using an external voltage source to bias the thermocouple input and adjusting the gain and offset of the amplifier stage. This requires careful calibration and testing to ensure accurate measurements.
To handle noise and EMI in a MAX31855A design, use a shielded cable for the thermocouple, keep the thermocouple traces away from noisy signals, use a ground plane, and add filtering capacitors to the power supply. Additionally, consider using a common-mode filter or a differential amplifier to reject common-mode noise.
MAX318ESA datasheet
by Maxim Integrated Products
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