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MAX31855 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX31855 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 13 page ����������������������������������������������������������������� Maxim Integrated Products 9 MAX31855 Cold-Junction Compensated Thermocouple-to-Digital Converter the same temperature as the board on which the device is mounted) can range from -55NC to +125NC. While the temperature at the cold end fluctuates, the device con- tinues to accurately sense the temperature difference at the opposite end. The device senses and corrects for the changes in the reference junction temperature with cold-junction compensation. It does this by first measuring its internal die temperature, which should be held at the same tem- perature as the reference junction. It then measures the voltage from the thermocouple’s output at the reference junction and converts this to the noncompensated ther- mocouple temperature value. This value is then added to the device’s die temperature to calculate the thermo- couple’s “hot junction” temperature. Note that the “hot junction” temperature can be lower than the cold junction (or reference junction) temperature. Optimal performance from the device is achieved when the thermocouple cold junction and the device are at the same temperature. Avoid placing heat-generating devices or components near the MAX31855 because this could produce cold-junction-related errors. Conversion Functions During the conversion time, tCONV, three functions are performed: the temperature conversion of the internal cold-junction temperature, the temperature conversion of the external thermocouple, and the detection of thermo- couple faults. When executing the temperature conversion for the inter- nal cold-junction compensation circuit, the connection to signal from the external thermocouple is opened (switch S4) and the connection to the cold-junction compensa- tion circuit is closed (switch S5). The internal T- reference to ground is still maintained (switch S3 is closed) and the connections to the fault-detection circuit are open (switches S1 and S2). When executing the temperature conversion of the external thermocouple, the connections to the internal fault-detection circuit are opened (switches S1 and S2 in the Block Diagram) and the switch connecting the cold- junction compensation circuit is opened (switch S5). The internal ground reference connection (switch S3) and the connection to the ADC (switch S4) are closed. This allows the ADC to process the voltage detected across the T+ and T- terminals. During fault detection, the connections from the exter- nal thermocouple and cold-junction compensation cir- cuit to the ADC are opened (switches S4 and S5). The internal ground reference on T- is also opened (switch S3). The connections to the internal fault-detection cir- cuit are closed (switch S1 and S2). The fault-detection circuit tests for shorted connections to VCC or GND on the T+ and T- inputs, as well as looking for an open thermocouple condition. Bits D0, D1, and D2 of the output data are normally low. Bit D2 goes high to indi- cate a thermocouple short to VCC, bit D1 goes high to indicate a thermocouple short to GND, and bit D0 goes high to indicate a thermocouple open circuit. If any of these conditions exists, bit D16 of the SO output data, which is normally low, also goes high to indicate that a fault has occurred. Serial Interface The Typical Application Circuit shows the device inter- faced with a microcontroller. In this example, the device processes the reading from the thermocouple and transmits the data through a serial interface. Drive CS low and apply a clock signal at SCK to read the results at SO. Conversions are always being performed in the background. The fault and temperature data are only be updated when CS is high. Drive CS low to output the first bit on the SO pin. A complete serial-interface read of the cold-junction com- pensated thermocouple temperature requires 14 clock cycles. Thirty-two clock cycles are required to read both the thermocouple and reference junction temperatures (Table 2 and Table 3.) Read the output bits on the falling edge of the clock. The first bit, D31, is the thermocouple temperature sign bit. Bits D[30:18] contain the converted temperature in the order of MSB to LSB. Bit D16 is nor- mally low and goes high when the thermocouple input is open or shorted to GND or VCC. The reference junction temperature data begins with D15. CS can be taken high at any point while clocking out conversion data. Figure 1 and Figure 2 show the serial-interface timing and order. Table 2 and Table 3 show the SO output bit weights and functions. |
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