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Figure 2. The local temperature detection IC (MAX6610) to determine the cold junction temperature. Detection IC near the thermocouple junction temperature (cold side), thermocouple and the cold end temperature sensor output voltage by 16 bit ADC (MX7705) conversion. When work in bipolar mode, the ADC can convert thermocouple is positive and negative signals, and the channel 1 output. ADC channel 2 will MAX6610 single-ended output voltage into a digital signal, and provided to the microcontroller. Temperature detection IC output voltage is proportional to the cold end temperature. In order to determine the temperature of hot end, need to first determine if the cold end temperature. Then provides a type K thermocouple by NBS lookup table will be converted to the cold end temperature corresponding to the thermoelectric voltage. This voltage and after PGA gain calibration of the thermocouple reading together, and then through the lookup table will sum the results converted to temperature, the result is the hot end temperature. Table 2 lists the temperature measurement result, the cold end temperature change range: to 40 ° C to + 85 ° C, the hot end keep in + 100 ° C. Actual measurement result depends largely on the local temperature detection precision of the IC and the oven temperature. Table 2. The circuit of figure 2 in different the cold end and heat end of the oven temperature measurement
Cold end temperature (° C) to measure the temperature of hot end * (° C)
Measured value # 1-39.9 + 101.4
Measured value # 2, 0.0 + 101.5
Measured value # 3 + 25.2 + 25.2
Measured value # 4 + 85.0 + 85.0
* hot side temperature is compensated values, were measured by the circuit. Example # 2 in the circuit shown in figure 3, the remote testing IC in the cold end temperature measurement circuit, temperature and local temperature detection IC IC is installed do not need to be close to the cold end, but rather through external connections into diode transistor cold end temperature measurement. Installed directly on the thermocouple junction transistor. Measure the temperature of the temperature detection IC will transistor is transformed into digital output. ADC channel 1 of the thermocouple voltage into digital output, channel 2 is not used, the input directly grounded. External benchmark 2.5 V IC provides reference voltage to ADC.
Figure 3. The remote diode IC don't have to be close to the cold end temperature detection, because it USES a diode testing temperature. MAX6002 provide 2.5 V reference voltage to ADC. Table 3 lists the temperature measurement result, the cold end temperature change range: to 40 ° C to + 85 ° C, the hot end keep in + 100 ° C. Actual measurement result depends largely on the remote diode temperature detection precision of the IC and the oven temperature. Table 3. The circuit of figure 3 in different the cold end and heat end of the oven temperature measurement
Cold end temperature (° C) to measure the temperature of hot end * (° C)
Measured value # 1-39.8 + 99.1
Measured value # 2-0.3 + 98.4
Measured value # 3 + 25.0 + 25.0
Measured value # 4 + 85.1 + 85.1
* hot side temperature is compensated values, were measured by the circuit. Example # 3 in figure 4 circuit 12 bit ADC with diode temperature detection, temperature detection of the environment temperature is converted into voltage diode, IC by processing the thermocouple voltage and diode testing voltage, to calculate the compensation after the hot side temperature. Digital output is the results of thermocouple testing after temperature compensation, at 0 ° C to + 700 ° C temperature range, the device temperature error within plus or minus 9 LSB. Although the device temperature measurement range is wide, but it can not measure temperature below 0 ° C. Figure 4. Integrate the cold junction compensation of ADC, the thermocouple voltage is converted to temperature, without external components.