The difficulties of thermocouple measurement - The voltage signal is too weak _ instrument

Will generate the thermocouple voltage transform into accurate temperature reading is not a easy thing, for many reasons: voltage signal is too weak, temperature voltage relationship is nonlinear, need the reference junction compensation, and the thermocouple can cause grounding problems voltage signal is too weak, common types of the thermocouple J, K and T. At room temperature, the voltage variation amplitude of 52 mu respectively 41 mu V/V / ° C, and 41 ° C u V / ° C. Other less common types of temperature variations in voltage or even smaller. The higher cost of weak signal in front of the modulus conversion gain stage. Because the voltage signal is weak, the gain of signal conditioning circuit generally need to about 100 or so, this is quite a simple signal disposal. More difficult thing is how to identify the actual pick up noise in the signal and the thermocouple wire. Thermocouple wire longer, often through the dense environment of electrical noise. Lead on the tiny thermocouple signal noise can be easily covered. General combines two plans to extract the signal from the noise. Using the differential input amplifier ( Such as instrumentation amplifier) To amplify the signal. Because most of the noise at the same time appear in the two wires ( Common mode) That difference measurement can be eliminated. Scheme is a low-pass filter to eliminate noise outside the band. Low-pass filter should also eliminate the radio frequency interference may cause amplifier rectifier ( More than 1 MHz) And 50 Hz / 60 Hz ( The power supply) The power frequency interference. A radio frequency interference filter is placed in front of the amplifier ( Or use belt filter input amplifier) It is very important. The location of the 50 hz / 60 hz filter doesn't matter - It can be combined with RFI filter on the amplifier and ADC, as ∑ - Part of Δ ADC filter, or can be used as the mean filter in software programming.