選擇放大器/調理電路與熱電偶接口

　　熱電偶是用來測量溫度的，是一種非常罕見的傳感器，它實際上產(chǎn)生電壓而不是改變電阻、電容或導電率。由于信號電平非常小，需要專門的高阻抗放大，以準確地拾取和放大熱電偶信號，而不會影響或影響其精度。雖然通用運算放大器可以使用，但有專門為熱電偶設計的一類專用放大器。

　　本文將研究熱電偶的特性，并著重于與它們相連的放大器和調節(jié)電路。這些部件可以放大、電平轉換，并抵消，補償熱電偶信息直接連接到A / D級嵌入式單片機，也可以用于驅動半導體分立器件如晶體管和繼電器作為安全閥或限位開關。在這兩種情況下，放大器負責確保精確的熱電偶信息被保存和傳播。

　　A tale of two metals

　　When two different metallic materials are in contact， the junction around the point of contact will generate a voltage that depends on the temperature difference between the junction and the rest of the conductors. Typically， induced voltages will be between 1 and 70 μV/°C for standard metal combinations. First noticed by Thomas Seebeck， this effect is commonly known as the thermoelectric effect， but it is also called the Seebeck effect in honor of the Estonian physicist.

　　In some cases， the voltage generated is proportional to the difference in temperature; other combinations of metals and alloys can produce a nonlinear proportional response. Both can create useful applications for thermocouples. The non-linear responses can be used as limit switches， while the （somewhat） linear responses can be used as a temperature sensor.

　　Since thermocouples measure the difference of a temperature between two distinct points， the voltage measurement is not a reflection of an absolute temperature. A cold junction can be maintained as a reference. This is a useful way of characterizing a thermocouple， since a simple medium， such as using ice as a calibration point， is easily replicated anywhere （Figure 1）。