To understand the concept of a remote temperature sensor and how it differs from a local temperature sensor, it’s best to first review some basics of temperature-sensor ICs. A silicon temperature-sensor IC takes advantage of the basic temperature-dependent behavior of silicon PN junctions.
Therefore, if you have two PN junctions of different areas and force a current through them, they produce two different forward voltage levels. With a constant current, the voltages rise or fall relative to the PN temperature junction. The difference between the voltages is proportional to absolute temperature:
Now that we understand the basic operating principle of all temperature-sensor ICs, the second issue concerns the differences between a remote temperature sensor and a local temperature sensor. A local temperature sensor’s PN junction is typically thermally connected to the ground pin of the package. Based on thermal equilibrium, the temperature experienced by the PN junction is essentially the same as that experienced by the printed-circuit board (PCB) where the ground pin is connected.
Remote temperature sensors sense the temperature locally like a local temperature sensor. In addition, they sense one or more remote diodes or transistors elsewhere in the system. In many cases, these remote elements are implemented in another IC, such as a processor, ASIC, or FPGA. The “remote diode” in this case is actually a PNP transistor with its collector tied to the device’s substrate.
This PNP is inherently available in most CMOS processes and, therefore, can be easily implemented in any complex, power-hungry circuit that may easily overheat. Alternatively, for a cost-effective system approach that must monitor multiple temperature points in a system, a standard transistor (such as a 2N3904 or 2N3906) could be utilized for each “remote” location.
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