How does a silicon temperature (temp) sensor IC actually measure temperature? These ICs take advantage of the basic temperature-dependent behavior of silicon PN junctions. 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:
Linear response is an important performance benefit of a silicon temp sensor over a thermistor (Figure 1). Additional processing is not required to calculate the actual temperature.
The earliest temp sensors produced an output voltage that was proportional to absolute temperature. Over time, engineers preferred data in °F or °C. With the correct circuitry (gain and/or level shifting), this absolute temperature can be converted to a voltage proportional to °C or °F. Analog output devices have either a positive (LM35 = 10 mV/°C) or negative (TMP20 = –11.77 mV/°C) gain. The LM34 is an example of a Fahrenheit-based temp sensor at 10 mV/°F. More recent devices have integrated an analog-to-digital converter (ADC) that allows for directly interfacing to a controller via a standard serial interface such as I2C or SPI.
Measuring temperature external to the IC
Since a temp sensor works based on the temperature experienced by the on-chip PN junctions, how do you measure the temperature of an object external to itself? Depending on which temperature you are trying to measure, for instance the IC on a PCB versus the air surrounding the unit determines where you place your sensor on the PCB and the size of the PCB. This goes back to the principle of thermal equilibrium from high school physics.
For a quick refresher, thermal equilibrium states that two thermally connected bodies at different temperatures will transfer heat until they reach the same temperature. Also the mass of the objects in question will affect the temperature flow. As you design a temperature measurement system, keep these two concepts in mind:
- Sensor must be thermally connected to the object
- Sensor's thermal mass should be smaller than the object's thermal mass
Measuring the temperature of another IC
In this example we measure the temperature of an on-board controller. During operation, the controller die heats up due to current flow. This increased die temperature results in a corresponding rise in the surrounding PCB temperature. Normally the largest thermal mass in the PCB is the ground plane. The rise in the die temperature causes the ground plane temperature to increase. The board layout determines how much the ground plane rises.
To accurately measure the increase in die temperature, the temp sensor IC must be thermally-coupled to the die's thermal mass: its ground connection. The temp sensor measures the board temperature at the point where its ground pad is soldered to the PCB. As long as the temp sensor and controller share a ground plane, the two should be in close thermal connection and equilibrium.
Measuring air temperature
In our second example we want to measure the air temperature. The PCB's thermal mass and how quickly it responds to a change in air temperature is critical. For best results, minimize the PCB's thermal mass where the temp sensor is mounted. Ideally, mounting the temp sensor on its own tiny board and connecting it to the main board with only the minimum wires keeps the temp sensor thermal mass to a minimum. This PCB should be thermally isolated from the main board and system housing to prevent them from affecting the results. The opposite is true relative to its thermal contact with air. Completely expose it to maximize the thermal connection.
Temp sensor ICs are very simple to use and work on very fundamental principles. Taking these principles into account allows a designer to achieve the most accurate result possible.
Please join us next time where we will discuss the finer details of the relationship between differential nonlinearity and missing codes in precision SAR data converters.
— Dan Harmon is Sensing Business Development Manager for TI's sensing group. In his 27-plus year career at TI, he has supported a wide variety of technologies and products including interface products, imaging analog front-ends (AFEs), and charge-coupled device (CCD) sensors. He also has served as TI's USB-IF Representative and TI's USB 3.0 Promoter's Group Chair. Dan earned a BSEE from the University of Dayton and a MSEE from the University of Texas in Arlington. You can reach Dan at .