What is Solid-state temperature sensing?

Temperature is by far the most commonly measured physical parameter in industrial, commercial, and even residential settings. It’s obviously about much more than just “what’s the temperature, so I will know what coat to wear?” Temperature readings are needed to control processes and systems via a negative feedback loop, PID algorithms, and comparators (References 1 through 3).

For many years, the most widely way to measure temperature electrically was by using thermocouples (Reference 4). However, in the past decades, an alternative component called a solid-state temperature sensor has emerged and been widely adopted. It is suitable for some — but not all — applications which were previously handled by thermocouples.

Part 1 of this FAQ will discuss the principles of solid-state temperature sensors, while Part 2 will look at how it is applied.

Q: What is a solid-state temperature sensor?

A: This sensor uses a topology of transistors and resistors in an innovative arrangement and is fabricated as a single IC. It produces an output that is proportional to absolute temperate (PTAT) over a wide range, although not as wide as thermocouples can handle on both cold and hot ends of the range.

Q: What is the basic output of the solid-state sensor?

A: Unlike the thermocouple, which is a millivolt-range voltage source (with fairly limited current capacity), the basic solid state sensor is a current source (Reference 5). Its output is a current which is PTAT. A typical solid-state sensor provides an output of 1 µA/K.

Q: For what range is the solid-state sensor practical?

A: It depends on the specific make and model, but some are available for operation from -55°C to +150°C, which covers a very wide range of applications. In comparison, thermocouples are available rated below -100°C on the low side and thousands of degrees on the high side.

Q: Are there special design-in considerations when using these sensors?

A: The basic sensor is very simple to use. It is a two-terminal device that operates from a fairly wide DC power supply, typically anywhere from +4 to +30 V. This voltage is applied to the sensor, which then “regulates” the current flow to the 1 µA/K proportionally.

Q: What is the principle of this sensor?

A: The underlying principle is the well-known temperature dependence of p-n junction of a diode or base-emitter of a transistor:

V=kT/q (ln I/I_s)

where k is Boltzmann’s constant, T is the temperature in degrees Kelvin, q is the charge of an electron, and I_s is a characteristic current of the junction. This is a nonlinear relationship between voltage and current and also varies with temperature (Figure 1).

temperature sensing — Fig 1: The solid-state temperature sensor exploits an inherent temperature dependency of the current/voltage relationship of the silicon p-n junction. (Image: Encyclopædia Britannica, Inc.)

Q: Is this relationship useful or a problem?

A: It’s both. The relationship was used for many years as the basis for low-cost but only moderately useful temperature sensors, usually to provide some sort of compensation in circuits that drifted. It was uncalibrated and so only useful over a limited range.

This relationship was also a headache, in that it hindered the development of accurate solid-state voltage references (band gap and others) as the reference voltage value would drift as the temperature varied. Special compensation circuitry and trims were needed to correct these errors. Still, this complex relationship also turned into an advantage

Q: Given the logarithm in this equation, how does this become a temperature sensor?

The first commercially available temperature sensor in this class — the AD590 from Analog Devices — was introduced about 25 years ago. It used the implications of the equation along with a very clever topology and laser trimming for calibration. This IC leveraged the fundamental properties of the silicon transistors from which it is made (note that there are only 12 transistors in the design) (Figure 2).

It develops its temperature-proportional characteristic this way: if two identical transistors are operated at a constant ratio of collector current densities, called r, then the difference in their base-emitter voltage is (kT/q)(In r). Because both k (Boltzman’s constant) and q (the charge of an electron) are constant, the resulting voltage is directly proportional to absolute temperature. In the AD590, this PTAT voltage is converted to a PTAT current by low-temperature-coefficient thin-film resistors. The total current of the device is then forced to be a multiple of this PTAT current.

Q: IS the AD590 the only solid-state temperature sensor available?

A: Although it is relatively old, it has been greatly improved and modified but is still available for that model number. Now, along with Analog Devices, many other vendors offer a wide array of sensors based on this principle, but with other output formats and functions in addition to the basic PTAT current.

Part 2 will discuss variations of this solid-state sensor and application issues.

References