Implementing the right thermal sensing option in your embedded design

We compare these solutions to silicon temperature sensor ICs and discuss how IC sensors provide flexible and cost-effective thermal-management solutions. We will also show to use integrated features such as integrated EEPROM, and under-/over- and critical-temperature monitoring to improve system performance.

Thermistors are the most common method used to sense temperature. Thermistors are made using semiconductor materials and can have either a positive or negative temperature coefficient (PTC or NTC, respectively).

The thermistor's resistance changes with respect to changes in temperature. PTC thermistor resistance increases when temperature increases, whereas NTC thermistor resistance decreases when temperature increases.

There are a few advantages to thermistor-based solutions. Thermistors are highly sensitive to changes in temperature, they have quick thermal response, and they are inexpensive. The biggest disadvantage is that thermistors are highly nonlinear over wide temperature ranges.

Figure 1: Shown is a thermistor circuit with a low-pass filter and a unity-gain buffer amplifier.

Figure 1 above shows a thermistor circuit with a low-pass filter and a unity-gain buffer amplifier. The low-pass filter (R2 and C1) network filters system noise from the sensor output, and the unity-gain buffer is used to drive resistive or capacitive loads.

The voltage across the thermistor (VTH) is proportional to the change in temperature. The graph indicates a linear response from 0 to 70 degrees Celsius . However, there is significant nonlinearity at temperature extremes. The change in resistance with respect to temperature is much less, when compared to the linear region. This requires some signal amplification, to improve measurement resolution at hot and cold temperature extremes.

Thermistors are inexpensive and provide accurate temperature monitoring, over a limited temperature range. To achieve high accuracy over a wider range requires a more complex design, including multiple gain stages at various temperature ranges using a programmable gain amplifier (PGA).

This provides a robust measurement solution but increases overall system cost. Additionally, thermistors require biasing current, which is set using R1 in Figure 1. Higher current increases temperature-measurement resolution.

However, it also increases temperature-measurement error. This is due to self-heat generated from the power that is dissipated across the thermistor. In most applications, other solutions such as IC sensors are better suited for extended-temperature applications.