Thermistors and analog temperature sensors are two commonly used temperature sensing solutions that can be used for most electronic applications. Deciding which technology is the best fit for your application can be a difficult task. However, I’m going to show you a few reasons why you should ditch the NTC thermistor and design in an analog temperature sensor instead.
Figure 1 shows an output voltage vs. temperature comparison. Notice that NTC thermistors requires to be used in a voltage divider circuit. Furthermore, other design techniques adding resistors in parallel to the NTC or proceeding to a polyfit calculation in the MCU help linearize their output. This is because their resistance vs. temperature characteristic has an exponential shape. Unlike NTC thermistors, analog temperature sensors do not require any additional circuitry as they have a virtually linear output voltage. For example, Texas Instrument’s TMP235 analog temperature sensor provides a very linear and accurate output voltage across the devices entire operating temperature range of -50°C to +150°C.
As you can see from the three NTC thermistor curves in figure 1, you can change the value of the bias resistor to adjust the location of the linear portion of the curve. Notice that this is a limited range and that the curves start to saturate at low and high temperatures. When interfacing with an ADC, this saturation will cause temperature errors if the resolution of the ADC is not high enough to detect a change in output voltage per degree Celsius. As a result, NTC thermistors tend to be less accurate across the entire operating temperature range and often require a higher resolution ADC in these cases.
Otherwise, if you want to keep the very small footprint of your thermistor and have as little disruption in your actual design as possible, TI released a series of pin to pin compatible thermistors for 0402 and 0603 NTCs, the TMP61. The TMP61 has the immense advantage of being linear all over its temperature range (-40°C to +125°C), and this at the same price point, This linearity simplifies the design of your system thanks to easy calibration, a smaller lookup table and no polyfit or external circuitry to linearize, as well as allowing you to reach better temperature accuracy over wide temperature ranges.
Figure 1: Output Voltage (V) vs. Temperature (°C)
Figure 2 shows a supply current vs. temperature comparison. The TMP235 has a typical value of 9µA and a max value of 14.5µA. NTC thermistor networks tend to dissipate more power as their supply currents are much higher and vary greatly over temperature. Notice that if you increase the resistance of the bias resistor, the supply current for the NTC thermistor network will decrease. But remember, the bias resistor is also chosen to ensure that the output voltage vs. temperature curve is linear for the desired temperature range. This is a tradeoff that can be ignored by using an analog temperature sensor as they have both a fairly constant low supply current and a virtually linear output voltage. Another disadvantage to NTC thermistors is that engineers must account for the self-heating effect as this will cause additional errors.
Figure 2: Supply Current (µA) vs. Temperature (°C)
You can avoid these NTC thermistor design issues by ditching them in favor of TI’s easy-to-use analog temperature sensors and linear thermistors. Check out my Engineer It video on the TI E2E community and learn more about TI’s analog temperature sensors.