This is the first article in a two-part series. This article will first discuss the history and design challenges of thermistor-based temperature measurement systems, as well as their comparison with resistance thermometer (RTD) temperature measurement systems. It will also describe the choice of thermistor, configuration trade-offs, and the importance of sigma-delta analog-to-digital converters (ADCs) in this application area. The second article will detail how to optimize and evaluate the final thermistor-based measurement system.
As described in the previous article series, Optimizing RTD Temperature Sensor Systems, an RTD is a resistor whose resistance varies with temperature. Thermistors work similarly to RTDs. Unlike RTDs, which only have a positive temperature coefficient, a thermistor can have a positive or negative temperature coefficient. Negative temperature coefficient (NTC) thermistors decrease their resistance as the temperature rises, while positive temperature coefficient (PTC) thermistors increase their resistance as the temperature rises. On fig. 1 shows the response characteristics of typical NTC and PTC thermistors and compares them to RTD curves.
In terms of temperature range, the RTD curve is nearly linear, and the sensor covers a much wider temperature range than thermistors (typically -200°C to +850°C) due to the non-linear (exponential) nature of the thermistor. RTDs are usually provided in well-known standardized curves, while thermistor curves vary by manufacturer. We will discuss this in detail in the thermistor selection guide section of this article.
Thermistors are made from composite materials, usually ceramics, polymers, or semiconductors (usually metal oxides) and pure metals (platinum, nickel, or copper). Thermistors can detect temperature changes faster than RTDs, providing faster feedback. Therefore, thermistors are commonly used by sensors in applications that require low cost, small size, faster response, higher sensitivity, and limited temperature range, such as electronics control, home and building control, scientific laboratories, or cold junction compensation for thermocouples in commercial or industrial applications. purposes. Applications.
In most cases, NTC thermistors are used for accurate temperature measurement, not PTC thermistors. Some PTC thermistors are available that can be used in overcurrent protection circuits or as resettable fuses for safety applications. The resistance-temperature curve of a PTC thermistor shows a very small NTC region before reaching the switch point (or Curie point), above which the resistance rises sharply by several orders of magnitude in the range of several degrees Celsius. Under overcurrent conditions, the PTC thermistor will generate strong self-heating when the switching temperature is exceeded, and its resistance will rise sharply, which will reduce the input current to the system, thereby preventing damage. The switching point of PTC thermistors is typically between 60°C and 120°C and is not suitable for controlling temperature measurements in a wide range of applications. This article focuses on NTC thermistors, which can typically measure or monitor temperatures ranging from -80°C to +150°C. NTC thermistors have resistance ratings ranging from a few ohms to 10 MΩ at 25°C. As shown in fig. 1, the change in resistance per degree Celsius for thermistors is more pronounced than for resistance thermometers. Compared to thermistors, the thermistor’s high sensitivity and high resistance value simplify its input circuitry, since thermistors do not require any special wiring configuration, such as 3-wire or 4-wire, to compensate for lead resistance. The thermistor design uses only a simple 2-wire configuration.
High-precision thermistor-based temperature measurement requires precise signal processing, analog-to-digital conversion, linearization, and compensation, as shown in fig. 2.
Although the signal chain may seem simple, there are several complexities that affect the size, cost, and performance of the entire motherboard. ADI’s precision ADC portfolio includes several integrated solutions, such as the AD7124-4/AD7124-8, which provide a number of advantages for thermal system design as most of the building blocks needed for an application are built-in. However, there are various challenges in designing and optimizing thermistor-based temperature measurement solutions.
This article discusses each of these issues and provides recommendations for solving them and further simplifying the design process for such systems.
There are a wide variety of NTC thermistors on the market today, so choosing the right thermistor for your application can be a daunting task. Note that thermistors are listed by their nominal value, which is their nominal resistance at 25°C. Therefore, a 10 kΩ thermistor has a nominal resistance of 10 kΩ at 25°C. Thermistors have nominal or basic resistance values ranging from a few ohms to 10 MΩ. Thermistors with low resistance ratings (nominal resistance of 10 kΩ or less) typically support lower temperature ranges, such as -50°C to +70°C. Thermistors with higher resistance ratings can withstand temperatures up to 300°C.
The thermistor element is made of metal oxide. Thermistors are available in ball, radial and SMD shapes. Thermistor beads are epoxy coated or glass encapsulated for added protection. Epoxy coated ball thermistors, radial and surface thermistors are suitable for temperatures up to 150°C. Glass bead thermistors are suitable for measuring high temperatures. All types of coatings/packaging also protect against corrosion. Some thermistors will also have additional housings for added protection in harsh environments. Bead thermistors have a faster response time than radial/SMD thermistors. However, they are not as durable. Therefore, the type of thermistor used depends on the end application and the environment in which the thermistor is located. The long-term stability of a thermistor depends on its material, packaging, and design. For example, an epoxy-coated NTC thermistor can change 0.2°C per year, while a sealed thermistor only changes 0.02°C per year.
Thermistors come in different accuracy. Standard thermistors typically have an accuracy of 0.5°C to 1.5°C. The thermistor resistance rating and beta value (ratio of 25°C to 50°C/85°C) have a tolerance. Note that the beta value of the thermistor varies by manufacturer. For example, 10 kΩ NTC thermistors from different manufacturers will have different beta values. For more accurate systems, thermistors such as the Omega™ 44xxx series can be used. They have an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C. Therefore, the range of temperatures that can be measured and the accuracy required over that temperature range determines whether thermistors are suitable for this application. Please note that the higher the accuracy of the Omega 44xxx series, the higher the cost.
To convert resistance to degrees Celsius, the beta value is usually used. The beta value is determined by knowing the two temperature points and the corresponding resistance at each temperature point.
RT1 = Temperature resistance 1 RT2 = Temperature resistance 2 T1 = Temperature 1 (K) T2 = Temperature 2 (K)
The user uses the beta value closest to the temperature range used in the project. Most thermistor datasheets list a beta value along with a resistance tolerance at 25°C and a tolerance for the beta value.
Higher precision thermistors and high precision termination solutions such as the Omega 44xxx series use the Steinhart-Hart equation to convert resistance to degrees Celsius. Equation 2 requires the three constants A, B, and C, again provided by the sensor manufacturer. Because the equation coefficients are generated using three temperature points, the resulting equation minimizes the error introduced by linearization (typically 0.02 °C).
A, B and C are constants derived from three temperature setpoints. R = thermistor resistance in ohms T = temperature in K degrees
On fig. 3 shows the current excitation of the sensor. Drive current is applied to the thermistor and the same current is applied to the precision resistor; a precision resistor is used as a reference for measurement. The value of the reference resistor must be greater than or equal to the highest value of the thermistor resistance (depending on the lowest temperature measured in the system).
When selecting the excitation current, the maximum resistance of the thermistor must again be taken into account. This ensures that the voltage across the sensor and the reference resistor is always at a level acceptable to the electronics. The field current source requires some headroom or output matching. If the thermistor has a high resistance at the lowest measurable temperature, this will result in a very low drive current. Therefore, the voltage generated across the thermistor at high temperature is small. Programmable gain stages can be used to optimize the measurement of these low level signals. However, the gain must be programmed dynamically because the signal level from the thermistor varies greatly with temperature.
Another option is to set the gain but use dynamic drive current. Therefore, as the signal level from the thermistor changes, the drive current value changes dynamically so that the voltage developed across the thermistor is within the specified input range of the electronic device. The user must ensure that the voltage developed across the reference resistor is also at a level acceptable to the electronics. Both options require a high level of control, constant monitoring of the voltage across the thermistor so that the electronics can measure the signal. Is there an easier option? Consider voltage excitation.
When DC voltage is applied to the thermistor, the current through the thermistor automatically scales as the thermistor’s resistance changes. Now, using a precision measuring resistor instead of a reference resistor, its purpose is to calculate the current flowing through the thermistor, thus allowing the thermistor resistance to be calculated. Since the drive voltage is also used as the ADC reference signal, no gain stage is required. The processor does not have the job of monitoring the thermistor voltage, determining if the signal level can be measured by the electronics, and calculating what drive gain/current value needs to be adjusted. This is the method used in this article.
If the thermistor has a small resistance rating and resistance range, voltage or current excitation can be used. In this case, the drive current and gain can be fixed. Thus, the circuit will be as shown in Figure 3. This method is convenient in that it is possible to control the current through the sensor and the reference resistor, which is valuable in low power applications. In addition, self-heating of the thermistor is minimized.
Voltage excitation can also be used for thermistors with low resistance ratings. However, the user must always ensure that the current through the sensor is not too high for the sensor or application.
Voltage excitation simplifies implementation when using a thermistor with a large resistance rating and a wide temperature range. Larger nominal resistance provides an acceptable level of rated current. However, designers need to ensure that the current is at an acceptable level over the entire temperature range supported by the application.
Sigma-Delta ADCs offer several advantages when designing a thermistor measurement system. First, because the sigma-delta ADC resamples the analog input, external filtering is kept to a minimum and the only requirement is a simple RC filter. They provide flexibility in filter type and output baud rate. Built-in digital filtering can be used to suppress any interference in mains powered devices. 24-bit devices such as the AD7124-4/AD7124-8 have a full resolution of up to 21.7 bits, so they provide high resolution.
The use of a sigma-delta ADC greatly simplifies the thermistor design while reducing specification, system cost, board space, and time to market.
This article uses the AD7124-4/AD7124-8 as the ADC because they are low noise, low current, precision ADCs with built-in PGA, built-in reference, analog input, and reference buffer.
Regardless of whether you are using drive current or drive voltage, a ratiometric configuration is recommended in which the reference voltage and sensor voltage come from the same drive source. This means that any change in the excitation source will not affect the accuracy of the measurement.
On fig. 5 shows the constant drive current for the thermistor and precision resistor RREF, the voltage developed across RREF is the reference voltage for measuring the thermistor.
The field current does not need to be accurate and may be less stable as any errors in the field current will be eliminated in this configuration. Generally, current excitation is preferred over voltage excitation due to superior sensitivity control and better noise immunity when the sensor is located in remote locations. This type of bias method is typically used for RTDs or thermistors with low resistance values. However, for a thermistor with a higher resistance value and higher sensitivity, the signal level generated by each temperature change will be larger, so voltage excitation is used. For example, a 10 kΩ thermistor has a resistance of 10 kΩ at 25°C. At -50°C, the resistance of the NTC thermistor is 441.117 kΩ. The minimum drive current of 50 µA provided by the AD7124-4/AD7124-8 generates 441.117 kΩ × 50 µA = 22 V, which is too high and outside the operating range of most available ADCs used in this application area. Thermistors are also usually connected or located near the electronics, so immunity to drive current is not required.
Adding a sense resistor in series as a voltage divider circuit will limit the current through the thermistor to its minimum resistance value. In this configuration, the value of the sense resistor RSENSE must be equal to the value of the thermistor resistance at a reference temperature of 25°C, so that the output voltage will be equal to the midpoint of the reference voltage at its nominal temperature of 25°C. C. Similarly, if a 10 kΩ thermistor with a resistance of 10 kΩ at 25°C is used, RSENSE should be 10 kΩ. As the temperature changes, the resistance of the NTC thermistor also changes, and the ratio of the drive voltage across the thermistor also changes, resulting in the output voltage being proportional to the resistance of the NTC thermistor.
If the selected voltage reference used to power the thermistor and/or RSENSE matches the ADC reference voltage used for measurement, the system is set to ratiometric measurement (Figure 7) so that any excitation-related error voltage source will be biased to remove.
Note that either the sense resistor (voltage driven) or the reference resistor (current driven) should have a low initial tolerance and low drift, as both variables can affect the accuracy of the entire system.
When using multiple thermistors, one excitation voltage can be used. However, each thermistor must have its own precision sense resistor, as shown in fig. 8. Another option is to use an external multiplexer or low-resistance switch in the on state, which allows sharing one precision sense resistor. With this configuration, each thermistor needs some settling time when measured.
In summary, when designing a thermistor-based temperature measurement system, there are many questions to consider: sensor selection, sensor wiring, component selection trade-offs, ADC configuration, and how these various variables affect the overall accuracy of the system. The next article in this series explains how to optimize your system design and overall system error budget to achieve your target performance.
Post time: Sep-30-2022