NXP KTY81/220 Silicon Temperature Sensors: Key Features and Application Design Guidelines
Silicon temperature sensors represent a robust and highly linear alternative to NTC thermistors and other temperature sensing technologies. Among the prominent solutions in this category are the KTY81/210 and KTY81/220 sensors from NXP Semiconductors. These sensors are renowned for their precision, reliability, and ease of integration into a wide array of automotive, industrial, and consumer applications. This article delves into the key characteristics of these devices and provides essential guidelines for their effective implementation.
Key Features of the KTY81/220 Sensor
The KTY81 series sensors are silicon PTC (Positive Temperature Coefficient) temperature sensors. Their operation is based on the well-characterized temperature dependence of the resistance of single-crystal silicon. The KTY81/220, in particular, offers a set of compelling features that make it a preferred choice for designers:
High Linearity: One of the most significant advantages over NTC thermistors is the excellent linearity of the resistance-temperature (R-T) characteristic. This simplifies the required signal conditioning circuitry and calibration processes, reducing both system complexity and cost.
Wide Operating Temperature Range: The sensor is designed to perform reliably across a broad temperature range from -55 °C to +150 °C, making it suitable for demanding environments, most notably in the automotive industry under the hood.
High Accuracy and Stability: The sensors exhibit high initial accuracy and excellent long-term stability. Their performance is resistant to aging effects, ensuring consistent readings over the product's lifetime.
Interchangeability: Devices within the same type number (e.g., KTY81/220) feature tight tolerances and are interchangeable. This is a critical factor for high-volume manufacturing, as it eliminates the need for individual sensor calibration on the production line.
Robustness and Reliability: Constructed in a robust SOD-70 (DO-34) glass package, these sensors are mechanically durable and resistant to moisture and other harsh environmental conditions.
Application Design Guidelines
Successfully integrating a KTY81 sensor into a system requires careful consideration of its operating principles.
1. Excitation Current: The sensor is a passive device, requiring a constant current source for excitation. A typical value is 1 mA, but this must be chosen carefully. Using a constant voltage source is not recommended, as the resulting current would change with the sensor's resistance, introducing non-linearity and measurement errors.
2. Signal Conditioning and Linearization: Although highly linear, the R-T curve of the KTY81 sensor still has a slight parabolic characteristic. For applications demanding the highest precision, this non-linearity can be compensated. The transfer function is defined by:
`R(T) = R0 (1 + CT T + CT2 T²)`
Where `R0` is the resistance at 25°C, and `CT` and `CT2` are temperature coefficients found in the datasheet. This equation can be implemented in the system's microcontroller software to convert the measured resistance into an accurate temperature value.
3. Interfacing with an ADC: A common configuration involves placing the sensor in a voltage divider circuit. The voltage across the sensor, which changes with temperature, is then fed into an Analog-to-Digital Converter (ADC) on a microcontroller.
Component Selection: The choice of the fixed series resistor (`Rseries`) in the divider is crucial. Its value should be chosen to optimize the voltage swing at the ADC input across the desired temperature range, maximizing measurement resolution.
Noise Immunity: For applications in electrically noisy environments (e.g., motor drives), employing filtering, such as a simple RC low-pass filter, on the ADC input line is essential to ensure stable readings.
4. Lead Resistance and Placement: In applications where the sensor is located far from the signal conditioning circuitry, the resistance of the connecting wires can introduce a significant measurement offset. Using a 3-wire or 4-wire (Kelvin) connection method can effectively eliminate the effect of lead resistance.
5. Thermal Management: To ensure accurate measurement of the target's temperature, the sensor must be in good thermal contact with the object. Furthermore, designers must consider self-heating effects caused by the excitation current. Keeping the current low (e.g., 1 mA) minimizes this error.
Conclusion
The NXP KTY81/220 silicon temperature sensor provides an optimal blend of accuracy, linearity, ruggedness, and cost-effectiveness. Its predictable behavior and ease of use make it an excellent solution for system designers who need a reliable temperature measurement solution without the complex linearization challenges associated with NTCs. By adhering to basic design principles—such as using a constant current source, understanding the transfer function, and mitigating noise and lead resistance—engineers can fully leverage the capabilities of this sensor in their applications.
ICGOODFIND: The NXP KTY81/220 stands out as a highly linear and robust silicon temperature sensor, ideal for automotive and industrial applications. Its key advantages of interchangeability, wide operating range, and simple signal conditioning make it a superior and cost-efficient choice for precise temperature monitoring.
Keywords: Silicon Temperature Sensor, PTC Sensor, Temperature Linearity, Signal Conditioning, Interchangeability.
