ALLPCB
1. Bandwidth
Bandwidth generally refers to the signal range where the sensor output falls to 1/sqrt(2) of its maximum response (half power). Informally, it is the range of signal frequencies the sensor can sample and respond to. Bandwidth describes the dynamic behavior of a sensor and whether it can follow the rate of change of the measured quantity. Effective bandwidth denotes the frequency range over which the sensor can guarantee the specified measurement accuracy. In frequency-domain terms, bandwidth is equivalent to frequency response: it reflects the sensor's ability to respond to external signals. From a transfer-function perspective, most sensors can be approximated as first- or second-order systems.
2. Sensitivity
Sensitivity is the ratio of the change in output delta y to the change in input delta x under steady-state conditions. Within the sensor's linear range, higher sensitivity usually produces a larger output signal for a given input change, which can improve the signal-to-noise ratio for subsequent signal processing. However, higher sensitivity can also amplify external noise that is unrelated to the measurement, degrading accuracy. Therefore, sensors should exhibit high intrinsic signal-to-noise ratio to minimize the impact of external interference.
3. Zero Drift
Zero drift occurs when the sensor output changes while the input is zero. Temperature drift is the most common cause of zero drift. Other causes include aging of the sensing element, mechanical stress, charge leakage, and temperature variations.
4. Resolution
Resolution is the smallest change in the measured quantity that the sensor can detect within a specified measurement range. It is the minimum detectable increment of the measurand and has units. For example, a ruler measures to the millimeter level, while a micrometer can detect changes on the order of 0.001 millimeter. If a temperature sensor has a resolution of 0.1 degrees Celsius and a full scale of 500 degrees Celsius, its relative resolution is 0.1/500 = 0.02%.
5. Accuracy
Accuracy refers to the closeness of the measured value to the true value. It can be expressed as the ratio of three times the standard deviation around the true value to the measurement range. If the goal is qualitative analysis, a sensor with good repeatability may suffice; for quantitative analysis, select a sensor whose accuracy meets the required specification.
Causes of systematic error include inherent limits of the measurement principle and algorithms, inaccurate calibration, environmental temperature effects, and material defects. Causes of random error include mechanical backlash, component aging, and similar factors.
6. Repeatability
Repeatability describes the variation in repeated measurements taken in the same direction under identical conditions. Also called repeat error or reproducibility error, smaller repeatability error indicates better repeatability and greater sensor stability.
7. Frequency Response Characteristics
A sensor's frequency response determines the range of signal frequencies it can measure without distortion. Sensor response typically involves some delay; shorter delays are preferable. Sensors with higher frequency response can measure a wider range of signal frequencies. Mechanical systems, due to inertia and structural characteristics, tend to have lower frequency response and are suitable for lower-frequency measurements.
8. Hysteresis
Hysteresis occurs when the input-output mapping differs between increasing and decreasing input values. Causes include material properties of the sensing element and mechanical factors such as friction and backlash.
9. Linear Range
The linear range of a sensor is the span over which output is proportional to input. Ideally, sensitivity is constant within this range, and a wider linear range means a larger usable measurement range. In practice, no sensor is perfectly linear; linearity is relative. When required measurement accuracy is modest, a sensor with small nonlinearity over a given span can be treated as linear for measurement convenience.
10. Sampling Frequency
Sampling frequency is the number of measurements the sensor can acquire per unit time and reflects the sensor's ability to respond quickly. It is a critical specification when measuring rapidly changing signals. Measurement accuracy can vary with sampling frequency: higher sampling rates can reduce per-sample accuracy in some sensors. Sensor datasheet accuracy is often provided at specific sampling speeds or under static conditions, so both speed and accuracy must be considered when selecting a sensor.
