Downloaded from https://www.everyspec.com ADS-79D-HDBK mounting location of the sensor. In addition, the sensor‘s cross-sensitivity response (or ―off-axis‖ sensitivity) should be 5% or less than the ―on-axis‖ sensitivity. Sensitivity is measured by the magnitude of the output signal corresponding to a unit input of the measured signal along the specified sensitive axis. It may be expressed as the ratio of the incremental output to incremental input, which is essentially a gain. Cross-sensitivity is the sensitivity along axes that are orthogonal to the direction of the sensitive axis. High sensitivity and low cross-sensitivity are characteristics of good sensors. 19 E.3.1.2 Dynamic range.
The dynamic range of the sensor should extend from the lowest signal amplitude required for detection to the largest expected amplitude such that the sensor signal does not saturate over the intended amplitude range of operation. If the amplitude range is dependent upon the location and orientation, or orientation at which the sensor is mounted, the determination of the required dynamic range should take this dependency into account. The dynamic range of a sensor is determined by the largest and smallest input signals that can be detected or measured by the device.
In most cases the lower limit is dictated by the amplifying electronics noise floor and the higher limit by the voltage rail used by the power supply. E.3.1.3 Linearity. The sensor‘s amplitude linearity should be 1% or less of full scale. Any associated bracketry required to install the sensor on the component of interest should be considered in the measure of linearity. Linearity is determined from the sensor‘s calibration curve which is a plot of the output amplitude versus the input amplitude within the dynamic range of the sensor. The degree to which the calibration curve is a straight line is its linearity.
Linearity is expressed as the maximum deviation of the calibration curve from the least squares straight-line fit of the calibration data in percent of the full scale range of the sensor. E.3.1.4 Drift. Sensor drift should be less than 1% over the expected range of ambient operating conditions. If the sensor drift is greater than 1%, then the parameters inducing the drift should also be measured to permit compensation for the drift. Over a period of time the characteristics of a sensor may change or drift with changes in temperature, pressure, humidity, the power supply, or with aging. Parametric drift is drift that results from parameter changes caused by instrument nonlinearities.
Change in a sensor‘s sensitivity due to temperature changes is an example of a parametric drift. E.3.1.5 Bandwidth. To ensure sufficient sensor response, the bandwidth or useful frequency range of the sensor should exceed the frequency range of interest for the component(s) being monitored. The bandwidth of a sensor is defined as the frequency range over which the magnitude of the ratio of the output to the input does not differ by more than ±3 dB from its nominal value (see Figure E- 1). In the case of an accelerometer, for example, the input is acceleration while the output is volts. Thus the magnitude ratio is in the form of volts/g which varies by no more than 3 dB over its bandwidth.
19 deSilva, Clarence, Control Sensors and Actuators, Prentice Hall, NJ, 1989, pp. 51-53. 102