A survey of MEMS accelerometers found that one product currently does not provide the lowest noise and lowest power consumption at the same time. When comparing a low-noise accelerometer such as the ADXL355 to a popular low-power accelerometer, the ADXL355 performs as follows:
►The noise density is 20 μg/Hz1/2, which is 9 times lower.
►Power consumption is 338 μW, approximately 13 times higher.
When the sensor is not in use, if the application supplies power to the sensor cycle to conserve power, the relationship between noise and power consumption can be quite different. This difference stems from the establishment time, which may surprise some people. In applications where a contiguous array of sensor data needs to be averaged to achieve critical uncertainty conditions, the time required to fill the array directly affects the total settling time. For example, the Allen's variance curve in the ADXL355 data sheet indicates that the 0.01 second mean time reduces its uncertainty to less than 100 μg. To achieve similar levels of uncertainty in low-power sensors, the required averaging time will be 81 times longer than the ADXL355 because the noise reduction of the averaging filter is proportional to the square root of the mean time.
Equations 1 and 2 quantify the trade-off relationship by using the energy required by each sensor/filter combination to support a single data record to achieve this level of accuracy. The results are quite interesting, and the much shorter mean time (tLP = 81 &TImes; tADXL355) results in an energy requirement (EADXL355) of the ADXL355 that is six times lower than the energy requirement (ELP) of a low-power device.
VMIN355
Equation 3 reflects the power consumption, that is, the above energy level divided by the interval time (T) of each measurement.
Observing the graph corresponding to this relationship (Figure 1), you can get several meaningful results. First, when the measurement cycle time (T) is less than 0.81 seconds, the low power device supports continuous operation. Second, the ADXL355 solution consumes less power when the measurement cycle time is greater than approximately 0.13 seconds. At the end of the day, we need to be open-minded, and sometimes the lowest-noise (highest performance) devices can achieve the lowest power solution.
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