Absolute maximum ratings LIS352AX
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3.1 Terminology
3.2 Sensitivity
Sensitivity describes the gain of the sensor and can be determined by applying 1 g
acceleration to it. Because the sensor can measure DC accelerations, this can be done
easily by pointing the selected axis towards the ground, noting the output value, rotating the
sensor 180 degrees (pointing towards the sky) and noting the output value again. By doing
so, a ±1 g acceleration is applied to the sensor. Subtracting the larger output value from the
smaller one, and dividing the result by 2, produces the actual sensitivity of the sensor. This
value changes very little over temperature (see sensitivity change vs. temperature) and over
time. The sensitivity tolerance describes the range of sensitivities of a large number of
sensors.
3.3 Zero-g level
Zero-g level describes the actual output signal if there is no acceleration present. A sensor
in a steady state on a horizontal surface will measure 0 g on both the X and Y axes,
whereas the Z axis will measure 1 g. A deviation from the ideal 0-g level (1250 mV, in this
case) is called Zero-g offset. Offset is to some extent a result of stress to the MEMS sensor
and therefore the offset can slightly change after mounting the sensor onto a printed circuit
board or exposing it to extensive mechanical stress. Offset changes little over temperature
(see “Zero-g level change vs. temperature” in Table 3: Mechanical characteristics). The
Zero-g level of an individual sensor is also very stable over its lifetime. The Zero-g level
tolerance describes the range of Zero-g levels of a group of sensors.
3.4 Self-test
Self-test (ST) provides a means of testing of the mechanical and electrical parts of the
sensor, allowing the seismic mass to be moved by through an electrostatic test-force. The
self-test function is off when the ST pin is connected to GND. When the ST pin is tied at
Vdd, an actuation force is applied to the sensor, simulating a definite input acceleration. In
this case the sensor outputs exhibits a voltage change in its DC levels. When ST is
activated, the device output level is given by the algebraic sum of the signals produced by
the acceleration acting on the sensor and by the electrostatic test-force. If the output signals
change within the amplitude specified in Table 3, then the sensor is working properly and
the parameters of the interface chip are within the defined specifications.
3.5 Output impedance
Output impedance describes the resistor inside the output stage of each channel. This
resistor is part of a filter consisting of an external capacitor of at least 2.5 nF and the internal
resistor. Due to the resistor level, only small inexpensive external capacitors are needed to
generate low corner frequencies. When interfacing with an ADC, it is important to use high
input impedance input circuitries to avoid measurement errors. Note that the minimum load
capacitance forms a corner frequency close to the resonant frequency of the sensor. In
general, the smallest possible bandwidth for a particular application should be chosen to
obtain the best results.
