Terminology

LIS332AX

8/14

Doc ID 16932 Rev 1

4

Terminology

4.1

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.

4.2

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.

4.3

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.

4.4

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.