FXLS8971CF datasheet(90/100 Pages) NXP

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NXP Semiconductors

FXLS8971CF

3-Axis Low-g Accelerometer

A proper flux cleaning process is desirable after reflow soldering.

Cleaning effectiveness can be facilitated by maximizing the gap between sensor and PCB. This is achieved

by removing any copper trace and Solder mask under sensor package, as well as using a thick solder paste

stencil.

20 Glossary

Term

Description

Cross-axis sensitivity

Cross-axis sensitivity is the ratio of the measured acceleration for an axis to the input

acceleration along each axis orthogonal to the measured axis. Cross-axis sensitivity leads to

undesirable nonorthogonality of X, Y, and Z axes in the frame of reference of the assembled

device when mapping to other frames of reference. Cross-axis sensitivity is expressed as a

percentage of the orthogonal input acceleration with six separate cross-axis sensitivity terms

known as Sxy, Sxz, Syx, Syz, Szx, and Szy.

Noise density and RMS-

integrated noise

Noise density is defined as the noise per unit of square root bandwidth and is typically

expressed in units of mg/√Hz or μg/√Hz for a consumer grade accelerometer. Noise is

measured with the device held stationary in a 1g field and isolated from environmental noise

and mechanical vibration. The RMS noise at a given ODR can be estimated as follows:

Nrms = ND * √BW

For example, in the ±2 g FSR, operating in HPM with an ODR of 400 Hz, the estimated RMS

noise would be:

Nrms = 230 µg/√Hz * √(400/2) Hz = 3.25 mg (~ 3.3 LSB).

Output data rate,

decimation factor, and

power consumption

The Output Data Rate (ODR) defines the rate at which acceleration data is output from

FXLS8971CF. Depending on the operating mode and ODR that is selected, different

decimation factors (oversampling ratios) are applied. The decimated output data is supplied

at the ODR rate, even though multiple samples of the sensor data may have been used to

calculate this average result. In FXLS8971CF lowest power mode, the decimation factor

is always 1, leading to the best power performance at the expense of the poorest noise

(resolution) performance. In high performance mode, the decimation factor is automatically

increased to the maximum possible value for a given ODR, resulting in the best noise

(resolution) performance at the expense of the poorest power consumption. In motion

detection mode with BT_MODE=VDD, use of HPM or FPM mode is not advised. Only the

default LPM mode should be used.

Primary

Device that initiates and drives the communication with secondary devices.

Secondary

Device or devices that responds to communication initiated by primary device.

Self-Test

The integrated self-test function can be used to verify correct transducer and signal chain

operation without the need to apply an external acceleration stimulus. When the self-test

function is activated for each axis, an electrostatic actuation force is applied to the proof mass,

simulating a small change in acceleration. The self-test function of the device is independently

exercisable for each axis, along with a selectable displacement direction (polarity). The device

need not be static while exercising the self-test function as it is insensitive to any external

physical acceleration. During a self-test sequence, 2 parameters are computed for each axis,

namely STOC and STOF. STOC is the half difference between Self-Test output for positive

direction (ST_POL=0) and Self-Test output for negative direction (ST_POL=1), whereas STOF

is the half sum between the same. Refer to AN13193[2] for more details on self-test.

Sensitivity

The accelerometer sensitivity, also known as scale-factor, represents the change in

acceleration input corresponding to 1 LSB change in output and is typically measured in either

mg/LSB or LSB/g.

Zero-g offset

The accelerometer zero-g offset describes the deviation of the sensor output from the ideal

0 g value when no motion or gravity is acting on it. With an accelerometer stationary and

placed on a level, horizontal surface, the ideal output is 0 g for the X and Y axes. Likewise,

Table 121. Glossary

FXLS8971CF

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Product data sheet

Rev. 2.1 — 7 March 2024

90 / 100

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