Sensors

Freescale Semiconductor, Inc.

23

MMA685x

3.5

Transducer

The MMA685x transducer is an overdamped mass-spring-damper system described by the following transfer function:

where:

ζ = Damping Ratio

ω

Reference Section 2.4 for transducer parameters.

3.6

Self-Test Interface

The self-test interface applies a voltage to the g-cell, causing deflection of the proof mass. The self-test interface is controlled

through SPI write operations to the DEVCFG_X register described in Section 3.1.7. The ENDINIT bit in the DEVCFG register

must also be low to enable self-test. A diagram of the self-test interface is shown in Figure 9.

Figure 9. Self-Test Interface

The raw self-test deflection can be verified against raw self-test limits using the following equations:

where:

ΔST

MIN

The minimum self-test deflection over temperature as specified in Section 2.4.

ΔST

MAX

The maximum self-test deflection over temperature as specified in Section 2.4.

SENS

The sensitivity of the device

ΔSENS

The sensitivity tolerance

Hs

()

ω

n

2

s

2

2

ξω

n

s

⋅⋅

⋅

ω

n

2

++

---------------------------------------------------------

=

SELF-TEST

VOLTAGE

GENERATOR

ENDINIT

ST

g-CELL

ENDINIT

ΔST

MINLIMIT

FLOOR

ΔST

MIN

() SENS 1 ΔSENS

–

()

⋅

[]

⋅

⋅

=

ΔST

MAXLIMIT

CEIL

ΔST

MAX

() SENS 1 ΔSENS

+

()

⋅

[]

⋅⋅

=