Sensor Device Data

4

Freescale Semiconductor

MMA6200 SERIES

PRINCIPLE OF OPERATION

The Freescale Semiconductor accelerometer is a surface-

micromachined integrated-circuit accelerometer.

The device consists of a surface micromachined capacitive

sensing cell (g-cell) and a signal conditioning ASIC contained in

a single integrated circuit package. The sensing element is

sealed hermetically at the wafer level using a bulk microma-

chined cap wafer.

The g-cell is a mechanical structure formed from semicon-

ductor materials (polysilicon) using semiconductor processes

(masking and etching). It can be modeled as a set of beams at-

tached to a movable central mass that move between fixed

beams. The movable beams can be deflected from their rest

position by subjecting the system to an acceleration (Figure 2).

As the beams attached to the central mass move, the dis-

tance from them to the fixed beams on one side will increase by

the same amount that the distance to the fixed beams on the

other side decreases. The change in distance is a measure of

acceleration.

The g-cell plates form two back-to-back capacitors

(Figure 2). As the center plate moves with acceleration, the dis-

tance between the plates changes and each ca

will change, (C = A

ε/D). Where A is the area of the plate,

ε is the dielectric constant, and D is the distance between

the plates.

The ASIC uses switched capacitor techniques to measure

the g-cell capacitors and extract the acceleration data from the

difference between the two capacitors. The ASIC also signal

conditions and filters (switched capacitor) the signal, providing

a high level output voltage that is ratiometric and proportional to

acceleration.

Figure 2. Simplified Transducer Physical Model

SPECIAL FEATURES

Filtering

These Freescale Semiconductor accelerometers contain an

onboard single-pole switched capacitor filter. Because the filter

is realized using switched capacitor techniques, there is no re-

quirement for external passive components (resistors and ca-

pacitors) to set the cut-off frequency.

Self-Test

The sensor provides a self-test feature allowing the verifica-

tion of the mechanical and electrical integrity of the accelerom-

eter at any time before or after installation. A fourth plate is used

in the g-cell as a self-test plate. When a logic high input to the

self-test pin is applied, a calibrated potential is applied across

the self-test plate and the moveable plate. The resulting electro-

The resultant deflection is measured by the accelerometer's

ASIC and a proportional output voltage results. This procedure

assures both the mechanical (g-cell) and electronic sections of

the accelerometer are functioning.

Freescale Semiconductor accelerometers include fault de-

tection circuitry and a fault latch. Parity of the EEPROM bits be-

comes odd in number.

Self-test is disabled when EEPROM parity error occurs.

Ratiometricity

Ratiometricity simply means the output offset voltage and

sensitivity will scale linearly with applied supply voltage. That is,

as supply voltage is increased, the sensitivity and offset in-

crease linearly; as supply voltage decreases, offset and sensi-

tivity decrease linearly. This is a key feature when interfacing to

a microcontroller or an A/D converter because it provides sys-

tem level cancellation of supply induced errors in the analog to

digital conversion process.

Acceleration