ADIS16201PCB datasheet(13/32 Pages) AD

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Data Sheet

ADIS16201

Rev. C | Page 13 of 32

THEORY OF OPERATION

The ADIS16201 is a complete dual-axis digital inclinometer/

accelerometer that uses Analog Devices’ surface-micromachining

process and embedded signal processing to make a functionally

complete, low cost dual-axis sensor.

The ADIS16201 offers a fully calibrated, dual–axis

micromachined sensor element that develops independent

analog signals representative of the acceleration levels applied to

the part. An on-board precision ADC samples the acceleration

signals, along with the power supply voltage, an internal

temperature signal, and the auxiliary analog input signal. These

signals are then processed and latched into addressable output

registers. The serial peripheral interface (SPI) provides

convenient, digital access to these registers.

In addition, the acceleration signals are further processed to

produce inclination angle data for both axes. The inclination

angle data represents the tilt away from the ideal plane, which

in this case, is normal to the earth’s gravitational force. This

calculation assumes that no force outside of the earth’s

gravitational force is acting on the device.

ACCELEROMETER OPERATION

The acceleration sensor used in the ADIS16201 is a surface-

machined, polysilicon structure built on top of a silicon wafer.

Polysilicon springs suspend the structure over the surface of the

wafer and provide a resistance against acceleration forces.

Acceleration causes a deflection in the differential capacitor

structure that includes both fixed plates and plates that are

attached to the moving mass. The fixed plates are driven by a set

of square waves that are 180o out-of-phase from one another.

Acceleration deflects the beam and unbalances the differential

capacitor, resulting in an output square wave whose amplitude

is proportional to acceleration. Phase sensitive demodulation

techniques rectify the signal and determine the direction of the

acceleration. The output of the demodulator is amplified,

digitized, and processed to remove any process variations and

sensitivities to supply variations.

INCLINOMETER OPERATION

The ADIS16201 inclinometer output data is linear with respect

to degrees of inclination and is dependent on no forces, other

than gravity, acting on the device. The ADIS16201 leverages a

simple geometrical relationship to convert its calibrated

acceleration measurements into an accurate inclination angle

estimate. Figure 31 displays the acceleration measurements

associated with each incline angle, along with the resulting

inclination angle estimate produced by the ADIS16201.

One important behavior to observe when using this approach is

the fact that the relationship between the acceleration

measurements and inclination angle is nonlinear. This non-

linear behavior results in larger quantization error changes as

the inclination angle approaches 90°. Figure 32 provides a closer

look at this behavior by illustrating the increase in step size as

the inclination angle estimate increases. Figure 33 offers a direct

relationship between the quantization error and the overall

inclination angle.

90.0

TILT (Degrees)

22.5

45.0

67.5

INCLINATION

ACCELERATION

Figure 31. Acceleration and Inclination Angle vs. Actual Tilt Angle

TILT (Degrees)

INCLINATION

ACCELERATION

Figure 32. Acceleration and Inclination Angle vs. Actual Tilt Angle

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