ADXL213
Rev. A | Page 8 of 12
THEORY OF OPERATION
EARTH'S SURFACE
04742-0-015
TOP VIEW
(Not to Scale)
PIN 8
X
OUT
= 50%
Y
OUT
= 80%
X
OUT
= 50%
Y
OUT
= 50%
PIN 8
X
OUT
= 50%
Y
OUT
= 20%
PIN 8
X
OUT
= 80%
Y
OUT
= 50%
PIN 8
X
OUT
= 20%
Y
OUT
= 50%
Figure 21. Output Response vs. Orientation
The ADXL213 is a complete dual axis acceleration measure-
ment system on a single monolithic IC. It contains a polysilicon
surface-micromachined sensor and signal conditioning
circuitry to implement an open-loop acceleration measurement
architecture. The output signals are duty cycle modulated
digital signals proportional to acceleration. The ADXL213 is
capable of measuring both positive and negative accelerations to
±1.2 g. The accelerometer can measure static acceleration forces
such as gravity, allowing the ADXL213 to be used as a tilt
sensor.
The sensor is a surface-micromachined polysilicon structure
built on top of the silicon wafer. Polysilicon springs suspend the
structure over the surface of the wafer and provide a resistance
against acceleration forces. Deflection of the structure is mea-
sured using a differential capacitor that consists of independent
fixed plates and plates attached to the moving mass. The fixed
plates are driven by 180° out-of-phase square waves. Accelera-
tion deflects the beam and unbalances the differential capacitor,
resulting in an output square wave whose amplitude is propor-
tional to acceleration. Phase sensitive demodulation techniques
are then used to rectify the signal and determine the direction
of the acceleration.
The output of the demodulator is amplified and brought off-
chip through a 32 kΩ resistor. At this point, the user can set the
signal bandwidth of the device by adding a capacitor. This
filtering improves measurement resolution and helps prevent
aliasing.
After being low-pass filtered, the duty cycle modulator converts
the analog signals to duty cycle modulated outputs that can be
read by a counter. A single resistor (R
SET
) sets the period for a
complete cycle. A 0 g acceleration produces a 50% nominal duty
cycle. The acceleration can be determined by measuring the
length of the positive pulse width (t1) and the period (t2). The
nominal transfer function of the ADXL213 is
Acceleration = ((t1/t2) – Zero g Bias)/Sensitivity
Where in the case of the ADXL213
Zero g Bias = 50% nominal
Sensitivity = 30%/g nominal
t2 = R
SET
/125 M
PERFORMANCE
Rather than using additional temperature compensation
circuitry, innovative design techniques have been used to ensure
that high performance is built in. As a result, there is essentially
no quantization error or nonmonotonic behavior, and
temperature hysteresis is very low (typically less than 10 mg
over the –40°C to +85°C temperature range).
Figure 9 shows the zero g output performance of eight parts (X
and Y axis) over a –40°C to +85°C temperature range.
Figure 12 demonstrates the typical sensitivity shift over
temperature for V
S
= 5 V. Sensitivity stability is optimized for
V
S
= 5 V, but is still very good over the specified range; it is
typically better than ±2% over temperature at V
S
= 3 V.
