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ADXL202E

Low-Cost ±2 g Dual-Axis Accelerometer with Duty Cycle Output

Analog Devices 

ADXL202E

1.06

1.04

1.02

1.00

0.98

0.96

0.94

–45 –30 –15 0 15 30 45 60 75 90

TEMPERATURE – ؇C

TPC 25. Normalized DCM Period (T2) vs. Temperature

DEFINITIONS

T1

Length of the “on” portion of the cycle.

T2

Length of the total cycle.

Duty Cycle Ratio of the “on” time (T1) of the cycle to the total

cycle (T2). Defined as T1/T2 for the ADXL202E/

ADXL210.

Pulsewidth Time period of the “on” pulse. Defined as T1 for

the ADXL202E/ADXL210.

THEORY OF OPERATION

The ADXL202E is a complete, dual-axis acceleration measurement

system on a single monolithic IC. It contains a polysilicon surface-

micromachined sensor and signal conditioning circuitry to imple-

ment an open loop acceleration measurement architecture. For

each axis, an output circuit converts the analog signal to a duty

cycle modulated (DCM) digital signal that can be decoded with

a counter/timer port on a microprocessor. The ADXL202E is

capable of measuring both positive and negative accelerations to

at least ± 2 g. The accelerometer can measure static acceleration

forces such as gravity, allowing it 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 measured

using a differential capacitor that consists of independent fixed

plates and central plates attached to the moving mass. The fixed

plates are driven by 180° out of phase square waves. An accelera-

tion will deflect the beam and unbalance 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 drives a duty cycle modulator

(DCM) stage through a 32 kΩ resistor. At this point a pin is

available on each channel to allow the user to set the signal band-

width of the device by adding a capacitor. This filtering improves

measurement resolution and helps prevent aliasing.

After being low-pass filtered, the analog signal is converted to a

duty cycle modulated signal by the DCM stage. A single resistor

sets the period for a complete cycle (T2), which can be set between

0.5 ms and 10 ms (see Figure 12). A 0 g acceleration produces a

nominally 50% duty cycle. The acceleration signal can be deter-

mined by measuring the length of the T1 and T2 pulses with

a counter/timer or with a polling loop using a low cost micro-

controller.

An analog output voltage can be obtained either by buffering the

signal from the XFILT and YFILT pin, or by passing the duty cycle

signal through an RC filter to reconstruct the dc value.

The ADXL202E will operate with supply voltages as low as 3.0 V

or as high as 5.25 V.

T2

T1

A(g) = (T1/T2 – 0.5)/12.5%

0g = 50% DUTY CYCLE

T2(s) = RSET(⍀)/125M⍀

Figure 1. Typical Output Duty Cycle

APPLICATIONS

POWER SUPPLY DECOUPLING

For most applications a single 0.1 µF capacitor, CDC, will

adequately decouple the accelerometer from signal and noise

on the power supply. However, in some cases, especially where

digital devices such as microcontrollers share the same power

supply, digital noise on the supply may cause interference on

the ADXL202E output. This may be observed as a slowly

undulating fluctuation of voltage at XFILT and YFILT. If additional

decoupling is needed, a 100 Ω (or smaller) resistor or ferrite

beads, may be inserted in the supply line of the ADXL202E.

FERRITE BEAD

100⍀

VDD

CDC

VDD

XOUT

ADXL202E

COM

YOUT

ST

T2

RSET

XFILT

YFILT

XFILT

YFILT

Figure 2.

–8–

REV. A

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