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ADXL210E

Low-Cost ±10 g Dual-Axis Accelerometer with Duty Cycle

Analog Devices 

ADXL210E

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 ADXL210E/

ADXL210.

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

the ADXL210E/ADXL210.

THEORY OF OPERATION

The ADXL210E 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 architec-

ture. 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 ADXL210E

is capable of measuring both positive and negative accelerations

to ± 10 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 mea-

sured 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

acceleration will deflect the beam and unbalance the differential

capacitor, resulting in an output square wave whose amplitude

is proportional 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 TPC 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 ADXL210E 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)/4%

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 ADXL210E 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 ADXL210E.

FERRITE BEAD

100⍀

VDD

CDC

VDD

XOUT

ADXL210E

COM

YOUT

ST

T2

RSET

XFILT

YFILT

XFILT

YFILT

Figure 2.

–8–

REV. 0

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