ADXL202 Просмотр технического описания (PDF) - Analog Devices

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ADXL202

Low Cost ±2 g/±10 g Dual Axis iMEMS® Accelerometers with Digital Output

Analog Devices 

ADXL202/ADXL210

USING THE ANALOG OUTPUT

Power Cycling When Using the Digital Output

The ADXL202/ADXL210 was specifically designed for use with An alternative is to run the microcontroller at a higher clock

its digital outputs, but has provisions to provide analog outputs rate and put it into shutdown between readings, allowing the

as well.

use of the digital output. In this approach the ADXL202/

Duty Cycle Filtering

An analog output can be reconstructed by filtering the duty

cycle output. This technique requires only passive components.

The duty cycle period (T2) should be set to 1 ms. An RC filter

with a 3 dB point at least a factor of 10 less than the duty cycle

ADXL210 should be set at its fastest sample rate (T2 = 0.5 ms),

with a 500 Hz filter at XFILT and YFILT. The concept is to ac-

quire a reading as quickly as possible and then shut down the

ADXL202/ADXL210 and the microcontroller until the next

sample is needed.

frequency is connected to the duty cycle output. The filter resis- In either of the above approaches, the ADXL202/ADXL210

tor should be no less than 100 kΩ to prevent loading of the

can be turned on and off directly using a digital port pin on the

output stage. The analog output signal will be ratiometric to the microcontroller to power the accelerometer without additional

supply voltage. The advantage of this method is an output scale components. The port should be used to switch the common

factor of approximately double the analog output. Its disadvan-

tage is that the frequency response will be lower than when

using the XFILT, YFILT output.

XFILT, YFILT Output

The second method is to use the analog output present at the

E XFILT and YFILT pin. Unfortunately, these pins have a 32 kΩ

output impedance and are not designed to drive a load directly.

An op amp follower may be required to buffer this pin. The

T advantage of this method is that the full 5 kHz bandwidth of the

accelerometer is available to the user. A capacitor still must be

added at this point for filtering. The duty cycle converter should

be kept running by using RSET <10 MΩ. Note that the acceler-

E ometer offset and sensitivity are ratiometric to the supply volt-

age. The offset and sensitivity are nominally:

0 g Offset = VDD/2

2.5 V at +5 V

ADXL202 Sensitivity = (60 mV × VS)/g 300 mV/g at +5 V, VDD

L ADXL210 Sensitivity = (20 mV × VS)/g 100 mV/g at +5 V, VDD

USING THE ADXL202/ADXL210 IN VERY LOW POWER

APPLICATIONS

O An application note outlining low power strategies for the

ADXL202/ADXL210 is available. Some key points are pre-

sented here. It is possible to reduce the ADXL202/ADXL210’s

average current from 0.6 mA to less than 20 µA by using the

S following techniques:

1. Power Cycle the accelerometer.

2. Run the accelerometer at a Lower Voltage, (Down to 3 V).

Power Cycling with an External A/D

B Depending on the value of the XFILT capacitor, the ADXL202/

ADXL210 is capable of turning on and giving a good reading in

1.6 ms. Most microcontroller based A/Ds can acquire a reading

O in another 25 µs. Thus it is possible to turn on the ADXL202/

pin of the accelerometer so the port pin is “pulling down.”

CALIBRATING THE ADXL202/ADXL210

The initial value of the offset and scale factor for the ADXL202/

ADXL210 will require calibration for applications such as tilt

measurement. The ADXL202/ADXL210 architecture has been

designed so that these calibrations take place in the software of

the microcontroller used to decode the duty cycle signal. Cali-

bration factors can be stored in EEPROM or determined at

turn-on and saved in dynamic memory.

For low g applications, the force of gravity is the most stable,

accurate and convenient acceleration reference available. A

reading of the 0 g point can be determined by orientating the

device parallel to the earth’s surface and then reading the output.

A more accurate calibration method is to make a measurements

at +1 g and –1 g. The sensitivity can be determined by the two

measurements.

To calibrate, the accelerometer’s measurement axis is pointed

directly at the earth. The 1 g reading is saved and the sensor is

turned 180° to measure –1 g. Using the two readings, the sensi-

tivity is:

Let A = Accelerometer output with axis oriented to +1 g

Let B = Accelerometer output with axis oriented to –1 g then:

Sensitivity = [A – B]/2 g

For example, if the +1 g reading (A) is 55% duty cycle and the

–1 g reading (B) is 32% duty cycle, then:

Sensitivity = [55% – 32%]/2 g = 11.5%/g

These equations apply whether the output is analog, or duty

cycle.

Application notes outlining algorithms for calculating accelera-

tion from duty cycle and automated calibration routines are

ADXL210 and take a reading in <2 ms. If we assume that a

available from the factory.

20 Hz sample rate is sufficient, the total current required to

take 20 samples is 2 ms × 20 samples/s × 0.6 mA = 24 µA aver-

age current. Running the part at 3 V will reduce the supply

current from 0.6 mA to 0.4 mA, bringing the average current

down to 16 µA.

The A/D should read the analog output of the ADXL202/

ADXL210 at the XFILT and YFILT pins. A buffer amplifier is

recommended, and may be required in any case to amplify the

analog output to give enough resolution with an 8-bit to 10-bit

converter.

–10–

REV. B

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