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ADXL210AE Datasheet(Arkusz danych) 11 Page - Analog Devices

Numer części ADXL210AE
Szczegółowy opis  Low-Cost 10 g Dual-Axis Accelerometer with Duty Cycle
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REV. 0
ADXL210E
–11–
Power Cycling with an External A/D
Depending on the value of the XFILT capacitor, the ADXL210E
is capable of turning on and giving a good reading in 1.6 ms.
Most microcontroller-based A/Ds can acquire a reading in
another 25
µs. Thus it is possible to turn on the ADXL210E
and take a reading in <2 ms. If we assume that a 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
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 ADXL210E 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.
Power Cycling When Using the Digital Output
An alternative is to run the microcontroller at a higher clock rate
and put it into shutdown between readings, allowing the use of the
digital output. In this approach the ADXL210E 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 acquire a reading as quickly as
possible and then shut down the ADXL210E and the microcon-
troller until the next sample is needed.
In either of the above approaches, the ADXL210E can be turned
on and off directly using a digital port pin on the microcontroller to
power the accelerometer without additional components.
CALIBRATING THE ADXL210E
The initial value of the offset and scale factor for the ADXL210E will
require calibration for applications such as tilt measurement. The
ADXL210E architecture has been designed so that these calibra-
tions take place in the software of the microcontroller used to decode
the duty cycle signal. Calibration 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 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 47% duty cycle, then:
Sensitivity = [55% – 47%]/2 g = 4%/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
available from the factory.
Table V. Trade-Offs Between Microcontroller Counter Rate,
T2 Period, and Resolution of Duty Cycle Modulator
Counter-
ADXL210E Clock
Counts
RSET Sample
Rate
per T2
Counts Resolution
T2 (ms) (k ) Rate
(MHz)
Cycle
per g
(mg)
1.0
124
1000
2.0
2000
80
12.50
1.0
124
1000
1.0
1000
40
25.00
1.0
124
1000
0.5
500
20
50.00
5.0
625
200
2.0
10000
400
2.50
5.0
625
200
1.0
5000
200
5.00
5.0
625
200
0.5
2500
100
10.00
10.0
1250 100
2.0
20000
800
1.25
10.0
1250 100
1.0
10000
400
2.50
10.0
1250 100
0.5
5000
200
5.00
USING THE ANALOG OUTPUT
The ADXL210E was specifically designed for use with its digital
outputs, but has provisions to provide analog outputs as well.
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 fre-
quency is connected to the duty cycle output. The filter resistor
should be no less than 100 k
Ω to prevent loading of the output
stage. The analog output signal will be ratiometric to the supply
voltage. The advantage of this method is an output scale factor of
approximately double the analog output. Its disadvantage 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
XFILT and YFILT pin. Unfortunately, these pins have a 32 k
output impedance and are not designed to drive a load directly.
An op amp follower may be required to buffer this pin. The
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 M
Ω. Note that the acceler-
ometer offset and sensitivity are ratiometric to the supply voltage.
The offset and sensitivity are nominally:
0 g Offset = VDD/2
ADXL210E Sensitivity = (20 mV
VS)/g
USING THE ADXL210E IN VERY LOW POWER
APPLICATIONS
An application note outlining low power strategies for the
ADXL210E is available. Some key points are presented here.
It is possible to reduce the ADXL210E’s average current from
0.6 mA to less than 20
µA by using the following techniques:
1. Power cycle the accelerometer.
2. Run the accelerometer at a lower voltage (down to 3 V).




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