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ADXL210AQC Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL210AQC
Szczegółowy opis  Low Cost -2 g/-10 g Dual Axis iMEMS Accelerometers with Digital Output
Pobierz  11 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL202/ADXL210
REV. B
–10–
USING THE ANALOG OUTPUT
The ADXL202/ADXL210 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
frequency is connected to the duty cycle output. The filter resis-
tor 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 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
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 volt-
age. The offset and sensitivity are nominally:
0 g Offset = VDD/2
2.5 V at +5 V
ADXL202 Sensitivity = (60 mV
× V
S)/g
300 mV/g at +5 V, VDD
ADXL210 Sensitivity = (20 mV
× V
S)/g
100 mV/g at +5 V, VDD
USING THE ADXL202/ADXL210 IN VERY LOW POWER
APPLICATIONS
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
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
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
in another 25
µs. Thus it is possible to turn on the ADXL202/
ADXL210 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 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.
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 ADXL202/
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.
In either of the above approaches, the ADXL202/ADXL210
can be turned on and off directly using a digital port pin on the
microcontroller to power the accelerometer without additional
components. The port should be used to switch the common
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
available from the factory.




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