2420-002 Datasheet, PDF (3/4 Pages) List of Unclassifed Manufacturers – TRIAXIAL DIGITAL ACCELEROMETER MODULE

Model 2420 Triaxial Digital Accelerometer Module
SIGNAL DESCRIPTIONS
CLK (input): Reference clock input. This hysteresis threshold input must be driven by a 50% duty cycle square wave signal.
Factory Calibration is performed at 250 kHz but the 2420 will operate at frequencies as low as 100 kHz or as high as 1 MHz.
CNT (output): Pin 10. Count output. A return-to-zero type digital pulse stream whose pulse width is equal to the input CLK logic
high time. The CNT pulse rate increases with positive acceleration. The device experiences positive (+1g) acceleration with
its lid facing up in the earth’s gravitational field. This signal is meant to drive an up-counter directly.
Pulses from the CNT output are meant to be accumulated in a hardware counter. Each pulse accumulation or sample, reflects
the average acceleration (change in velocity) over that interval. The sample period or “gate time” over which these pulses are
accumulated determines both the bandwidth and quantization of the measurement.
Quantization (g' s) =
g_SPAN • f _SR
fCLK
f _CNT
=
f
CLK



1
2
+
g _FORCE


g_SPAN 
g _FORCE
=

g
SPAN


f _CNT
f _CLK
−
1
2
Where:
gSPAN = 2 * (full scale acceleration in g' s)
f _SR = CNT sample rate in Hertz
fCLK = accelerometer clock rate in Hertz
fCNT = CNT pulse rate in pulses / sec
g_FORCE = acceleration in gravity units
1 g = 9.807 m / s2 or 32.175ft / s2
The first equation above shows that as the sample rate is reduced (i.e. a longer sample period), the quantization becomes finer
but bandwidth is reduced. Conversely, as the sample rate is increased, quantization becomes coarser but the bandwidth of the
measurement is increased. The second and third equations show how the CNT pulse frequency equates to the applied g-force.
When using a frequency counter to monitor the CNT output pulse rate, a counter with a DC coupled input must be used. The
CNT output is a return-to-zero signal whose duty cycle varies from zero to fifty percent, from minus full scale to positive full scale
acceleration. A frequency counter with an AC coupled input will provide an erroneous reading as the duty cycle varies
appreciably from fifty percent.
CABLE LENGTH CONSIDERATIONS
Cable lengths of up to 45 meters (150 feet) can be added to the model 2420's standard 1 meter cable when it is being used with
a Model 3320 G-LOGGER_TM. The extension cable must however, be a longer version of the G-LOGGER_TM 9 conductor flat cable
instead of the 2420's round cable to reduce crosstalk between the CNT and CLK signals. 15 meter (50 foot) cable lengths of
DB9 male to DB9 female flat extension cable are available on special order.
SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE
Silicon Designs, Inc. ! 1445 NW Mall Street, Issaquah, WA 98027-5344 ! Phone: [phone] ! Fax: [phone]
web site: www.silicondesigns.com
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Sep 07