SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE
Silicon Designs, Inc.
! 1445-NW Mall Street, Issaquah, WA 98027-5344 ! Phone: 425-391-8329 ! Fax: 425-391-0446
web site: www.silicondesigns.com
[page 3]
Jun 07
f
R C
0
2
1
1
2
=
π
Model 2220 Analog Accelerometer Module
CABLE SPECIFICATIONS & LENGTH CONSIDERATIONS
The cable consists of four 28 AWG (7x36) tin plated copper wires with Teflon FEP insulation surrounded by a 40 AWG tin plated
copper braided shield. The shield jacket is Teflon FEP with a nominal outer diameter of 0.096”. Cable lengths of up to 15 meters
(50 feet) can be added to the model 2220's standard 1 meter cable without the need to test for output instability. For lengths
longer than 15 meters we recommend you check each individual installation for oscillation by tapping the accelerometer and
watching the differential output for oscillation in the 20kHz to 50kHz region. If no oscillation is present then the cable length being
used is OK. From the standpoint of output current drive and slew rate limitations, the model 2220 is capable of driving over 600
meters (2000 feet) of its cable type but at some length between 15 and 600 meters, each device will likely begin to exhibit
oscillation.
CONVERTING THE 2220's DIFFERENTIAL OUTPUT TO SINGLE ENDED
C1 = C2 (See below for value calculation)
R1, R2, R3 & R4 = 20k to 50k
R1 = R3 to within 0.1% for good common mode rejection
R2 = R4 to within 0.1% for good common mode rejection
R2 / R1 ratio accurate to within 0.1% for gain control
R4 / R3 ratio accurate to within 0.1% for gain control
To achieve the highest resolution and lowest noise performance from your model 2220 accelerometer module, it should
be connected to your voltage measurement instrument in a differential configuration using both the AOP and AON output
signals. If your measurement instrument lacks differential input capability or you desire to use a differential input capable
instrument in single ended mode, then the circuit above can be used to preserve the low noise performance of the model
2220 while using a single ended type connection.
This circuit converts the ± 4 Volt differential output of the model 2220 accelerometer, centered at +2.5 Volts, to a single
ended output centered about ground (0.0 Volts). It provides the advantage of low common mode noise by preventing the
accelerometer’s ground current from causing an error in the voltage reading.
The op-amp should be located as close as possible to your voltage monitoring equipment so that the majority of the signal
path is differential. Any noise present along the differential path will affect both wires to the same degree and the op-amp
will reject this noise because it is a common mode signal. The op-amp type is not critical; a µA741 or ¼ of a LM124 can
be used. Both plus and minus supplies are needed for the op-amp to accommodate the positive and negative swings of
the single ended output.
For this design, always set R
1 = R3, R2 = R4 and C1 = C2.
The gain of the circuit is then determined by the ratio R
2/R1.
When R
1 = R2 = R3 = R4, the gain equals 1 and the output swing will be ± 4 Volts single ended with respect to ground. To
obtain a ± 5 Volt single ended output, set R
2/R1 = R4/R3 = 5/4 = 1.25.
The single ended output of the op-amp will be
centered at ground if R
2 and C1 are tied to ground; using some other fixed voltage for this reference will shift the output.
The value of the optional capacitors C
1 and C2 (C1 = C2) can be selected to roll off the frequency response to the frequency
range of interest. The cutoff frequency f
0 (-3 dB frequency) for this single order low pass filter is given by:
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