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 7]
Aug 05
APPLICATION NOTE
Model 1221 Accelerometer
$'',1*$6,1*/((1'('287387727+(02'(/
',))(5(17,$/287387$&&(/(520(7(5
R1 = R2 = 5.00K ±0.5% for precision 2.50V ref.
R3, R4, R5 & R6 = 20k
6 to 50k6
R3 = R5 to within 0.1% for common mode rejection
R4 = R6 to within 0.1% for common mode rejection
R4 / R3 ratio accurate to within 0.1% for gain control
R6 / R5 ratio accurate to within 0.1% for gain control
C1 = C2 = 0.01
F ceramic
To achieve the highest resolution and lowest noise performance from your model 1221 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 1221 while using a single ended type connection.
This circuit converts the ± 4 Volt differential output of the model 1221 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 ground current of the model 1221 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
A741 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 signal.
The gain of the op-amp is determined by the ratio R4/R3 (where R4=R6 and R3=R5). If R3 through R6 are all the
same value, 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 R4/R3 = R6/R5 = 5/4 = 1.25. The single ended output of the op-amp will be centered at
ground if R4 and C3 are tied to ground; using some other fixed voltage for this reference can shift the output. The value
of the optional capacitors C3 and C4 (C3=C4) can be selected to roll off the frequency response to the frequency range
of interest.
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