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 8]
Mar 07
SINGLE POLE RC FILTER
TWO POLE ACTIVE FILTER
APPLICATION NOTE
Model 1010 Digital Accelerometer
CONVERTING
DIR TO AN ANALOG VOLTAGE
It is best to use a model 1210 or 1221 for analog applications but if a proportional
analog voltage is desired from a model 1010, one can be easily generated by the
connection of a low-pass filter to the DIR output as shown in the schematic
(below).
The table (at right) lists values for R
1 and C2 for various cutoff
frequencies (-3 dB frequency). R
1 is chosen to be at least 100 times the
maximum output impedance of DIR which is 200 . The circuit or instrument that
the 0 to +5V analog output is connected to, must have an input impedance at
least 100 times the value of R
1.
This simple passive RC filter can be used as
long as it provides sufficient rejection of the switching noise present on the DIR
output for the specific application.
CUTOFF
FREQUENCY
(Hz)
R1
(k )
C2
(µF)
200
36.0
.022
800
35.7
.0056
1000
34.0
.0047
1600
30.1
.0033
2000
36.0
.0022
If greater rejection of switching noise is needed, a two pole active filter can be used as shown in the schematic (below).
This circuit has the added advantage of providing a very low output impedance compared with the single pole circuit. Its
disadvantages include greater complexity and the need for 1 or 2 additional supply voltages for the op-amp. For both filter
types, tight tolerance, temperature stable resistors and capacitors should be used. To reject common mode noise over
long signal transmission line lengths, DIR and its complement can be used to drive a pair of wires with a differential filter
placed at the far end of the wires.
Cutoff Frequency
(Hz)
R1 & R2
(k )
R3
(k )
R4 & R5
(k )
C6
(µF)
C7
(µF)
200
21.5
36.0
93.1
.012
.068
800
19.1
43.0
105
.0027
.018
1000
18.2
42.2
102
.0022
.015
1600
17.4
38.3
93.1
.0015
.010
2000
21.5
36.0
93.1
.0012 .0068
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