Campbell Scientific CR9000X Measurement and Control System User Manual
Page 120
Section 3. CR9000X Measurement Details
The digital implementation of the CR9052 FIR filters maintains a group delay
that is independent of frequency (linear phase response). In addition, the
digital filter performance does not change with time, temperature, or
component tolerances. The on-board DSP automatically chooses the
appropriate low-pass filter to anti-alias the input data for the user's desired
measurement rate. If desired, users may load their own coefficients into the
on-board DSP to tailor the FIR filter's frequency response to their own needs
(band pass, band reject, etc.).
CR9052IEPE DC Frequency Response
The CR9052IEPE module has two programmable time constants available: 5
seconds and 0.5 seconds. The advantage of the 0.5 second time constant is that
if you have a step in the voltage (either from a shock to the sensor or when
initially supplying excitation) it will only take 0.5 seconds for 63% of the
voltage step to discharge, while with the 5 second time constant, it would take
5 seconds. See Chart 3.3-2 Step Discharge Rate.
Chart 3.3-2 Step Discharge Rate
0
0 .2
0 .4
0 .6
0 .8
1
1 .2
- 1
0
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 0
2 1
S E C O N D S
V/V_ST
EP RE
SPO
NSE
T a u = 5 .0 S e c o n d s
T a u = 0 .5 S e c o n d s
T im e C o n s t a n t
= 5 S e c o n d s
T i m e C o n s t a n t
= 0 . 5 S e c o n d s
The advantage of the 5.0 second time constant is that it will not result in lower
frequencies being attenuated as much (3 dB at 0.03 Hz) as the 0.5 second time
constant (3 dB at 0.3 Hz). See Chart 3.3-3 Frequency Response for
attenuation plot comparison.
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