Structural Health Monitoring Using Smart Sensors - ideals ...

8.32F
3V
3.45F
3V
Input
1K
1K 1K 1K
Output
1.22F
2.94F
Figure 4.5. Sallen-Key fourth-order active low-pass filter.
Figure 4.6. Four-pole AA filter board.
Magnitude (dB)
0
-10
-20
-30
0 20 40 60 80
Frequency (Hz)
100 120
Figure 4.7. Four-pole AA filter board transfer function.
Design
Measured
was set as 50 Hz by appropriate choice of resistors and capacitors. Note that filters with
other cutoff frequencies can be easily constructed by changing resistors and capacitors.
The final printed circuit board (see Figure 4.6) was manufactured at the Electronics
Services Shop (ESS, 2007) based on the cascaded two second-order filter design. The
filter's transfer function was verified as shown in Figure 4.7. The difference between the
design value and the measured value of the transfer function is within the expected range
of error due to finite precision of the capacitors on the board.
Moreover, another AA filter, an 8-pole Butterworth low-pass filter with a cutoff
frequency of 25 Hz, was designed and fabricated (PCBExpress, 2007; Screaming Circuit,
2007). This filter board has a stopband cutoff frequency, f sb
, of about 60 Hz. Aliasing is
completely eliminated when sampled at 120 Hz or at a higher frequency, at the expense of
46