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A filter is necessary to remove power line frequency and ambient noise at 60
Hz. Since 60 Hz is within the bandwidth of electromyography signals, a notch filter
must be implemented to only attenuate frequencies in the 60 Hz range. The notch
filter is shown in Fig. 23.
VCC
9V
D1
1N4001GP
7
U1A
6
R5
LM3900D
1
26.5kΩ
26.5kΩ
14
VDD
C4
0.2uF
V1
120 Vrms
60 Hz
0°
OUTPUT
5
R6
-9V
R8
100kΩ
R7
13.2kΩ
C5
0.1uF
C6
0.1uF
Figure 23. 60 Hz notch filter
This filter is a derivative of the Twin T notch filter design. This was selected
because it has a high Q factor and an infinitely deep notch. The LM3900 from Texas
Instruments was selected due to its excellent performance characteristics shown in
Multisim 11.0. The AC simulation is shown in Fig. 24.
Figure 24. 60 Hz notch filter AC simulation
It is evident from Figure 24 that the filter sufficiently attenuates 60 Hz noise,
while passing nearby frequencies.
Protoboard testing was conducted on the 60 Hz notch filter and the results
are shown in Fig. 25.
TMJD Diagnostics System
Senior Design Spring 2011
23