Sensor Circuits - The Electronic Nose Artificial Olfaction Technology - page 189

Fig. 7.14 Full wave rectifier
circuit using op-amp

Fig. 7.15 Low pass R-C

filter

capacitors. Active filters are suitable for low frequency, small signals but require
power supply to operate. Passive filter has the added advantage of low noise. An
active filter is also based on op-amp circuits.

• The filter which removes all frequencies above the cut off frequency is defined
as low-pass filter. (Typically used for noise removal and data smoothing.) A
low-pass R-C filter circuit is shown in Fig. 7.15 .

The high-pass filter removes all frequencies below the cut-off frequency. (Used
for DC or low-frequency drift). The high-pass filter is designed to contain a lower
corner frequency close to zero and the cut-off frequency at near to several higher
values. The number of capacitor/resistors joining up decides the number of poles
and the degree of cut-off sharpness. It also operates exactly reverse to low-pass
filter. Also note that a combination of high-pass and low-pass filters may be used
to create a notch filter to attenuate a narrow band of frequencies. Figure 7.16
demonstrates the circuit of high-pass filter.

Fig. 7.16 High pass filter