Sensor Circuits - The Electronic Nose Artificial Olfaction Technology - page 190

Fig. 7.17 Band pass filter

Fig. 7.18 Band reject filter
(Notch filter)

• Band-pass filter removes all frequencies outside (f 1 -f 2 ). (Used often in EEG
measurements) A series combination of the low-pass filter and the high-pass
filter results in a band-pass filter, which amplifies frequencies over a desired
range and attenuates higher and lower frequencies. (A band-pass filter attenuates
both low and high frequencies). A schematic of band-reject filter is shown in
Fig. 7.17 .
• Band reject or notch filter removes all frequencies between f 1 and f 2 . (60 Hz
noise removal). A schematic of band reject pass filter is shown in Fig. 7.18 .

This notch filter can be helpful in rejecting unwanted signals that are on a
particular frequency. The response provided by the filter includes a low level of
attenuation away from the notch frequency. As a signal shifts nearer to the notch
frequency, the level of attenuation rises, giving the typical notch filter response.
Response of various types of filters is illustrated in Fig. 7.19 .
When during the signal analysis, the physical quantity being measured is either
constant or only changing slowly with time, at that time the system noise fre-
quency is high. So there is a requirement of low-pass filter. In a few cases, the
measured signal itself has a high frequency, for instance when mechanical
vibrations are being monitored, and the signal processing required is the appli-
cation of a high-pass filter to attenuate low-frequency noise components. Band-
stop filters can be used where a measurement signal is corrupted by noise at a
particular frequency. Such noise is frequently due to mechanical vibrations or
proximity of the measurement circuit to other electrical apparatuses.

4. Analog-to-Digital Converters for Signal Conditioning Circuit
The conversion of analog signals into digital form is essential in any system that
will use digital methods, i.e., for display, counting, or any other logical actions.
(Conversion means the processing of an analog signal into a set of digital signals).