2. Filtering

Output signals from sensors contain noise due to various external factors like improper hardware connections, environment etc. Noise gives an error in the final output of system. Therefore it must be removed. In practice, change in desired frequency level of output signal is a commonly noted noise. This can be rectified by suing filters. Following types of filters are used in practice:

1. Low Pass Filter
2. High Pass Filter
3. Band Pass Filter
4. Band Reject Filter

2.1 Low Pass Filter

Low pass filter is used to allow low frequency content and to reject high frequencycontent of an input signal. Its configuration is shown in Figure 2.6.5

Figure 2.6.5 Circuitry of Low Pass Filter

Figure 2.6.6 Pass band for low pass filter

In the circuit shown in Figure 2.6.5, resistance and capacitance are in series with voltage at resistance terminal is input voltage and voltage at capacitance terminal is output voltage. Then by applying the Ohm's Law,we can write,

(2.6.7)

(2.6.8)

From equation 2.6.8 we can say that if frequency of Input signal is low then jωCR would be low. Thus would be nearly equal to 1. However at higher frequency jωCR would be higher, then would be nearly equal to 0. Thus above circuit will act as Low Pass Filter.It selects frequencies below a breakpoint frequency ω = 1/RC as shown in Figure 2.6.6. By selecting suitable values of R and C we can obtain desired values of frequency to pass in.