SWITCHED CAPACITOR FILTERS

The principle circuit of a switched capacitor filter has been demonstrated in the figure 25.5 shown below The basic low-pass filter is made up of 2 capacitors, along with CMOS switches that are turned on alternately by a built-in clock oscillator. The clock runs at more than a few tens of kilohertz.

The switch C1 samples the input voltage when the 1st switch is closed & the 2nd switch is open. This voltage may be +ve or -ve depending on the phase of the input signal at that instant. An instant later the switches change state & the charge on C1 is equalized with the charge on C2, which came from the preceding sampling.

The generated voltage on C2 is read by the op-amp, which is connected as a unity gain voltage follower. The op-amp has a very high input resistance, so virtually no charge is lost from C2. The output from the op amp is the filtered signal.

The process continues with switch C1 again and again sampling the input, and then sharing its charge with C2. The mathematics of the circuit is a bit complex but it can be shown that it acts like a low-pass filter. At low frequencies, there are only small voltage changes between samplings so the filter is able to easily respond to these changes. At higher frequencies, it is less able to respond so the signal is reduced or lost.

The significant point about the switched capacitor-filter is that its frequency response depends on frequency of its clock. The negative 3 dB cut-off frequency can be correctly tuned by means of adjusting the clock rate. normally the cut-off frequency is fclock/100.

There are Filters having more complicated design but they are operating on the same principle are manufactured as integrated circuits. A representative example is the MAX7480 8th order filter with a Butterworth response, which posses a very flat pass-band and sharp cut-off. It is tunable over the range 1 kHz to 2 kHz. Its action is equivalent to a passive filter built up from 4 capacitors & 4 inductors. The saving of space & weight gives the switched capacitor filter a very big benefit.