What are the the advantages of a switched capacitor filter over a digital filter?

Question

The main advantage that switched capacitor filters seem to have over digital filters is the potential to achieve a high dynamic range, since the signal does not have to be quantized.

As discrete time filters, they share the digital filter's frequency aliasing problem, and may need multiple clocks at different frequencies as well. Digital filters seem more flexible and easy to design, and analog filters are still needed to deal with frequencies above the Nyquist limit.

What other advantages do switched capacitor filters have over digital filters?

Answer 1

The primary advantage of switched capacitor filters is that they can be easily implemented on an integrated circuit. You can get performance similar to an analog RC op-amp based filter using a switched capacitor topology, while avoiding the need for an ADC, DSP, and DAC on a chip.

Switched capacitor circuits use capacitors and switches to emulate the behavior of resistors. Additionally, the frequency response is determined by the ratio of the capacitors, so even low frequency filters can be easily realized on-chip. The real benefit for IC implementations is that while the absolute value of capacitances and resistances have a poor tolerance, the matching between similar devices is very good. This makes it possible to implement relatively high precision analog filters on a chip.

In an integrated circuit, you would choose a switched capacitor filter for the following reasons:

• Minimizing chip area is a priority
• You will not be doing significant digital processing on the chip
• The output of the DSP would be an analog signal

Practically, you would not use a discrete switched capacitor filter (using op-amps, capacitors, and analog switches) at the board level - you would use an active RC continuous-time filter. There are switched capacitor chips that can provide good filtering results with just a few additional components. Using a general-purpose DSP on a board level design will require additional programming, and may not have analog outputs.

Answer 2

Another advantage is that the filter's frequency characteristics can be changed on the fly by changing the clock speed. The value of the simulated reistors are a function of the capacitance and the clock speed. By varying the clock speed, you can adjust the effective resistance on the fly electronically, something that is hard to do in a controlled and reliable way in analog otherwise.

This feature of switched capacitors can also be used outside of filters for other purposes, like electronically controlled variable gain.

Answer 3

With my years of experimentation with digital and switched capacitor filters the conclusion is:

1. SC filters are essentially digital filters. They also work in discrete time. The only difference is, say the coefficients a,b,c, etc. in a recursive filter difference equation are replaced by functions of Ri s and Ci s in the circuit.
2. As mentioned they save the use of an ADC therefore come in as an advantageous solution for a processor-less or basic processor boards.
3. SC filters are preferred over analog filters because they are tunable. Where adaptive filtering is needed this is a very important issue.
4. In the olden days, SC filters were a more popular topic because digital filters were slower (I have a friend who completed a PhD in SC filters.) With today's options, SC filters are only advantageous over digital filters when space and cost is limited to options which exclude use of ADC circuits.
5. With the abundance of fast processors, digital filters also have the option of running forth and back in time just by data reversal. Hence it is possible to obtain zero phase delay version of any filter at the cost of doubling the filter time.
6. Digital filters do not depend on component tolerances like SC filters. They only rely on good ADC performance which again is in generous supply these days.
7. SC filter pass band cutoff frequency tuning and component values and clock the frequency are interrelated. Changing cutoff frequency must be accompanied usually by a change in the component values assuming a change in clock rate is less desirable (only limited clock rate division choices being available.) This sometimes becomes a costly and severe problem in finding the right component values.
8. Digital filters give complete freedom over tuning, the only restriction being working safely away from the Nyquist frequency.

As a result of all considerations, do try to use a digital filter rather than a SC filter unless really pressed by cost and space in low resolution environments. SC filters also have good stability over analog and digital that a bug in S/W except producing a ridiculous clock rate cannot disturb.

Answer 4

1. Higher order filters (4th order and up) are implemented by simply cascading additional packages, and all the classical filters (such as Butterworth, Bessel, Elliptic, and Chebyshev) can be realized. with 60dB range easily at low cost up to 100KHz.

2. It is easy to make harmonic filters.