In the wonderful world of integrated circuits, filters play a key role in many important functions within a system design. There are many types of filter architectures from which to choose, but one of the myriad of decisions which a designer must work through is the choice of using an analog vs. digital filter.
Let's explore some of the pros and cons of different filter types. Below are some excerpts from several experts on filter design. These comments only touch on a small sampling of the characteristics from which to choose. This discussion will hopefully stir some educational and experiential comments from you, the analog field of experts.
Walt Kester/ADI 
Walt Kester from Analog Devices has this to say about the need for analog filter techniques:
Even in highly oversampled sampled data systems, an analog antialiasing filter is still required ahead of the ADC and a reconstruction (anti-imaging) filter after the DAC. Finally, as signal frequencies increase sufficiently, they surpass the capabilities of available ADCs, and digital filtering then becomes impossible. Active analog filtering is not possible at extremely high frequencies because of op amp bandwidth and distortion limitations, and filtering requirements must then be met using purely passive components.
Walt also talks about digital and analog filtering pros and cons. That information is summarized in Table 1.
One comparison example of an analog vs. digital filter
Let's look at a low-pass filter at 1 kHz. Steven W. Smith observes the following analog vs. digital filter comparisons: Flatness
Typically, an analog filter flatness is governed by the accuracy of the discrete resistors and capacitors used. Residue ripple of 1 percent is not unexpected. The flatness of digital filters is primarily limited by round-off error, making them hundreds of times flatter than their analog counterparts. Frequency response
A digital filter will usually have better rolloff and stopband attenuation. Step response
A digital filter's step response has linear phase; i.e., symmetry between the lower and upper parts of the step waveform. The analog filter has nonlinear phase and is not symmetrical in the lower and upper parts of the step waveform. Overshoot
The analog filter typically has more overshoot on one side of the step, but the digital filter also has overshoot evenly on both sides of the step — but if added together, in total, the overshoot can be close to the analog filter in overall total peak voltage level. Where analog filters shine
Steven W. Smith says, “There are times that an analog filter must be used; for example, as an anti-alias filter in front of an ADC. Analog filters should be used since analog has better speed : digital is slow; analog is fast. For example, a personal computer can only filter data at about 10,000 samples per second, using FFT convolution. Even simple op amps can operate at 100kHz to 1MHz, 10 to 100 times as fast as the digital system.” Switched capacitor filters
Enter the modern switched-capacitor architecture, which uses both digital and analog functions to achieve compactness and tunability.
VLSI circuits use MOS transistors and pico-farad range MOS capacitors (CMOS technology gives an added feature of lowering the power in the IC). So audio and instrumentation applications are hindered due to limited ability of the R and C component values and their accuracy. Designers have overcome these problems by replacing resistors with MOS switches that are rapidly turned on and off and MOS capacitors. Time constants are given as capacitance ratios and capacitors can be matched really well in silicon and will track each other accurately with temperature . See Figure 1.
(Image courtesy of Technology Interface )
Switched capacitor filters have the added features of tenability by simply changing the frequency of the clock pulses that drive the circuit. Footprints are reduced since modern IC packages are small and can support many filtering functions in a single monolithic chip.
Now, high value resistors can be created in a small area on silicon. For example, a 1MΩ resistor can be created by switching a 10pF capacitor at a 100kHz rate.
The switched-capacitor biquadratic filter 
This type of filter, in a conventional design, is usually architected with a lossy inverting integrator, a lossless inverting integrator, and a unity gain inverting amplifier and uses three op amps.
By using a switched-capacitor technology architecture, we can do this with only two op amps. One for the lossy inverting integration function and the other op amp for the lossless inverting integration. See Figure 2.
(Image courtesy of Technology Interface )
The switched-capacitor biquad filter has far improved capacitance ratios as compared with a conventional filter architecture.
Integrated filters and signal processing 
Professor Paul Hasler states the following: Basic directions for integrated circuit filters
- Continuous or Discrete: Time and/or Amplitude
- High level specifications of filters
- Obtaining a filter function (H(s) or H(z))
- Implementing the filter function into basic blocks (first and second-order filter sections, integrators, delays, etc.)
Essentially, digital filters are binary-valued and analog filters can be continuous or multi-valued using a sampled data or continuous time architectures.
Where do we draw the boundary between analog and digital?
To get a sense of where we can put the boundary between filtering via analog techniques and digital techniques, see Figure 3.
(Image courtesy Prof. Paul Hasler )
Analog filters tend to have low SNR and digital has high SNR, but at what cost? It's not so straightforward. See Figure 4.
(Image courtesy Professor Paul Hasler)
For a larger image, click here.
National Instruments weighs in
National Instruments comments on Digital vs. Analog filters in this way:
An analog filter has an analog signal at both its input x(t) and its output y(t) . Both x(t) and y(t) are functions of a continuous variable t and can have an infinite number of values. Analog filter design requires advanced mathematical knowledge and an understanding of the processes involved in the system affecting the filter.
Because of modern sampling and digital signal processing tools, you can replace analog filters with digital filters in applications that require flexibility and programmability, such as audio, telecommunications, geophysics, and medical monitoring applications.
Digital filters have the following advantages compared to analog filters:
- Digital filters are software programmable, which makes them easy to build and test.
- Digital filters require only the arithmetic operations of addition, subtraction, and multiplication.
- Digital filters do not drift with temperature or humidity or require precision components.
- Digital filters have a superior performance-to-cost ratio.
- Digital filters do not suffer from manufacturing variations or aging.
Stanford University's “two cents”
Here's what a Stanford University Center for Computer Research in Music and Acoustics (CCRMA) has to say about digital vs. analog filters:
For our purposes, an analog filter is any filter which operates on continuous-time signals. In other respects, they are just like digital filters. In particular, linear, time-invariant (LTI) analog filters can be characterized by their (continuous) impulse response h(t), where t is time in seconds. Instead of a difference equation, analog filters may be described by a differential equation. Instead of using the z transform to compute the transfer function, we use the Laplace transform (introduced in Appendix D). Every aspect of the theory of digital filters has its counterpart in that of analog filters. In fact, one can think of analog filters as simply the limiting case of digital filters as the sampling-rate is allowed to go to infinity.
In the real world, analog filters are often electrical models, or “analogues”, of mechanical systems working in continuous time. If the physical system is LTI (e.g., consisting of elastic springs and masses which are constant over time), an LTI analog filter can be used to model it. Before the widespread use of digital computers, physical systems were simulated on so-called “analog computers.'' An analog computer was much like an analog synthesizer providing modular building-blocks (such as “integrators'') that could be patched together to build models of dynamic systems.
I hope to have stirred your creative juices. So please let us know of your experiences with filters of both types and examples that you may have had in some of your designs.
- The Technology Interface
- Walt Kester, “Digital Filters” ADI
- Signal Processing Design of Integrated Analog and Digital Filters, Prof. Paul Hasler
- The Scientist and Engineer's Guide to Digital Signal Processing By Steven W. Smith, Ph.D.