Engineers' thoughts on analog filter design

When talking about filter design, we need to understand two issues within the scope of our current knowledge:

The first is what the essential characteristics of this analog filtering are;

The second is whether the system I want to design is digital filtering or analog filtering.

Understanding the essential difference between digital filtering and analog filtering. Here, I define it from my own point of view. Analog filtering is equivalent to keeping the enemy out of the country for interference signals, while digital filtering is equivalent to opening the door to let the wolf in first and then closing the door to fight the wolf. The two are consistent in strategy, both of which kill the interference signal; the difference in tactics is so great that many times because of this issue, digital filtering and analog filtering design are opposed to each other. As a result, each side says that its own side is good, especially today, the computing speed of single-chip microcomputers is faster, the price of DSP has dropped, and FPGA has become more and more popular, making more digital system engineers more inclined to use digital filters when understanding filter design, and more people would rather ideally think that the biggest advantage of using digital filters is that they can be adjusted unlimitedly through software until their characteristics meet the design requirements. This sounds exciting, but it is not the case.

Take the low-pass filter as an example. The analog low-pass filter filters out the high-frequency noise and peak noise in the signal before it reaches the A/D converter, while digital filtering often cannot filter out the peak noise in the analog signal. When the peak noise is close to the full scale of the A/D, the analog modulator of the A/D converter may enter the saturation state. At this time, even if the average value of the input signal is within the range, this saturation will still occur. Some types of A/Ds have a feeling of "it's easier to invite a god than to send him away" for this kind of peak noise. After entering saturation, it is difficult to desaturate the A/D as a whole, and sometimes it is even necessary to turn off the power to solve the problem.

Of course, we can never say that digital filtering is bad, or directly deny it. After all, digital filtering can reduce noise in the frequency band by using its averaging technology and oversampling technology. Digital filtering can successfully filter out the noise introduced in the A/D conversion process, such as quantization noise, while analog filtering cannot complete such tasks. The programmable design of digital filtering has greater flexibility than analog filtering design. Today, various forms of filters implemented by digital filtering, including Butterworth, Bessel, Chebyshev, elliptical, FIR, IIR, and FFT-type digital filters, have flexible applications that are beyond the reach of analog filters in some performance. Of course, the development of chip technology has also spawned the complementary advantages of the two. The emergence of new switched capacitor filters seems to indicate that the two are also absorbing the good side of each other. At present, MAXIM, LINER, TI and other companies have their own unique switched capacitor filter products, and the performance is also good. CYPRESS has independently integrated switched capacitor modules and analog modules, digital modules and single-chip microcomputer cores. You can flexibly configure its PSOC to implement various filters according to your needs, which represents a design direction.

The filters I focus on here are analog filters below M level widely used in automatic detection, automatic control and electronic instruments. The prosperity of digital filters does not hinder my love for analog filter applications. The emergence of digital filters and switched capacitor filters cannot completely replace the analog filter design that has been used for decades and has not been fully understood yet. Otherwise, there is no need for us to learn analog filter design. There must be a way to defend against the enemy outside the country, and there must be a way to steal, but everyone's way is different, which does not hinder my understanding of my own design.