Analog and Digital Filters, Pros and Cons

[Saad mahsoub]

I am interested in a simple comparison of analog & digital filters in general as to frequency response , hardware requirements, delay, limitations of both filters,....etc, and in general their pros and cons. I am much obliged if anyone can help me with this comparison or guide me to sources of such matter but rather simple detailed ones not the mathematical theory but only pros and cons and much of them.
Regards and many thanks
S. Mahsoub

 

[th3gr8]

1) Analog filters are designed with R, L and C where Digital filters are designed via coding ..
2) To change anything in Analog filter, whole design has to be changed whereas in Digital filters, you just need to change the code ..
3) Digital filters are more reliable and easy to design as compare to Analog filters ..
4) A high order Analog filter may get bulky whereas a high order Digital Filter doesn't get bulky as it is designed via code ..
5) Analog filters are much more affected by surrounding noise as compared to Digital Filters ..

 

[Audioguru]

I have used a switched capacitor filter IC to make an excellent Butterworth lowpass filter and a notch filter. Is it analog or digital? I think it is digital.

 

[Saad mahsoub]

1) Analog filters are designed with R, L and C where Digital filters are designed via coding ..
2) To change anything in Analog filter, whole design has to be changed whereas in Digital filters, you just need to change the code ..
3) Digital filters are more reliable and easy to design as compare to Analog filters ..
4) A high order Analog filter may get bulky whereas a high order Digital Filter doesn't get bulky as it is designed via code ..
5) Analog filters are much more affected by surrounding noise as compared to Digital Filters ..

can you please elaborate more . do analog filters have merits over digital ones?

 

[FvM]

have used a switched capacitor filter IC to make an excellent Butterworth lowpass filter and a notch filter. Is it analog or digital? I think it is digital.

The example shows the need for a more precise discrimination. Digital filters involve both time and amplitude discrete signal processing. Signal processing in switched capacitor filters is time discrete and amplitude continuous. The latter feature qualifies it as analog. But because it's discrete time, the same z-domain calculation methods as for digital filters have to be applied.

 

[LvW]

I have used a switched capacitor filter IC to make an excellent Butterworth lowpass filter and a notch filter. Is it analog or digital? I think it is digital.

I rather think that switched-capacitor filter are classified as "sampled data systems" - and as such they are "between" analog and digital filters ("time discrete and amplitude continuous " as explained by FvM).
Moreover, in response to the answer in post#2 I like to add that - as far as analog filters are concerned - we discriminate between two major classes: Passive RLC and active-RC.

 

[LvW]

To Saad Mahsoub:
Perhaps the following link can answer some of your questions:

http://complextoreal.com/wp-content/uploads/2013/01/filters.pdf

 

[andre_luis]

can you please elaborate more . do analog filters have merits over digital ones?

Digital filters are as precise as its clock rate can be, generaly few more than crystal oscillator near to 0,001%, differently from analog filters that are limited to the combination of multiple components precision, which idividually varies from 0,1% for resistors up to 2-5% for capacitors.

This fact ensures a better accuracy expected on digital filters rather than analog filters.

 

[Saad mahsoub]

Digital filters are as precise as its clock rate can be, generaly few more than crystal oscillator near to 0,001%, differently from analog filters that are limited to the combination of multiple components precision, which idividually varies from 0,1% for resistors up to 2-5% for capacitors.

This fact ensures a better accuracy expected on digital filters rather than analog filters.

Digital filters seem to be better than analog, but on what expense? what are the extra and actual requirements of digital filters?

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To Saad Mahsoub:
Perhaps the following link can answer some of your questions:

http://complextoreal.com/wp-content/uploads/2013/01/filters.pdf

Thanks for your effort. I am reading the file now,it is good.

 

[Saad mahsoub]

I wish i can get a comparison table in a concise manner with points of comparison as to complexity & hardware requirements, stability, linearity in frequency and phase responses, limitations in hardware and probably software . I need to know the drawbacks and limitations of digital filters in general in particular.

 

[andre_luis]

Except for the need to have a reasonable microcontroller system to accomplish the filtering digitally, I see no more drawbacks. For both Analog and Digital approaches, there are currently available lots of powerful tools which significantly reduces development time to somewhat negligible.

 

[LvW]

Except for the need to have a reasonable microcontroller system to accomplish the filtering digitally, I see no more drawbacks.

In many many cases the signal to be filtered is analog.
In case of digital filtering techniques you need (a) analog prefiltering, (b) A-D converter, (c) the filter, (d) D-A converter, (e) analog post filtering.

 

[andre_luis]

Stages (a) and (e) above are required for both implementations, either analog or digital. Furthermore, considering built-in hardware modules available on current microcontrolers ( particularly A/D and PWM ), the external conversion circuitry can be somewhat reduced.

 

[LvW]

Stages (a) and (e) above are required for both implementations, either analog or digital

Pre- and post filtering also for analog???

 

[andre_luis]

You´re right, I guess you was referring to the pre filtering process mentioned above as the pre conditioning for the signal. Anyway, returning to the original question, I would not properly consider these steps as issues, once done digitally stays on the programming scope, and no additional cost is accounted due to the extra core processing.

 

[Saad mahsoub]

Thanks for your comments. One thing I forgot to ask, is what are the points that I should consider when comparing the two filters? I know of linearity, phase response, steepness between cut and pass ranges. If you can, please suggest others.
Thanks.

 

[betwixt]

Nobody has mentioned the upper frequency limitations of digital filters. To process in the digital domain requires a system capable of multiple calculations within the wavelength of the signal, analog does not. Although there is no technical limit to the upper frequency of digital filters, it is decided by the speed the calculations can be performed and hence currently available technology. At the moment, low VHF is about as good as it gets but analog networks can continue to filter effectively well into microwave frequencies.

Brian.

 

[LvW]

For my opinion - such a comparison makes not too much sense. Why?
It´s hard to imagine that in case of signal processing in the analog world (input and output signals of the filter pure analog) somebody decides to replace an analog filter with a digital one. What should he gain? The circuitry certainly will be more complex (in an ealier post I already have mentioned the necessity of analog pre- and post-filtering). However - if continuous tuning of the pole frequecy is desired (what is problematic for pure analog) there is one technology just "between" analog and digital: Switched capacitor filters.

 

[andre_luis]

For filters having critical requirements with respect to the accurate location of poles and zeros, I fear switched capacitor filters can be a problem due to the inherent inaccuracy in characterizing the value of a capacitor implemented in silicon, or even external.

In fact, for each application one or other implementation could be the more appropriate.

 

[Audioguru]

The switched capacitor Butterworth lowpass and notch filter ICs I have used (from National Semiconductor) were very accurate. Their frequency is tuned by changing the clock frequency.