Thumb rule for selecting filter frequency cut off

[garimella]

Hi

I am experimenting with analog and digital filters. Would like to know the thumb rule for selecting filter frequency cut off.
I want to realize 4th order filter by combining analog + digital

How should I choose analog filter cut off, sampling rate and digital filter cutoff. Requirement is 100Hz 4th order

 

[BradtheRad]

thumb rule for selecting filter frequency cut off.

The raw formula for capacitors:

f= 1 / (2 Pi R C)

Besides a resistor or resistors which are in plain sight, often a neighboring component has some unknown or unseen resistance, which must be taken into account.

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Inductors also are used in filters. There is a formula for cutoff frequency with inductors. However capacitors tend to be more convenient to use for high-pass or low-pass, especially at small Ampere levels.

 

[KlausST]

Hi,

The above formula is correct, but only for first order RC filter.
You asked about 4th order.

My personal opinion: There is no rule of thumb.
It depends.

First please give some basic informations:
* LPF, BPF, HPF?
* filter at the ADC side or DAC side
* does the filter frequency need to be adjustable?

Now you say you want 4th order. It's telling me you have requirements that call for a 4th order filter. But we don't know this requirements. I assume you already know that there is no ideal filter
* A filter always causes amplitude- and phase errors at the passband.
* and a filter always has a finite steepness at the stop band
Thus the cutoff frequency (usually at -3dB ) needs to have some distance to yor frequency range of interest
And the cutoff frequency needs to have some distance to the frequencies you want to suppress.

Thus it's urgent that you first define your requiremens in values with units, no vague text.

* pass band: min and max frequency, max amplitude error.
Optional: phase requirements, setup time

* Stop band: frequency range you want to suppress, and how much you need to attenuate them.

Also tell us more about your application.
Is it audio, or do you want to build a scope, do you have a fixed sample frequency, what resolution, what processing power, what's the input signal, what's the information you are interested in?

Klaus