Difference of using digital filters in place of these analog filters in the circuit

[patan.gova]

Hello,
I am using the circuit as shown here(the first stage of filtering) https://embedded-lab.com/blog/?p=5508with the highpass and lowpass filter.I am planning to implement these filters digitally using microcontroller.
Can I know what are the advantages and disadvantages of analog filters over digital filters or vice versa for these kind of filters as shown in the circuit.
Thanks.

 

[crutschow]

The main advantage of a digital filter is the filter characteristics are precisely determined by the digital filter program and the accuracy of the µC clock frequency. You don't have to be concerned with part tolerances affecting the filter frequency as you do with analog filters.

A disadvantage is that you still need an analog anti-aliasing filter in front of the digital filter to remove any frequencies about 1/2 the digital filter sample frequency or they will appear in the signal as noise. Per the Nyquist criteria, the sample frequency must, of course, be at least twice the highest signal frequency of interest. Also you need a µC with an A/D input and a D/A output operating at the sample rate if you want an analog input and output signal.

 

[chuckey]

And a nice high level signal to get a good S/N at the output, so you will need the IC circuit as a pre-amp!!
Frank

 

[crutschow]

Yes, chuckey makes a good point. The analog filter in that circuit has a gain of 101 so you will still need an amplifier for that.

 

[patan.gova]

Thanks fro explaning such an important things.I got a cuple of doubts as
1)In the post4 it is mentioned like each stage of circuit has a gain of 101, can't it be possible to use both digital filters stages(each having a gain of 101) that replaces two stages of analog.Will there be any difference.
2)And willl it work fine with 1st stage of analog and 2nd stage with digital filters.
3)What can be the minimum sampling freq that should be used/needed for the ADC sampling if the signal to be sampled has a freq of 1Hz.
Can you explain the above things.
Thanks.

 

[chuckey]

A to Ds work by comparing the incoming analogue with a known step. say you have an eight A to D with a range of 0 -> 5V, then each step will be 5 / 2^8 = 5/256 ~ 20 mV. If your incoming analogue signal is less then 20 mV, the A/D will remain stubbornly at 00000000!, a 45 mV signal would give you 00000010. So it is important that your analogue signal uses the whole range of the A/D. So if the maximum analogue is expected to be say 100 mV, then you need an analogue amplifier to raise this to the 5v = gain of 50.
The minimum sampling frequency is twice the maximum incoming frequency, this is a theoretical figure and it should be be greater to stop errors. Imagine a 1 HZ sine wave, If you sample it at twice the one HZ (2 HZ), You measure exactly at the peaks, so your sample gives you, +v, -v, +v, -v. .. This looks OK. Suppose the incoming frequency is raised to 2 HZ, when sampled at 2HZ, will give you +v, +v, +v, +v. . Which means that your A/D is reading the incoming AC as DC. !!.
You must precede the A/D with an anti-aliasing filter to stop this effect, this is a filter that limits the high frequency signal input to the A/D. This filter obviously limits the bandwidth of the response of the A/D, so select your maximum frequency that you want to measure, design the filter with a cut off slightly higher, say +20% to allow for the filter not being perfect. So now you have your 1 HZ bandwidth and you want to be able to measure at 4000 places during its one cycle (1000 up to positive peak, thousand down to zero, same for negative peak) , this then gives you a sampling speed of 4000 in a second so each one must be every 250 microseconds. If you use a better A/D, say 14 bit, then the sampling speed must be that much higher.
Can you see that what with an analogue amplifier front end, followed by an anti aliasing filter, you have virtually built what you want?
Frank

 

[crutschow]

In summary a digital filter will not significantly save any circuitry in your application. Digital filters can be an advantage for high order or complex filters which require precise and stable corner frequencies. For simple filtering such as here, analog is probably best.

 

[LvW]

In summary a digital filter will not significantly save any circuitry in your application. Digital filters can be an advantage for high order or complex filters which require precise and stable corner frequencies. For simple filtering such as here, analog is probably best.

Yes - I support this statement. There are many applications - although it seems that we live in a "digital world" - where a pure analog solution is more economical than a digital one.