Working of two similar filtering stages

[patan.gova]

Hello,

I already posted so many threads in related to know the working. https://embedded-lab.com/blog/?p=5508

But I don't understand the usage of two similar filtering stages.can someone please explain me

what will be the difference in the output ,if the second signal conditioning stage is replaced by an amplifier stage without using filters.Will the output of the newly used second stage contains some frequencies that were filtered in the first stage.

Thankyou.

 

[chuckey]

The pulses out of the first stage are 5V in amplitude. Further amplification will not increase this, it will just sharpener up the edges and make the waveform " more square".
Frank

 

[patan.gova]

@Frank:I think you didn't understood my question correctly.I will try to explain it more clearly.
The first stage signal conditioning output is around 1.6V.
The second stage signal conditioning output is around 4.7V.
I want to know the difference between the below two
1) first stage signal conditioning + (followed by) second stage signal conditioning
2)first stage signal conditioning + (followed by) Just an amplification stage(without any filters).

Can someone please epxlain me about the above.

thanks.

 

[LvW]

I want to know the difference between the below two
1) first stage signal conditioning + (followed by) second stage signal conditioning
2)first stage signal conditioning + (followed by) Just an amplification stage(without any filters).
.

I think, the basic contents of your question is if one stage filtering will have the same effect as two-stage filtering, correct?
Two active low-pass filter stages in series, of course, will result in a better suppression of higher frequencies if compared with one stage only (40 dB/dec roll-off instead of 20dB/dec above the cut-off).
However, the -3dB cut-off fequency of the series combination will be lower in comparison with the one-stage approach.

 

[patan.gova]

can I please get some information on this- the effect of "3dB cut-off fequency of the series combination will be lower in comparison with the one-stage approach" on the output signals if frequency in the range of 0.7Hz to 2.3Hz(as mentioned in the embedded blog).

 

[Audioguru]

One filter produces a -3dB cutoff frequency. The filters are completely separate so two of then will add and make their total cutoff frequency -6dB so the -3dB cutoff is at a lower frequency.
A Sallen-Key Butterworth filter with two RC stages uses positive feedback to boost the level of the cutoff frequency of each stage so it is -3dB instead of -6dB.

 

[patan.gova]

Thanks but what will be the effect on the output or how the signal changes if the
a) -6dB cutoff frequency
b)-3db cutoff frequency is used

 

[Audioguru]

-3dB is used as the cutoff level of a filter because it is small but is noticeable. 6dB is twice as much of a change in level which cannot be called the cutoff frequency.
Two identical filters in series produce a -3dB cutoff frequency that is droopy (it does not produce a sharp corner) and at a frequency that is half or double the -3dB cutoff frequency of a single filter. You probably need a Butterworth filter that has a sharp corner.

 

[patan.gova]

can someone please tell me a software/tool to practically(with the simulation results) understand this difference in -3db cutoff frequency( and the 40 dB/dec roll-off instead of 20dB/dec above the cut-off).
Thanks.

 

[Audioguru]

One RC stage produces 6db/octave or 20dB/decade rolloff and two RC stages produce 12dB/octave or 40dB/decade rolloff.
The calculation for the cutoff frequency of a stage or of a good filter shows -3dB.

Here is a Sallen-Key 2nd-order Butterworth bandpass filter showing the -3dB (calculate it yourself) and the 12dB per octave rolloff.
The Sim software is LTspice IV.

 

[patan.gova]

Hello Audio guru,
Thankyou for your explanations and suggestions. Now I am clear that the two stages don’t provide sharp cutoff frequency and there is need to use another type of filter.
1)Can you please explain me the steps to build a new filtering stage (that replaces the two stage filtering stages: each one with a HP followed by active-LP filter) that allows a signals in the frequency range of 0.35 to 3.3Hz.
2)Also can I get some links or materials related to the above of sallen-key 2nd-order butterworth bandpass filter (posted image by you).

 

[Audioguru]

The Sallen-Key Butterworth filter details can be found in articles in Google.

 

[crutschow]

If you want some software to help design active filters of various orders and types, FilterPro is a free download from Texas Instruments.

 

[Audioguru]

A problem with FilterPro is that its selection of resistors and capacitors is not logical.
It picks any old value (but you can select the value of one of its parts) as long as the parts calculate correctly.

Usually the opamp has a gain of 1.
A logical Sallen-Key Butterworth lowpass filter has equal capacitor values and one resistor value has double the value of the other resistor.
A logical Sallen-Key Butterworth highpass filter has equal resistor values and one capacitor has double the value of the other capacitor.
Or the resistors can have equal values and the capacitors can also have equal values but the gain of the opamp must be about 1.6 times.
The opamp provides some positive feedback at the cutoff frequency so that the response is flat then drops sharply at the cutoff frequency.

 

[LvW]

In case FilterPro does not meet your requirements there are other alternatives which are more versatile/flexible:

* Filter Free from Nuhertz Technologies (www. filter-solutions.com).
* Filter Wiz Pro from Schematica Software (www.schematica.com).

Both evaluation versions are for free.

 

[crutschow]

And yet another free filter program is FilterLab from Microchip.