uoficowboy
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Hi Keith - thanks for the suggestion. I just played around with FilterPro and it is giving me some strange results. I asked for it to give me a -97db attenuation at 10KHz with 500Hz being my passband.Nuhertz is good - I have it myself. However, you requirements would probably be satisfied by Texas Instruments' Filter Pro which is free.
Keith
Actually, this is probably a better example. It is a 4 pole Butterworth and shows the first stage response, the second stage and the overall response. The peak in the second stage is to help to "sharpen up" the cut off point while still giving a 4 pole slope after the cut-off frequency. The Filter Pro help may have some useful information about the different filter types and how they are made up.
Keith.
I think you and LvW are right - a bessel filter is probably the way to go. My application is data acquisition on a active load/battery analysis system. I don't know exactly what the noise will look like just yet, so it's not clear to me how aggressively I should be filtering, and what the dangers are of excessive filtering.Also the time domain behaviour of the filter may be interesting for measurement applications, a bessel (linear phase) filter is also good regarding step or impulse response.
The requirement for a steep pass- to stopband transition is usually set by the application, as in the case of an audio ADC, that wants to achieve 18 or 20 kHz bandwidth with 44 kHz sampling rate.
hey..I used the filter software of microchip company and it helped me a lot. You can try that hope that will also help you a lot.
What is not simulated or calculated is the coupling between the input and output from the physical layout. It is very difficult to get more than 60 dB rejection because of this coupling. The solution is to have two filters in tandem and have them physically separated.
After a bunch of reading, I think I can answer this question now: I just have to make sure that the phase shift at the frequency my (closed loop) control loop is sampling at is less than -180 degrees. Otherwise I could have instability. Does this sound right?Will the phase shift of the circuit cause any issues? My control theory kung fu is a bit weak, but I thought going past 180 degrees phase shift caused major issues. The 4 pole filter suggested gives 360 degree phase shift. Is there any reason to avoid adding more poles? (besides additional component count)
I think you are right that you want low phase shift at your frequency of interest so the phase shift in your stop band is not the critical one.
I haven't measured the stop band of my filters I have built but I think it would be a bad layout/design that could only achieve 60dB. 120dB should be achievable. I have achieved 80dB isolation between a transmitter and receiver of the same board on the same channel.
Keith
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