WEBENCH® Tools: Filters

Sarang,

Thanks for using the Filter Designer tool from the WEBENCH suite.

I wlll be looking into this problem for you, but I am afraid that it may take a day or two. Please stay tuned.

Dear Bonnie Baker,
Thank you for the prompt reply.

1. Good to know that negative frequency does not allow Filter Designer to simulate.
2. Yes, I am able to design with center frequency = 316Hz, passband bandwith = 500 Hz.

However, request you to help me design bandpass filter with lower/upper 3dB frequency of 20Hz and 5KHz using WEBENCH. Any references to documents would be really helpful. I am looking forward to 2nd order filter.

Regards,
Sarang Suryawanshi

Sarang,

Thank you for continuing to look at Filter Designer solutions for your filters. In terms of the filter that you are looking for, the limits of 20 HZ with the 3dB bandpass bandwidth of 5 kHz is impossible to design with a 2nd order filter. Actually, because your 3dB bandwidth is so wide,this will require a very high order filter to implement these conditions; higher than an order of 20. To get a filter that you can use you need to reduce your 3 dB bandwidth and possibly increase the order.

For instance, here are some filters for you circuit

Center Frequency     Passband 3 dB Bandwidth    Order

20                              100                                        12, 14, 16, 18, 20

20                              50                                          6, 8, 10, 12, 14, 16, 18, 20

20                              20                                          2, 4, 6, 8, 10, 12, 14, 16, 18, 20          

I have found these solutions using Filter Designer.                            

Sarang,

What you are trying to design is what is called a Wide-Band Bandpass Fitler. This is where the ratio of your 3 dB frequencies is 2 or greater. This is the case with your filter. The approprate strategy is to design your bandpass with a low-pass and high-pass filters cascaded to acheive your bandpass response. With this design, you can get your 2nd order filter easily by making the low-pass and high-pass filter both 1st order filters.

I am curious about your filter specifications of cneter frequency equaling 20 Hz and the 3 dB bandwidth of 5 kHz. Are these values based on a particular application? Please provide more information.

Dear Bonnie Baker,

I shall go with the approach of 2nd order high-pass filter cascaded with 2nd order low-pass filter to get 2nd order bandpass filter (Fc=316Hz, Lower&Upper 3dB frequency= 20Hz & 5KHz).

I am building a medical application wherein I shall inject 0.5mA current into the body. The received signal has to go through bandpass filter with the above specifications.

Regards,
Sarang Suryawanshi

Sarang,

This is excellent. I hope you realize that with this configuration you have described you actually have a 4th order bandbass filter, which is very good. This would require 2 op amps. Additionally the selection of 20 Hz and 5 KHz is very appropriate. If you want to flatten out the gain of fthe circit in the flatband region (or at the top of the freuqency response) you may consider using 4th order lowpass and highpass filters. This would actually give you an 8th order bandpass filter, requiring 4 op amps.

Sarang,

Another question, have you considered what amplifier(s) you will use for this circuit? I am assuming that you are using Filter Desiger. If this is the case please look at the selected amplifier in the summary page. If you need further assistance, please ask with this forum.

Dear Bonnie Baker,

Thank you for the assistance.
I am using Filter Designer. As suggested, shall check for the amplifier in the summary page.

Regards,
Sarang Suryawanshi

Sarang,

You are very welcome. If you have any other issues please get back to us.