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THS4551: TI pdf document contains error? Band pass filter details
I design a narrow band pass filter using filter pro. I have selected the Multifeedback single ended, chebyshev filter, with order 8, stages 4 gain of 1.121 V/V central frequency fc at 2250 Hz, band width 50Hz. Kindly suggest me best Opamp for this design. Also as i used the single ended 10 stage opamp (OPA37E) based amplifier before this filtering stage so do i need to select single ended multi feedback amplifier or i can go with fully differential as like ( @TIDA-01036 High-Q Active Differential Band-Pass Filter Reference Design for Instrumentation Qualification). My design is attached kindly check in the attachment.
narroe band BPF filter pro.pdf
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In reply to kai klaas69:
In reply to Parag chourey:
Kai has explained well the issues with attempting to realize a very high-Q band-pass response. You can see from your original FilterPro printout that the individual stage Q values were running about 133, which the standard single op amp Multiple Feedback (MFB) and Sallen-Key filter topologies can't realistically achieve. It is often recommended to keep the stage Q to a maximum of 10 for the MFB, and about 25 for the Sallen-Key, providing moderately low stage gains are used.
Most often, when very high-Q stages are needed in a filter a state-variable analog filter topology is used. The topology utilizes 3 op-amps for each 2nd-order stage, and because of that much higher Q can be supported. TI offers the UAF42 Universal Active Filter and likely could meet the requirements for a 1 dB, 2.25 kHz fc, 50 Hz BW, Chebyshev filter. You may want to have a look at its datasheet:
I did look for a simpler approach to your filter requirements and tried simulating some different filter topologies. Interestingly, a variation on the MFB that incorporates a degree of positive feedback produced a pretty accurate response. You will find the schematic for the filter as follows:
The filter has a center frequency of 2.25 kHz and a gain of 1.58 V/V (4 dB), and the bandwidth is close to 50 Hz. I used OPA209 op amps having a unity gain-bandwidth of 18 MHz. The op amps need to have a gain-bandwidth of at least 15 MHz to minimize distorting the response. The one characteristics that is missing is the ripple in the passband associated with the 1 dB Chebyshev filter. I've found that to capture the filter ripple the op amps must have very high gain bandwidth in the hundreds of Megahertz, or even in the Gigiahertz.
Unfortunately, high-order filters such as this 8th-order filter are extremely sensitive to component values. I provided exact values for the resistors and capacitors. If they are off from the ideal value by a fraction of a percent, the response can be badly altered. Precision resistors are available, but precision capacitors are harder to come by.
Alternately, I recall there being a two op-amp per stage filter topology that supports high-Q filter band-pass responses in a handbook. I am traveling for TI right now so I don't have access to the Electronic Filter Design Handbook, by Arthur Williams, but I do believe that is where I came across it.
I attached my TINA Spice circuit for the 8th-order band-pass if you would like to try it.
Precision Amplifiers Applications Engineering
In reply to Thomas Kuehl:
Now that I am back in my office I have access to my Electronic Filter Design Handbook, that I mentioned previously. The High-Q band-pass topology that I was thinking about is covered in the Bandpass Filters Section, under the Title, "Dual-Amplifier Bandpass (DABP) Structure." The information indicates that it is usable with Qs up to 150, the Qs are easily adjustable, and component sensitivity is small. The handbook walks you through the design process. It references filter tables that the book includes and provides the equations you will need to apply.
Some time ago, I designed a high-Q 144 kHz bandpass filter for another customer. I have copied an image of it, an 8th-order DABP below so that you can see what a complete filter looks like created from the handbook information.
Smart decision to get the Electronic Filter Design Handbook. It is my "go-to" book when one of TI's active filter design programs doesn't quite have what I need. I have designed many filters based on the information that it provides.
If you do need help selecting op amps for the finalized filter do drop us another e2e post. If for now you can close this e2e inquiry that would be helpful to us.
I have designed one similar design as you mentioned above. Kindly see the schematic and help me to improve it if you found some mistakes.
I reviewed your DBAP filter and its response certainly looks like that of a narrow bandwidth bandpass filter. When I simulate your TINA Spice file I find the filter has an fc = 2.25 kHz, a gain of 6 db (2 V/V), an fl (-3 dB) of 2.213 kHz and an fh (-3 dB) of 2.292 kHz. That results in a -3 dB bandwidth of 79 Hz, and an overall filter Q of 28.5. I am not sure what response type you selected, but possibly a Butterworth. If these are the design goals you had in mind, then the filter should do what you need.
It looks like you are applying the DBAP design procedures provided in the filter handbook correctly.
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