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FILTERPRO: Ask for recommendation on differential band-pass filter applications

Yu-Hung
Expert 1130 points

Part Number: FILTERPRO

Hi,

I would like to find a differential band-pass filter with 37.5KHz cutoff freq and variable gain(1~30 V/V).

But there is single-end filter only in the TI's Filter Design Tool.

Does it has differential filter in Filter Design Tool?

YHH

  • 0
    TI__Guru* 93105 points

    Hello YHH,

    TI's new Filter Design Tool is in the beta test phase and doesn't include FDA topologies yet. That will be coming in a version after the beta testing has completed.

    The original FIlterPro V3.1 filter tool does include the multiple feedback (MFB) FDA topology and you should be able to synthesize a 37.5 kHz bandpass filter using it. You can access the FilterPro V3.1 tool here:

    Please be aware that there is an error in the FDA filter schematics that the program provides. They show a wire connection between the FDA differential outputs, which isn't correct; that wire shouldn't be included in the actual circuit. Otherwise, the rest of the filter information and schematic are correct.

    Do not attempt to make the FDA active filters have variable gain, but instead decide on a fixed gain for the filter. Most filter topologies are not intended for a variable gain and some that do often compromise other characteristics of the filter response; gain, Q, bandwidth, etc.

    It would be best to set the filter gain to the minimum level, and then use a separate gain stage that can have its gain setting changed from 1 to 30 V/V as needed.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    Expert 1130 points

    Hi Thomas,

    Yes, there is FDA option in FilterPro V3.1.

    Thank you for your recommendation.

    What is "They show a wire connection between the FDA differential outputs" means?

    Regards,

    YHH

  • 0 in reply to Yu-Hung
    Guru 47485 points

    Hello YHH,

    Also, it is pretty easy to adapt single ended designs to differential out of any tool as shown in the attached file, just duplicate every element on each side and take the resistor to ground on the single ended design and double it to appear differentially across the two side,

    Single ended to differential BP MFB changes.docx

    Also, in the example you sent, those are unrealizable R values and Q's.

    Your FDA options for implementation would be, in ascending speed,

    THS4531

    THS4131

    THS4551

    THS4541

  • 0 in reply to Michael Steffes
    Expert 1130 points
    Hello Michael,

    Thanks for your info.
    I will try to make a test by TI-TINA.
    Hope everything fine. :)

    YHH
  • 0 in reply to Yu-Hung
    TI__Guru* 93105 points

    Hi YYH,

    It looks like your FilterPro schematic for the FDA filter is good. My version of the software may be older and has the short showing up across the outputs in the schematic. I've added a red "X" to the wires that would need to be opened.

    I am posting this information for any one else who might have the issue show up when they synthesize an FDA filter using FilterPro.

    Regards, Thomas

    Precision Amplifiers Applications Engineering