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I've recently been doing a bunch of research on analog filtering in preparation for a data acquisition system I'm building. For context, while I'm fairly comfortable with FPGAs and digital/computer engineering, I have far less experience with analog circuitry. I've been reading Analog Filter and Circuit Design Handbook as a reference, as well as a lot of material online.

There's one question I have been unable to find an answer to in all these sources. I would like to use differential signals as far as possible in this circuit, ideally all the way to the ADCs. However, I can't figure out how to construct active, multi-stage filters with fully differential signals. I used Analog Device's filter wizard to spec out a 4th order Chebyshev response comprised of two 2nd order band-pass multiple feedback stages.

Here's the result: Original circuit

I'm now trying to 'translate' it to fully differential form. I had two ideas on how to do this. The first is to use the first stage to convert to single-ended, which isn't ideal and also I don't know if it would work. The second idea uses differential signals the whole way through, which would require swapping out the op-amps recommended by the filter wizard for fully differential ones (no big deal). My question is: will either of these work? It seems highly sketchy to get rid of their grounding scheme like that, but I don't know enough about this to decide if it'll work. Any help or direction to other resources would be appreciated. Here's the sketch of my two ideas.

My ideas

Thanks! -Seth

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  • \$\begingroup\$ Do you plan to implement the analog filters on the FPGA? or on a PCB? \$\endgroup\$
    – analogsystemsrf
    Commented Aug 10, 2019 at 4:33
  • \$\begingroup\$ No - your handwritten circuits do not work as desired. For both inputs you need two identical feedback networks. Try a google search for "fully symmetrical filters" (pictures) - and you will find many different circuits..... \$\endgroup\$
    – LvW
    Commented Aug 10, 2019 at 8:09

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The first proposal deals with common-mode noise only on the first stage. It might be good enough, or might not, depends on your internal grounding.

The second proposal is something like this: http://www.ti.com/lit/ug/tiducc9a/tiducc9a.pdf This is a high-quality front end for a DAQ system, maybe more what you're after.

That said, you will need to re-arrange the MFT filter stages to have feedback on both sides for the second proposal to work. The TI link above shows this.

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  • \$\begingroup\$ Thanks so much! This is a huge help. \$\endgroup\$
    – sethgi
    Commented Aug 10, 2019 at 14:53

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