0
\$\begingroup\$

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

\$\endgroup\$
  • \$\begingroup\$ Do you plan to implement the analog filters on the FPGA? or on a PCB? \$\endgroup\$ – analogsystemsrf Aug 10 '19 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 Aug 10 '19 at 8:09
0
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ Thanks so much! This is a huge help. \$\endgroup\$ – sethgi Aug 10 '19 at 14:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.