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I'm designing an audio DAC based on the PCM1690 from TI. Datasheet

One requirement of the DAC is differential outputs. Since the PCM1690 has differential outputs, this is not a problem. However, the design of the low pass output filter is unclear for me. Figure 40 of the datasheet shows an example circuit, but this circuit converts the signal also from differential to single ended. What changes are needed to make this circuit a differential low pass filter alone? My assumption is to remove C1 and R1, R2 and R3 connected to the + input of the opamp. Is this correct? enter image description here

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    \$\begingroup\$ I'd use that circuit and then tee off an inverting op-amp circuit to make the diff output. \$\endgroup\$
    – Andy aka
    Commented Oct 14, 2019 at 14:52
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    \$\begingroup\$ Are you just guessing, or do you have some reason to believe that would work? I would suggest that you get familiar with a circuit simulator, and use that to test your ideas. \$\endgroup\$
    – Dave Tweed
    Commented Oct 14, 2019 at 14:52
  • \$\begingroup\$ These filters are less than unity gain. If you want to maintain RF CMRR, use a ferrite balun CLC filter with coaxial cable. otherwise with high impedance inputs, DM signals can be redundant. \$\endgroup\$
    – D.A.S.
    Commented Oct 14, 2019 at 16:23

2 Answers 2

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One option is to use a fully differential amplifier for your op amp. The MFB filter for that type of amplifier looks like (source):

Multiple feedback (MFB) topology FDA Filter

As you can see, it uses the same number of parts as the filter you reference, with the added advantage that you can also set the output common-mode voltage.

Full design details can be found in this application note: https://www.ti.com/lit/an/sloa054e/sloa054e.pdf

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In addition to Andyaka's simple suggestion, there are specialized ICs that can drive differential outputs, such as ADI's SSM2142 (NRND, but still available). This gives you a true "floating" differential output, much like you'd get from a transformer.

Anything you cobble together from discrete components will rely very heavily on matching values closely in order to maintain a high CMRR.

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