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I have a single-ended audio signal that I want to low-pass filter and convert to differential (with GND as the OCM voltage — that is, when one output is X Volts, the other output shall be −X Volts).

Can I accomplish this with just one fully-differential amplifier, for example the LME49724? I'm trying to avoid one additional stage in the signal path.

One idea that comes to mind is to just pretend that I have a normal (non-differential) op-amp where I treat the non-inverting output as if it was the output of a normal op-amp and build a Sallen-Key circuit; then, simply use the inverting output as part of my differential pair, without connecting any feedback network from it.

Will this work? Normally the fully-differential op-amps use symmetric feedback networks. However, as I understand, the differential amplifier internally forces the two outputs to be the complement of each other, so it seems intuitive that it will work. Since the input is single-ended, I don't see any issues with common-mode signals (which is what I've seen as the possible issue when there is a mismatch between the two feedback networks). However, a quick LTSpice simulation using the LTC1992 seems to disagree with this — the non-inverting output looks ok, but the inverting output saturates.

How do I make a Sallen-Key filter with fully differential outputs?

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  • \$\begingroup\$ What is Vcm input ? 0V or V+/2 ... If V+/2 then connect V- input R to V+/2 then Vcm output =0 ... TI also has a better Filter design software with any N order LPF HPF BPF APF FilterPro Desktop \$\endgroup\$ – Sunnyskyguy EE75 Apr 6 '17 at 3:58
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    \$\begingroup\$ For a real fully differential sallen key version there is no easy workaround. Search for DDA - differential difference amplifier with sallen key structure \$\endgroup\$ – Don Apr 6 '17 at 17:51

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