DACs and the Differential Dilemma

Which way?

Most high-performance modern DAC ICs have differential outputs, even those targeting single-ended consumer applications. It’s a great way to reduce even-order nonlinearities in the chip’s conversion and output stages, and it gets you around 3dB better SNR.

But in most systems that support both “balanced” (i.e., differential) and RCA (i.e., single-ended) outputs, there is an existential battle between the two for primacy.

To design for a differential (“balanced”) application, you can simply pass each side of the DAC chip’s outputs through a combined filter and buffer and let the downstream electronics do the summing.

Differential output stage design from the AKM AK4493 English Datasheet.

To get a single-ended output, application guides and evaluation boards often show an added a summing stage to the above. But experience has shown that with voltage output DACs at least, things seem to work best when you use a single gain stage that both sums and filters the outputs of the IC.

Single-ended output stage design from the AKM AK4493 English Datasheet

When taking this approach, there’s a good amount of optimization you need to do that might not be initially apparent. But when the needed care is taken, it works very well. I suspect it works better than the filter-then-sum approach because the signal passes through fewer active cells (typically opamps). This leads not just to the potential for lower accumulated nonlinearity but also for less complex distortion spectra. These stated reasons are incredibly speculative at this point, but I am certain of what my ears consistently prefer.

So, there’s an easy solution when you want a differential output, and an almost as easy solution when you what a single-ended output. But what do you do when you want both? Initially it seems you have to decide which output gets priority: You either prioritize the differential output and convert that to single-ended or you prioritize the single-ended output and convert that to differential.

It turns out there’s an approach that as far as I know no one is yet using commercially that solves this dilemma perfectly.1I recently completed a design for a client using this approach that I’m expecting to be released soon. I’ll cover what that is in my next post.

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