DAC chips with more current!

Paralleling DACs to get more current works fine until we get bored with soldering so many chips down to the PCB. I've also been wondering if more juice can be had without tying up so much PCB real-estate. So I've been looking for DACs which deliver more current, and in particular, more output current in relation to supply current. In Celibidache I'm using 20 TDA1387s and the supply current is 110mA @5V to deliver 20mA output swing, per channel. Hence 5.5mA per 1mA output current but in its favour there are two independent channels.

Communications DACs are quite a long way from the world of audio DACs but I've long suspected they might turn out to work fine reproducing audio. Their THD specs tend to be poorer without a doubt as they don't use DEM methods to improve DNL/INL. (There are one or two recent designs that incorporate self-calibration though.) Recent designs tend to be more expensive and in much trickier to use (higher pin-density) packages so I tend to favour older models in SOP28 and the like. Things that appeal to me about comms DACs are - they're stripped down to bare essentials converters, no fancy features like digital filters or sample rate converters on-chip. They are cheap when bought recycled (under $1 a piece typically) and they go up to insanely high sample rates. This latter aspect gives me confidence that we can run them heavily oversampled if need be without loss of significant performance as even 32X OS is off to the far-left of the spec graphs of these devices. Their extreme speed capability means they're always parallel-input this is a minor inconvenience in terms of an extra logic overhead.

I have a short tube of AD9764 in my possession - this chip delivers 20mA balanced output swing for a supply current of under 30mA, depending on sample rate. Better than 1.5mA of supply per 1mA output current. Here are its lack-lustre characteristics - bear in mind this is a 14bit part :

and here's its block diagram :

Its a multiplying DAC so its reference input can be varied (over a 10:1 range) to create a rudimentary volume control for the first 20dB.

The output noise spec is 50pA/rtHz which translates to 2.5nV/rtHz in a 50R resistor, giving a dynamic range of 120dB, assuming no intrusive 1/f noise. This is rather a lot better than even TDA1541 (110dB) and it has a high Zout (100k) and a wide compliance range of over 1V. So relatively tolerant of I/V stages. We just need to find ways to improve the resolution from 14bits and perhaps to smooth out the INL/DNL somehow.

 

[abraxalito]

The only other DIYer I'm aware of using comms DAC chips for audio purposes is xx3stksm : https://www.diyaudio.com/community/threads/two-way-dac-multibit-and-dsm-from-scratch-with-input-options-sdmicro-toslink-and-iis.331079
In this post he compares the noise performance of various examples and notes that great RF noise numbers do not necessarily translate to audio band performance : https://www.diyaudio.com/community/...thd-etc-in-multi-bit-dacs.321189/post-5400877. Also setting output current to the max doesn't necessarily yield the best results - he uses 6mA from a 20mA max chip.

He's particularly interested in ultra-low THD for instrumentation purposes. I'm not so interested in chasing that as I'm not convinced its audible but I am open to persuasion. The DAC chip he's settled on is AD9717, this is a modern one (tricky package) that has internal calibration to improve its INL/DNL specs.

Here's the relevant line in the spec table, AD9717 is the highest resolution member of a family of devices :

Look at how good the DNL gets relative to the number of bits - at 8 bits, each bit width is within 0.3% of the others, very impressive. Going to 10bits, each LSB is a quarter the width so if the precision were relatively the same we'd expect 1.2% and to one decimal that's what we do get. However look at what happens when we add two further bits, to 12. I'd expect 4.8% but in fact we have 20%. So even though there are 4 times as many bits, they're even less precisely weighted than for the 10bit DAC. Things get worse still at 14bits. From the point of view of how precisely the bits are weighted, we seem to have a sweet-spot at 10bits, beyond which measured performance goes to pot.

As regards noise, the output noise isn't specified in the same way as with the older DACs so I'll need to do some number crunching to see if I can come up with an apples-apples comparison. The ADI price of AD9717 is ~$14 (it is a dual channel DAC though) and I've not found any recycled ones on Taobao so I'll keep searching for a cheaper and easier to solder alternative to this chip.

 

[abraxalito]

Its interesting to compare an earlier generation of chips, since these parts are subject to continuous refinement. AD9717 dates back to 2008, its predecessor AD9707 is from two years earlier. Here's how the older parts stack up on INL/DNL (I'm guessing by arranging them in opposite resolution order and swapping the order of DNL/INL they're discouraging direct comparisons) :

Interesting that the performance of the older 14bit part is superior both prior to and after calibration. And not by a small amount either - the AD9707 is very comfortably monotonic after calibration but AD9717 is marginal. Perhaps there are different contexts to explain this, I'll delve into the small print to explore further. The first thing to notice is the fullscale current is 2mA for both tables, but the older parts go up to 5mA and the newer, 4mA.

 

[abraxalito]

On the older chips still specified for output noise the simple way, there's a pA/rtHz spec and almost without variation that number is 50pA @ 20mA, declining to 30pA @ 2mA. I've been studying DAC datasheets for quite some years now and have collected a few - so it stuck out like a sore thumb when I found this spec :

This comes from TI's THS5661A 12bit DAC DS but their other offerings in the same family give the same figures so it can't be a typo. I have some of these devices so this really needs an investigation, given ostensibly these DACs deliver 10dB better SNR on paper. Of course it could be that HF noise has been reduced at the expense of LF, only experiments will reveal the underlying truth.

Incidentally I've just noticed 'moar' got changed to 'more' in the title of this thread. Stealth autocorrect or some human intervention?

 

[abraxalito]

Here's my test-bed for THS5661A taking shape. The 8 chips top right are HC595s doing serial/parallel conversion. I am veering towards putting the cascode MOSFETs on a plug-in board as in the past when I've experimented with paralleled chips, the common-gate MOSFETs have a tendency to oscillate and hence self-destruct. There's really too much current for it all to be channeled through a single MOSFET unless its heatsunk so I greatly prefer paralleled smaller FETs to share the load but the oscillation issue is a serious one.