Multi-bit, but not R2R

[mansr]

3 minutes ago, tmtomh said:

I would love it if you two and/or others could elaborate further. Not being versed in electrical engineering. I've always felt my understanding of DACs and their number of bits was abstract at best.

 

I know that there are 1-bit/bitstream DACs that as Miska notes are basically DSD DACs. And I've read repeatedly that most current bitstream DACs are actually more PCM/DSD hybrids that are bitstream(?) but with 4 or 5 bits instead of 1 bit. Apologies if I've garbled that summary - as I say, I'm not sure I fully understand.

Most modern DAC designs convert normal 24-bit PCM data to a much higher sample rate with a relatively small number of levels, typically in the 5 to ~65 range. This can be converted to analogue using a resistor array with equal weighting, eliminating the need for extreme tolerances. A greater number of levels makes noise shaping easier while complicating the conversion stage. As with so many things in engineering, it's a trade-off. Some DACs (e.g. TI) use a hybrid approach with R2R (or equivalent) for the top few bits and sigma-delta for the remainder.

 

3 minutes ago, tmtomh said:

As for older multi-bit designs like the 1980s Philips 14-bit and 16-bit DACs, I'd always assumed all of those were R2R and had no idea that such a DAC could be anything else.

Many DACs from that era are switched capacitor rather than R2R designs.

[mansr]

8 hours ago, opus101 said:

I'm still trying to figure out what the market is for this chip - the fact that it has an SPI interface tells me its more industrial than audiophile. 'Lab and field instrumentation' it says but what about those applications needs R2R rather than (significantly cheaper) S-D ? Could it be performance inside a control loop, given there's an example shown on the first page? 'Professional audio amp (rack mount)' looks like an outlier.. The maximum supply and reference of 40V suggests some seriously impressive SNRs might be possible, the output noise figure of 7nV/rtHz  implies 1uV in the audio band, signal may go up to 14VRMS,

It's obviously aimed at industrial and scientific applications. For audio applications, the specified THD of -105 dB of this chip is surpassed by many a sigma-delta design. Even if the analogue noise level is low, you're still limited to -115 dB or so by the 20-bit resolution. At low signal levels, glitching could also be a problem that needs to be dealt with. Regarding the "professional audio amplifier" application, I can't figure out how they're envisioning this device to be used there. The linked page mentions the somewhat similar DAC8812 in addition to a regular audio DAC, but the reference designs do not include it.