Modern Multi-Bit DAC vs Delta Sigma, specifically AKM's newest flagship, but also others

[Philbo King]

Sorry for veering off-topic...
This thread is sort of interesting to me. I was involved in designing and prototyping an optical D/A about 10 years ago using 10+ GHz pulsed lasers through fiber cable to directly sample RF, reaching the design goal of an ENOB of 10 bits. (Patent 8928510). It was a radically different approach to A/D. I haven't kept up to date on the progress since I retired, but one eventual goal was to implement the design on a silicon photonic chipset.

 

[KSTR]

So how does that work? Clearly the AK4191 generates multibit PWM data, not PCM. The modulator is there, not in the AK4499EX.

As explained in my previous post, the AK4499EX is a PCM flash DAC with parallel 7-bit input interface (multiplexed when used for stereo input).
Inside the DAC, the 7-bit PCM is expanded to 128-bit thermometer code with DWA.

The output from the AK4191 is also PCM with just 7 bits but a high sample rate, produced by the multibit DS-modulator.

 

[voodooless]

The output from the AK4191 is also PCM with just 7 bits but a high sample rate, produced by the multibit DS-modulator.

I think you might be wrong. An older product brief shows it's a 3rd-order modulator, which would correspond well to a 7-level output stream. Otherwise, you'd have to downsample the DS data again to then expand to 7-bit PCM. No such mechanism is described in the datasheets.

 

[DonH56]

I am not aware of any direct link to modulator order and DAC levels... A 7-bit DAC should have 127 levels, with oversampling and delta-sigma modulation pushing up the data rate and pushing the quantization noise to higher frequency to be filtered away later.

 

[KSTR]

I think you might be wrong. An older product brief shows it's a 3rd-order modulator, which would correspond well to a 7-level output stream. Otherwise, you'd have to downsample the DS data again to then expand to 7-bit PCM. No such mechanism is described in the datasheets.

Block diagram from the full AK4499EX datasheet clearly states its a "pure" DAC with a two's complement PCM parallel input (with no processing whatsoever except for the DWA and thermometer decoder), which implies that the output from the AK4191 also has the same format.

That's the whole idea of the AK4499EX: an excellent quality DAC output core which can be used in many ways, for example you can use it for a fast arbitrary waveform generator with 7-bit resolution, being fed with direct PCM.

 

[voodooless]

Mkay, seems I was wrong. Thanks guys!

 

[KSTR]

Mkay, seems I was wrong. Thanks guys!

No problem. I do admit that AKM datasheets are not always straight-forward to read/understand and often are a bit vague terminology.

Fun fact: In the above block diagram from AK4499EX datasheet there still is a string "Modulator" above the string "DWA" but it is whitened out. Looks like there have been misunderstandings what this thing actually does even inside AKM ;-)

 

[mocenigo]

For multibit DS, you’ll need more than on level to be “on” at a time. You can’t do that with 127 levels and 7-bits.

The 7 bits are not the thermometer encoding. That's a binary enoding (two's complement, in fact, as described in the data sheet).
Oh yes, the 4499EX expands 7 bits to a 128 (actually, I think 127) levels of "thermometer" encoding, but the assignment to the various resistors is scrambled in a simple way. I am not sure what order of DWA (data weighted averaging) are using, but it is probably 2 or 3.