Using Video-DACs for audio

[rfbrw]

Originally posted by peufeu
This is actually what dCs does in their Elgar series. The digital signal is oversampled a lot, noise-shaped, and fed to a 8 bits or so DAC which is (I think) implemented in discrete parts.

While the dCs ringDAC does oversample it is at a relatively low 64x and the dac is only around 5bits and the current sources all have the same weight unlike a normal multibit dac.

using a high-frequency DAC (most of which, but not all, are video dacs)

Wrong. The high MHz/low GHZ end of the market is occupied by communications and data acquistion ADC/DACs. These speeds are not needed for video.

[ojg]

Originally posted by andrei
I have been toying with the idea of building a DAC around video DAC chips (AD768 in particular) for a while too... Not a first order Delta-Sigma though, but a straightforward non-oversampled design (which are in fashion nowadays). That needs some glue logic to do serial to parallel conversion and data retiming, which I was planning to stuff into a single CPLD, if I ever got around to it.

If you are into non-OS DACs then this could be interesting. Converting the audio data to parallel is fairly simple and can even be done with discrete logic. From the top of my head you would need 4 8-bit serial-in parallel-out shift registers, one 4-bit counter and some inverters.

Originally posted by rfbrw
The AD768 and the AD9752 are NOT video dacs.

Yeah, I was probably wrong there, seems like they are intended for communication like VDSL.

Originally posted by peufeu
However I'd advise against using 1LSB dither, rather use dither over several LSB's ; you'll get more noise (won't matter anyway) but more spreading of the LSB nonlinearities in the DAC (which is better).

I have thought about this and I think you are right. With a 12-bit dac it is better to use 2-4 bits for dither.

Originally posted by rfbrw
While the dCs ringDAC does oversample it is at a relatively low 64x and the dac is only around 5bits and the current sources all have the same weight unlike a normal multibit dac.

I believe you are right, but then how is this different from being just a discrete implementation of a PCM1738/1792?

Thanks for everybodys input!

[jewilson]

You know that some of the DACs that are on the market have multiple internal ladder DAC's inside already. I mean what you really need to make this fly are DACs that that allow you to use your own voltage reference. Then you can scale the bits accordingly.

The other day I saw that TI now has a 16 bit video DAC, I kfor got the speed of the device.

[rfbrw]

Originally posted by ojg


I believe you are right, but then how is this different from being just a discrete implementation of a PCM1738/1792?

Thanks for everybodys input!

A normal multibit dac has a bit weight that scales with the position of the bit i.e. the weighting of the lsb of a 16 bit dac is 1/65536.
As the wordlength increases the values required for the current sources get increasing smaller and more difficult and expensive to fabricate.
The dCs ringdac has unitary weighted current sources ie. they all have the same value. The output current is formed by selecting the appropriate number of current sources and this is governed the control software.

[peufeu]

The dCs ringdac has unitary weighted current sources ie. they all have the same value

And the used current sources are "rotated" hence the name "ring-dac". With 5 current sources, you can make any value from 0 to 5 (by activating 0 to 5 sources), but the value 1 does not always correspond to the same source because of the rotation. This helps balancing any inconsistencies between the sources.

Anyway, this still is around 2.5 bits (=log2 (6)) times 64x oversampling, and I find it hard to believe that it can reproduce the full information contained in 16 bits samples...

[jewilson]

If your looking to build a accurate high speed DAC you can do this discretely using the R2R ladder approach. I have found the controlling charge injection and current source to limit the accuracy in the design of Delta Sigma DAC.

The problem with building a R2R DAC is scaling the R2R ladders and producing a voltage reference that is stable with very low noise and low impedance. However, with S102 Vishay resistors the ladder not a big deal it just becomes costly. So building a DAC in this method is normally out of the question for the Burr Brown and Analog Devices due to cost, hence cheap Delta Sigma converters. There use to be several companies that made Hybrids DAC but they seem to have bit the dust; Hybrid Systems, MicroNetworks and Analogic were some of these companies.

In addition, there use to be a number of resistor companies making R2R network on thin film substrate. Anyway, I have found during design and testing that the accuracy of these DAC's to be far superior to the Delta Sigma's since low level accuracy is compromised. So with a little effort multiple R2R DAC’s can be series together with a summing junction.

[peufeu]

Could you post a simple schematic for a R2R dac ? I know I'm looking stupid, but I don't know whats that

I know it's a ladder with many times R and 2R, I saw stepped attenautors like this, but what is used for switching ? CMOS switches as usual ? (charge injection etc) or something else ?

[jewilson]

R2R ladder DAC are used in all types of applications where accuracy is needed.

[jewilson]

Here is another R2R DAC one that may help you. As you can see the device allows you to use any Ref voltage and has options for bipolar outputs.