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[Tomas Danko]

crna59 wrote on Tue, 03 February 2009 04:22

Tomas Danko wrote on Mon, 02 February 2009 11:37 Noone here said ADC's are better because they are pricier. . Sorry....  my attempt a humor..  geez.... Regards,

Looked more like sarcasm at a first glance, though.


...and Terry still says emoticons are bad!

[Ted Olausson]

OTR-jkl wrote on Sat, 31 January 2009 14:28

Which is harder to do right? D/A -or- A/D

Definately AD.
But AD is often done internally in the chip as a DA with a comparator.
Check page 5-6 in this datasheet for an example http://www.national.com/ds/DC/ADC0808.pdf

PS: Does anyone know how to make a AD without a DA? I cant think of any way to do that... Neither analog or digital.

[zmix]

Ted Olausson wrote on Tue, 03 February 2009 07:25

OTR-jkl wrote on Sat, 31 January 2009 14:28 Which is harder to do right? D/A -or- A/D Definately AD. But AD is often done internally in the chip as a DA with a comparator. Check page 5-6 in this datasheet for an example http://www.national.com/ds/DC/ADC0808.pdf PS: Does anyone know how to make a AD without a DA? I cant think of any way to do that... Neither analog or digital.

Using a D/A in the design of an A/D converter is known as a "Successive Approximation"  converter.

Successive approximation converters were used in the very early days of digital audio.
They generally are not able to produce a high enough resolution for high fidelity audio reproduction.
(the example in the PDF you posted is an 8-bit converter).


There are several ways to make an A/D converter without a D/A

Here is a typical design, called a "parallel" or "flash" A/D:




And here is the ever popular "Delta Sigma":

[Ted Olausson]

zmix wrote on Wed, 04 February 2009 23:59

There are several ways to make an A/D converter without a D/A Here is a typical design, called a "parallel" or "flash" A/D:

This one i did "invent" in my head when i wrote the question, but it still has a ladder DA with comparators.
I also have an idea of how to make a Successive Approximation converter in the analog domain, but it will just get rid of the priority encoder and not the DA itself.

Quote:

And here is the ever popular "Delta Sigma":

Funny how often my knowledge is outdated when it comes to digital
But do you know any good description of how it works? I couldnt directly find out by looking at the schematic.

[bruno putzeys]

Depends on the technology. A ladder DAC is easier than a SAR ADC for the simple reason that a SAR ADC contains the DAC as a subcircuit and runs it much faster than would be necessary for a DAC.

In sigmadelta the difference is much smaller. In continuous-time sigmadelta (the sort I make) the loop filter in the ADC is more complicated than the output filter in the DAC but its noise and distortion contribution is much, much smaller. Meaning I find it harder to do a transparent DAC output filter (the ADC not having any) than a loop filter for an ADC. On the other hand, settling time must be much better controlled on the ADC end because any "intersymbol intermodulation" is going to affect the modulation process and cause any variety of whistles, tones and noise modulation.

In switched-cap sigmadelta (the ones you find on chips), the s/c output filter in a DAC has roughly the same complexity as the loop filter in an ADC. Aforementioned settling time problems are present there as well, but it's much more practical to do multilevel schemes on chip.

In short: for ladder/SAR type converters the ADC is definitely the toughest to crack. For sigmadelta converters both have their challenges and there isn't a clear difference as global complexity goes.

[Tomas Danko]

A very informative and appreciated contribution to the thread, indeed. Thanks, Bruno!

[klaukholm]

jdg wrote on Sat, 31 January 2009 22:31

its hard to me to think of a good way to test a D/A, as there is no real way to hear a D/D, as it has to turn to analog at some point.  unlike an A/D, where i've heard A->A and A->D... but now the A/D has to be heard out of a D/A...

I should think that with Digital to analogue, you can design a wave in the digital realm and accurately predict what it should look like in the analogue realm.
Obviously this is not enough, but there should be predictable absolutes.