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lingDAC - cost effective RBCD multibit DAC design
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Old 15th July 2019, 03:58 PM   #441
Markw4 is offline Markw4  United States
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Originally Posted by abraxalito View Post
...you don't have too much choice about your I/V stage, given the numbers for THD+N you're targetting. Its either opampA or opampB as no discrete circuit will come close in terms of measured linearity...
That is, and has been, the case. It may be that my analog designer friend will want to try a discrete approach, I know he has been doing some thinking about it. Will have to wait and see how things turn out.

However, while measured distortion is very low, subjective sound quality is very good. It just takes a lot of work (and some expense) with SD dacs to get it right. Most designs you see around here have their little peculiarities, one might say (IMHO). Even today, people are still finding additional considerations for minimizing sources of distortion. There are no Self or Cordell books listing every possible known problem for dacs.

Last edited by Markw4; 15th July 2019 at 04:05 PM.
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Old 16th July 2019, 09:53 AM   #442
abraxalito is offline abraxalito  United Kingdom
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Default Parallelling DACs progress report

The 36 DAC prototype I featured a few posts back is now up and running. I've yet to come up with a relatively foolproof way of paralleling lots of chips so it looks like a PCB is going to be the best solution. Here's a board that's just gone out for manufacture - once I've proven it I'll share the gerbers here.


Today I've updated my Hackaday project pages with a couple of logs, if anyone's interested in some of the background thinking behind the use of a transformer in an Iout DAC then you might find these links useful : Going beyond PhiDAC | Details | Hackaday.io How a transformer helps... | Details | Hackaday.io
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Old 17th July 2019, 05:11 AM   #443
laserscrape is offline laserscrape
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Im gonna give your discrete Ling I/V a try, one question, other than I/V resistor would the circuit need adjustments if you were to use stacked chips? probably 4 at most
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Old 17th July 2019, 05:24 AM   #444
abraxalito is offline abraxalito  United Kingdom
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Certainly it would need adjustments not simply limited to the I/V resistor.


The main reason for changes is because there are current sources involved which pre-bias the transistors to help improve the circuit linearity and set the DC operating point. The most critical is the first transistor, right after the filter - if you add more DACs this transistor deals with bigger current swings and a higher DC bias current. Therefore its input impedance will become lower and it'll need a change to its emitter degeneration resistor. This resistor's function is to pad the input transistor's Re to match the output impedance of the filter - at higher current the Re is lower and you'll need a higher degen R.
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Old 17th July 2019, 07:31 AM   #445
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I wasnt planning to build the reconstruction filter for now, just use software upsampling and gentle filter after I/V, same as with the op amps I was using.
Would you just leave the resistor out if there is no filter?
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Old 17th July 2019, 07:36 AM   #446
abraxalito is offline abraxalito  United Kingdom
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Yes - change that resistor to 0R if not using the filter. You'll still need to verify the DC offset is mid-way (3.9V) on the output if you change the number of DAC chips.
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Old 17th July 2019, 07:51 AM   #447
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ok, thanks for the help. I'll just stick with the one chip as intended
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Old 18th July 2019, 03:23 AM   #448
hollowman is offline hollowman  United States
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Originally Posted by weissi View Post
The latest SAA7220 I have is '92. So I guess not much longer... They must have counted on digital filters integrated in the other ASICs, otherwise they would have accounted for NON-OS operation?
Most of the stock of 1387s were intended for SoundBlaster soundcards (mid to late 1990s).
If anyone knows the schema of the SB cards, you may have your answer.
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Old 18th July 2019, 09:45 AM   #449
laserscrape is offline laserscrape
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finished the I/V, works but dont think its set up quite correctly. There is only 1.5V on the output of all 4 I/Vs, I think it may be due to the LED used for bref, its some amber LEDs which I had and it doesnt light up, if it supposed to or not.
The schematic doesnt specify a type but I assume its important.

btw what is best way to support you for your work here?
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Old 18th July 2019, 12:00 PM   #450
abraxalito is offline abraxalito  United Kingdom
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Yes the LEDs are all supposed to light up as they're run with forward bias of a milliamp or more. Check the voltages across all the LEDs - they're fairly critical as they set the DC bias points.

I'm not sure of your question at the end - you want to support me, financially? I don't look on what I do here as 'work' rather its fun pushing the boundaries of affordable but good-sounding DACs.
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