Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

Building the ultimate NOS DAC using TDA1541A
Building the ultimate NOS DAC using TDA1541A
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 10th April 2018, 08:07 AM   #6061
zenelectro is offline zenelectro  Australia
diyAudio Member
 
zenelectro's Avatar
 
Join Date: Sep 2009
Quote:
Originally Posted by andrea_mori View Post
Hi John,

just a curiosity about ternary logic switches, are you using discrete devices or logic IC (flip-flop, latch) like Soekris or Rockna's devices?

I'm also working on a discrete DAC (DIY design, not a commercial device) and I have approached in the same way using ternary swithces to switch between positive, ground and negative.
I have not yet decided which is the best way, there are advantages and disadvantages in both cases. With discrete parts you can reach very low RDS-on (20-30 mOhm or so), but they are slower that logic ICs. Also the logic to drive discrete switches is more complex and p-channel devices are tipically different from n-channel devices (RDS-on, input and output capacitance, speed).
Conversely logic ICs are simpler to drive, are faster but have greater RDS-on (10 to 20 ohm or so), that finally affects the ladder network precision (monotonicity).

Andrea
Just an idea - what about using a high speed BJT follower after the logic gate. An RF BJT will have from <1pF to 2pF load on the logic and it will have lower OP Z of a few ohms or less if you load it with more current.

This would be very easy to implement and the extra class A loading could be a simple resistor to a - supply.

T
  Reply With Quote
Old 10th April 2018, 08:58 AM   #6062
ecdesigns is offline ecdesigns  Netherlands
diyAudio Member
 
Join Date: May 2006
Location: Holland
Hi andrea_mori

Quote:
just a curiosity about ternary logic switches, are you using discrete devices or logic IC (flip-flop, latch) like Soekris or Rockna's devices?
I use 7400 series bus storage registers for the Mosaic UV and 7400 bus storage registers and 7400 series octal D flip-flops for the Mosaic II. These are similar devices as used in most discrete R2R ladder DACs.

Quote:
I have not yet decided which is the best way, there are advantages and disadvantages in both cases. With discrete parts you can reach very low RDS-on (20-30 mOhm or so), but they are slower that logic ICs. Also the logic to drive discrete switches is more complex and p-channel devices are tipically different from n-channel devices (RDS-on, input and output capacitance, speed).
Conversely logic ICs are simpler to drive, are faster but have greater RDS-on (10 to 20 ohm or so), that finally affects the ladder network precision (monotonicity).
We have to face the fact that there are no perfect components, ok we might get parts with say 1 Ohm RDSon but these will cause new problems (higher surge currents, higher ground-bounce, more pollution) especially with the given parasitics of a relatively large discrete circuit.

The same imperfect devices can be used in different ways to improve performance. The Mosaic converter circuit -looks- similar to conventional discrete R2R ladder DAC circuits, except, it is no R2R ladder DAC.

Most popular discrete R2R ladder DACs are based on simple voltage attenuators based on two resistors (R and 2R).

For higher precision Wheatstone bridges are used:

Wheatstone bridge - Wikipedia

That's what I used for the Mosaic matrix converter and that's why accuracy is no longer an issue.

The remaining small bit errors are below the audibility threshold. These errors can be easily nulled by trimming.


I personally would be much more concerned about the (interface) noise spectra that are unavoidable with digital audio.
  Reply With Quote
Old 10th April 2018, 09:13 AM   #6063
Alexandre is online now Alexandre
diyAudio Member
 
Join Date: Jan 2004
Location: Brazil
Hi John!

I just wanted to point that I mentioned you, and your research, in a recent post... here:
Integral nonlinearity (INL), THD, etc., in multi-bit DACs
  Reply With Quote
Old 10th April 2018, 11:12 PM   #6064
andrea_mori is offline andrea_mori  Italy
diyAudio Member
 
andrea_mori's Avatar
 
Join Date: Jan 2005
Location: Italy
Building the ultimate NOS DAC using TDA1541A
Quote:
Originally Posted by ecdesigns View Post
For higher precision Wheatstone bridges are used:

Wheatstone bridge - Wikipedia

That's what I used for the Mosaic matrix converter and that's why accuracy is no longer an issue.

The remaining small bit errors are below the audibility threshold. These errors can be easily nulled by trimming.
Well, I have a little preference for the discrete devices.
They have some drawbacks but since they have to switch at a relatively low frequency (44 to 192 or maybe 384kHz) they are not so bad.
In the attached picture the simulation (48kHz and 384kHz) and the real measurement (45 kHz and 443kHz) of a mosfet pair in SO-8 package.
The waveform is beatiful at 45kHz and still looks good enough at 443kHz. The ground bounce can be decreased with a better driver circuit.
The advantage of this configuration is that they have very low output impedance, 38 mOhm for the N-channel and 66 mOhm for the P-channel, that means less than 0.001% with a 10K ladder (17-18 bit accuracy).

BTW, I can't figure out how a Weathstone bridge can help with DAC accuracy. You need anyway precise resistors to balance the bridge (or to unbalance it at certain current/voltage) unless you trim individually each bridge or you use some artifacts to get a dynamic correction of the error due to the resistances tolerance.
Moreover, let say a 74LVC574 has an output impedance around 23 Ohm when it switches low and around 25 Ohm when it switches high, at least 350 times higher than the mosfet pair. This resistance is in series with the ladder, so with a 10k ladder it weights at least 0.25%.
This resistance could be part of the bridge so it does not affect the balance, but:
- the other resistances of the bridge have to be very precise
- there is anyway a difference around 2 ohms between the low/high logic states (at least 0.01% with a 10k ladder)

What is missing?
Attached Images
File Type: jpg Sim_Mosfet_pair_48kHz.jpg (77.0 KB, 150 views)
File Type: jpg Sim_Mosfet_pair_384kHz.jpg (78.1 KB, 142 views)
File Type: jpg Mosfet_pair_45kHz.jpg (17.7 KB, 147 views)
File Type: jpg Mosfet_pair_443kHz.jpg (18.7 KB, 141 views)

Last edited by andrea_mori; 10th April 2018 at 11:16 PM.
  Reply With Quote

Reply


Building the ultimate NOS DAC using TDA1541AHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


New To Site? Need Help?

All times are GMT. The time now is 02:13 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki