Open-source USB interface: Audio Widget - Page 218 - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

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 8th April 2013, 09:19 PM   #2171
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
Quote:
Originally Posted by 1audio View Post
1. I think that board is really interesting. Would it be possible to bring the ADC lines out to a header (62,64,66, clocks and grounds)? There is a lot of interest in a capture variant. Perhaps Borge can comment.

2. One thing to be very careful of is the loop around a current output DAC. That link is very sensitive to external magnetic fields. it cannot tell a different between internal currents and induced currents. If the peak current is 10 mA then -120 dB is 10 nA, not very much. I would make that extremely small and using an airplane board would be a good idea (to keep the currents at orthogonal to any others in the system).
Thanks for your comments!

1. A header to capture sounds easy, especially if it can be handled in the digital part - I am just reluctant to put too much stuff on the clock in the analog section.

2. Good point about the currents. Sorry for my ignorance, but what is an "airplane board"?
  Reply With Quote
Old 9th April 2013, 02:52 AM   #2172
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Blog Entries: 3
Airplane board- a board "floating" above the main board on a connector. If its perpendicular to the main board its even better.

The capture part of this board would need the master clock to be in sync. The capture card could be a slave to the host and the clocks would come from the digital section.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 14th April 2013, 07:15 PM   #2173
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
I have been thinking a lot about this - but not reached a conclusion. Perpendicular airplane board is probably not going to fly, because some of the components I was thinking about putting on the IV stage are bulky... how is the common mode rejection typically on current out dacs? Couldn't the issue be solved by routing the diff pair close and ensure nearby gnd return path?
  Reply With Quote
Old 14th April 2013, 07:28 PM   #2174
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Blog Entries: 3
Quote:
Originally Posted by cviller View Post
how is the common mode rejection typically on current out dacs?
Nonexistant. . .

The common mode rejection comes from the next stage. For a current out DAC the next stage needs to create a virtual ground at the output node. A reference would come from the dac as well. The area inside that loop will be sensitive to interference. Also keep in mind how fast the current needs to slew. That calculation which needs to include the out of band energy at the output pins is the biggest challenge. If yopu ignore it the current will go somewhere, possibly charging caps and active device nodes waiting to discharge. This is all before the image rejection filter. You could add a passive filter at the DAC output but that would adversely impact the VI stage if its source Z is increasing. Lots of interesting experiments possible.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 14th April 2013, 08:26 PM   #2175
borges is offline borges  Norway
diyAudio Member
 
Join Date: Dec 2003
Location: Oslo, Norway
OT:
I'm actually struggling with the same kind of issues these days. Laying out a new OPA1704 IVC I spend 10x the time looking at where the current goes as I do actually connecting the thing. And when currents are conserned I spend 10x the time considering a return current than I spend on the actual intended currents. Too many designers believe slapping on a ground plane or two at the end will do the trick.

BÝrge
  Reply With Quote
Old 14th April 2013, 08:34 PM   #2176
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
Regarding current loop on the layout I posted, is it correct to focus on the pink or green loops (drawn for only one channel)?
If these are the loops, I feel that it is fairly straight forward to keep them close with no tracks inside - or am I missing something?
  Reply With Quote
Old 14th April 2013, 08:51 PM   #2177
borges is offline borges  Norway
diyAudio Member
 
Join Date: Dec 2003
Location: Oslo, Norway
Not sure if I see pink and green loops, could you link to the actual layout again?

When I design analog parts my checklist tends to be something like this:
1 - Place decoupling caps with the smallest value closest to the load, place increasingly larger caps more and more peripherally
2 - Minimize the loop made up of chip's ground pin (of relevant analog or digital domain), chip's power pin and the two pins on the decoupling
3 - Make sure there is very good ground conductance underneath the chip with all ground pins connected both toward decopling on the outside and the ground plane underneath it
4 - Try to route power to the peripheral largest caps in the decoupling stack associated with each loa pin
5 - Move blocks of chip+decoupling around to minimize the loop area of any signal correlated current.
6 - Signal correlated current doesn't just appear. It comes out of the decoupling cap, flows into the chip's power pin, through an internal driving resistor and then out the output pin. That same current may be consumed by an op-amp output somewhere.
7 - Whenever I get lost in this I model my chip outputs as variable voltage sources between power pin and output pin and draw the full schematic of decoupling and analog signals.

... or you may just say screw it, pour the ground plane and pour yourself a drink :-)

BÝrge
  Reply With Quote
Old 14th April 2013, 08:57 PM   #2178
cviller is offline cviller  Denmark
diyAudio Member
 
cviller's Avatar
 
Join Date: Dec 2005
Location: Copenhagen
Blog Entries: 2
I had the drawing ready, but forgot to attach it...
Attached Images
File Type: png IVconnector.png (66.3 KB, 149 views)
  Reply With Quote
Old 14th April 2013, 09:05 PM   #2179
borges is offline borges  Norway
diyAudio Member
 
Join Date: Dec 2003
Location: Oslo, Norway
I haven't really separated DAC and IVC like that in any of my designs. At least have ground on either side of the track pair (in the plane) and below them on the board. Make sure that ground is well connected to the ground underneath the IC (ref.prev.msg.)

Now I understand Demian's point a lot better. Anything induced into a loop between A+ and A- will appear amplified on your output. High-frequency noise may be demodulated by non-linearities in following stages.

Some times it helps to read up on how people build radios, and then do the opposite :-)

BÝrge
  Reply With Quote
Old 14th April 2013, 09:38 PM   #2180
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by borges View Post
I haven't really separated DAC and IVC like that in any of my designs. At least have ground on either side of the track pair (in the plane) and below them on the board. Make sure that ground is well connected to the ground underneath the IC (ref.prev.msg.)

Now I understand Demian's point a lot better. Anything induced into a loop between A+ and A- will appear amplified on your output. High-frequency noise may be demodulated by non-linearities in following stages.

Some times it helps to read up on how people build radios, and then do the opposite :-)

BÝrge
Lol

I used to say that too. But now I think following RF rules will take someone a long way.
Try treating ever trace as a transmission line. Less reflection will result in lower noise.
I work on RF transmission lines every day.
__________________
David.
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Async 192Khz USB - the SDR-Widget collaborative project SunRa PC Based 5 26th April 2011 07:38 PM
usb audio interface david12 Equipment & Tools 14 10th October 2010 03:58 AM
Cheap Audio Interface (USB?) to PC agm2003 Instruments and Amps 11 16th September 2007 08:48 AM
Open call for suggestions on Open Source DIY Audio Design gfergy Everything Else 1 15th April 2007 08:33 AM
USB Interface Perfect?- Computer Audio fmak Digital Source 3 4th December 2004 11:24 PM


New To Site? Need Help?

All times are GMT. The time now is 07:01 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2