I2S line termination on dual mono DAC - 4 or 8 resistors? - diyAudio
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Old 24th October 2013, 11:00 AM   #1
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Default I2S line termination on dual mono DAC - 4 or 8 resistors?

I'm planning the build of a WM8741 dual mono differential DAC. When it comes to the I2S line termination I wonder how many resitors to use - 4 or 8?

i2s_termination_1.jpg

i2s_termination_2.jpg

Last edited by Groundloops; 24th October 2013 at 11:02 AM. Reason: Typo
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Old 24th October 2013, 12:27 PM   #2
marce is offline marce  United Kingdom
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2nd one values would have to be half of 33R.
These are series termination and should be placed as close to the driver as possible, and really have to be matched to the PCB layout to be effective.
Other factors come into play such as receiver and transmitter impedances etc. The pics below show the effect of series termination resistors on a real signal, and how matching the value to the line gives the best result.
Attached Images
File Type: png 22R_Term_AP.png (41.6 KB, 155 views)
File Type: png 47R_Term_AP_1.png (36.2 KB, 155 views)
File Type: png 56R_Term_AP.png (37.2 KB, 152 views)
File Type: png 68R_Term_AP.png (39.3 KB, 151 views)
File Type: png 75R_Term_AP.png (36.7 KB, 150 views)
File Type: png 82R_Term_AP.png (36.5 KB, 19 views)
File Type: png 100R_Term_AP.png (35.6 KB, 30 views)
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Old 24th October 2013, 12:38 PM   #3
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Thank you, introduced in a very concise way. If I2S source and DAC impedances are not stated, how do I achieve good results? Fetch my scope and then trial and error?

Where to apply the probe(s) - at the DAC input pins and DAC digital ground point? Probe every signal line and figure out each R individually?

Last edited by Groundloops; 24th October 2013 at 12:44 PM.
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Old 24th October 2013, 01:13 PM   #4
marce is offline marce  United Kingdom
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Yep, its the only way unless you have simulation software.
Sometime driving two sources can be a problem as you are halving the impedances so either simulation or actual measurements is the best way.

http://www.ni.com/white-paper/3854/en/

http://www.altera.co.uk/literature/wp/wp_sgnlntgry.pdf

Howard Johnsons book on high speed design is a good reference also.

Last edited by marce; 24th October 2013 at 01:23 PM. Reason: added links etc
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Old 24th October 2013, 05:52 PM   #5
marce is offline marce  United Kingdom
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Der, I should have said the two res solution should be twice the value, to get the same value as 1 res. my brain is not working well today.
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Old 25th October 2013, 09:55 PM   #6
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Quote:
"I should have said the two res solution should be twice the value..."
Mate, you gave me a hard time ... Before starting this thread I've read the thread stated below and so I thought: If Marce says so, there may be some truth behind it.



I2S and digital signal path - in-line resistors?
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