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Old 3rd May 2012, 03:48 AM   #1
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Question I2S and digital signal path - in-line resistors?

Hello all,
I am currently working on a USB DAC with PCM2707 and PCM1794 and they are connected via I2S.

I am looking at the data sheets and some designs online and I am curious about one thing... some designs include in-line resistors in the I2S signals between the chips. I tried looking as to why are people doing that and data sheets are omitting it.

I also noticed that there are in-line resistors on the USB lines at D+ and D- at 22ohms. I just put them there because that is what was show in the datasheet and other design but without really thinking about it.

So what are the reasons behind those in-line resistors on the digital lines? and how I can pick the optimal values/types of resistors and position?

I assume it has to deal with stabilizing the signal so we get cleaner/stable square waves by putting an in-line load on the digital line.


Thank you,
IT
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Old 3rd May 2012, 03:56 AM   #2
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Resistors are used to match impedance in high speed digital circuit to avoid reflecting between driver and receiver.
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Old 3rd May 2012, 04:03 AM   #3
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Should be placed at the source end of the I2S signal path. (Provides a terminating impedance for reflections from the driven device so they do not corrupt the transmitted signal.)

From experience they don't work too well [read as "really badly"] at the receiving end particularly if the traces/wires are a few inches long.

47 - 100 ohms work well and should be low inductance types such as smd resistors.
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Old 3rd May 2012, 04:06 AM   #4
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Default doh.... :(

Quote:
Originally Posted by siliconray View Post
Resistors are used to match impedance in high speed digital circuit to avoid reflecting between driver and receiver.
Thank you for that obvious answer..... I am stupid, should have known better

Anyways some tips on impedance matching practices would be useful

I always have a problem with impedance matching and never quite get how to calculate it and sometimes it seems as simple as just putting the same resistor values on the sending and receiving ends? but sometimes it doesn't make sense in some circuits


I am gonna have more of those problems with the amp input and output impedance... i know it :/
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Old 3rd May 2012, 04:08 AM   #5
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I think 47-100 is too much since the impedance of transmition line is around 50Ohm. For example,the dirving impedance of TTL is 13 Ohm, put a 37 Ohm in series will match the impedance for 50 Ohm. It's correct that the resistor should be put at the driver end.

In my opinion it's not necessary for I2S at all. Several MHz Is far from high speed digital. It really count in USB2.0 or 3.0.
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Old 3rd May 2012, 04:11 AM   #6
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Quote:
Originally Posted by zxgravediggerxz View Post
<snip>

I always have a problem with impedance matching and never quite get how to calculate it and sometimes it seems as simple as just putting the same resistor values on the sending and receiving ends? but sometimes it doesn't make sense in some circuits


I am gonna have more of those problems with the amp input and output impedance... i know it :/
I2S series terminate at the transmitting end ONLY. (No parallel termination at the receiving end - remember this is logic level stuff and you don't want to divide logic levels by 2..)

For general audio just remember that the load impedance should generally be >10X the driving impedance for best performance. Smaller ratios are possible with some loss of gain and potential for increases in distortion particularly at high signal levels.

Exception: Most tube power amplifiers are matched to their load and the source impedance is often rather significant relative to the loudspeaker driven particularly in the case of SE amps with no global feedback. This is often expressed as damping factor which in tube amps is often quite low frequently 10 or less.
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Old 3rd May 2012, 04:12 AM   #7
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Impedance matching works in the same way in high speed digital circuit as the audio amplifier, the only thing need to be concerned is that you must think in complex impedance instead of resistance in high speed circuit.
Quote:
Originally Posted by zxgravediggerxz View Post
Thank you for that obvious answer..... I am stupid, should have known better

Anyways some tips on impedance matching practices would be useful

I always have a problem with impedance matching and never quite get how to calculate it and sometimes it seems as simple as just putting the same resistor values on the sending and receiving ends? but sometimes it doesn't make sense in some circuits


I am gonna have more of those problems with the amp input and output impedance... i know it :/
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Old 3rd May 2012, 04:41 AM   #8
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Quote:
Originally Posted by siliconray View Post
Resistors are used to match impedance in high speed digital circuit to avoid reflecting between driver and receiver.
This isn't really about impedance matching. Without termination, there will be unavoidable reflections from the far end. So if it were about impedance matching there would be terminations. In digital circuits these can be done without compromising the logic thresholds too much by putting a C in series with the termination R.

Designers (myself included) use series resistors on digital lines for bandwidth limitation and hence noise mitigation.
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Old 3rd May 2012, 04:45 AM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by siliconray View Post
I think 47-100 is too much since the impedance of transmition line is around 50Ohm. For example,the dirving impedance of TTL is 13 Ohm, put a 37 Ohm in series will match the impedance for 50 Ohm. It's correct that the resistor should be put at the driver end.

In my opinion it's not necessary for I2S at all. Several MHz Is far from high speed digital. It really count in USB2.0 or 3.0.
Look at the bit clock, data and system clock for I2S with 24 bit 192kHz audio, it's not quite as trivial as you think.. SCLK in my dac runs at close to 25MHz (128FS or 24.576MHz) and the edges have rise and fall times of much less <10nS. Data is 9.216Mbits/sec, LRCLK half this..(Assuming I have done my math right.) I'm using Wolfson WM8804 and a pair of PCM1794A in mono mode and was having a lot of trouble with SCLK. (MCLK on 8804)

FWIW few dacs I have seen use controlled impedance lines for I2S, and cmos logic with low gate capacitance have high input impedances. The results of reflections may be seen. Perhaps this is less of an issue with old chips but certainly with the WM8804 the termination is important - I learned this the hard way.

Almost all high speed digital hardware (embedded systems, ATE) I have seen uses 47 ohm series terminations.. TTL/CMOS device output impedance is a function of device family - I've never seen 13 ohms quoted in any data sheet I have seen, did you measure it and if so what device(s)?
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Old 3rd May 2012, 05:34 AM   #10
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13 ohms would be fairly accurate for AC logic family standard gates running at 5V. Its not directly quoted in the datasheets but can be estimated from the output voltage vs load current graphs they show. It does depend on output logic level - in general logic 0 is sourced from a lower impedance than a logic 1, no matter what the family.
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