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SPDIF balanced output
SPDIF balanced output
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Old 14th June 2010, 06:51 PM   #11
Joseph K is offline Joseph K  Hungary
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Quote:
Originally Posted by jkeny View Post
Differential = no ground
Ehm. Not true.
Differential: three signals: Signal+; Signal-; AND GROUND

Ciao, George
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Old 14th June 2010, 07:01 PM   #12
jkeny is offline jkeny  Ireland
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Does differential not ignore the ground as the reference for the signal & instead use the difference with complimentary signal?

George, I see you over here now - I hope we can both remain civil?
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Old 14th June 2010, 08:00 PM   #13
Bill Fuss is offline Bill Fuss  United States
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Quote:
Originally Posted by Joseph K View Post
Ehm. Not true.
Differential: three signals: Signal+; Signal-; AND GROUND

Ciao, George
Then my entire system works by magic. Amazing!
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Old 14th June 2010, 08:02 PM   #14
jkeny is offline jkeny  Ireland
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Yes but George is a respected E'ee (I think?) so let's hear what he has to say, I'm sure he wasn't being flippant.

Last edited by jkeny; 14th June 2010 at 08:12 PM.
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Old 14th June 2010, 11:21 PM   #15
jkeny is offline jkeny  Ireland
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Moving on - Another question guys:
I'm looking to work out the impedance of the output stage using a multimeter.
Here's my proposed technique:
- measure the voltage out when the outputting digital silence of the output pins without any caps in-line
- using a multimeter across the pins adjust a potentiometer also connected across the pins until half the voltage is read across the pins
- the reading of the potentiometer is approx the impedance of the output stage
- better than guessing?

Will this work?

Last edited by jkeny; 14th June 2010 at 11:46 PM.
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Old 14th June 2010, 11:35 PM   #16
stormsonic is offline stormsonic  Slovenia
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you want to measure characteristic impedance with multimeter?
or want to measure Zout of your chip?
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Old 14th June 2010, 11:40 PM   #17
jkeny is offline jkeny  Ireland
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Well if I want to try and get a 75ohm impedance match on the line which do I need to take into account the output stage impedance? What is the difference with Zout? I need to add this to a first series resistor that I'll be using as part of a 75ohm H-pad resistive network attenuator - is this correct?

Edit: Sorry, I re-read my post & see that I left out one important bit - I'm using a potentiometer as an adjustable R across the output pins & so acting as a voltage divider, doh

I've adjusted the post above but repeat it here:
Here's my proposed technique:
- measure the voltage out across when the outputting digital silence of the output pins without any caps in-line
- using a multimeter & potentiometer across the pins adjust the potentiometer until half the voltage is read
- the reading of the potentiometer is approx the impedance of the output stage

Does the output stage being balanced cause any problem here?

Last edited by jkeny; 15th June 2010 at 12:02 AM.
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Old 15th June 2010, 12:03 AM   #18
stormsonic is offline stormsonic  Slovenia
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Huh, there is a long way infront of you

Start with Zout for your chip.
Look into datasheet and search for VOH and VOL parameters (Voltage Output Low and Voltage Output High).
Take into consideration your power supply voltage (Vcc) and look into IOH and IOL values (output current).
You have voltage, you have current, use Ohm law to calculate resistance.
There will be different resistance for VOH and VOL.
Sum those 2 values together and divide with 2 to get average value.
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Old 15th June 2010, 12:10 AM   #19
jkeny is offline jkeny  Ireland
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I've done this already but get crazy figures:
High-Level Output Voltage, VOH IO = –30mA VDD–0.7(min) VDD–0.4(typ) VDD(Max)
Low-Level Output Voltage, VOL IO = +30mA 0(min) 0.4(typ) 0.7(max)

I (mA) V R (ohm)
30 4.6 153.333333 for High
30 0.4 13.333333 for Low

So 83 ohm but this seems too high, no? I think I heard 20 to 30 mentioned somewhere as a reasonable figure?

What's wrong with my technique above?

Last edited by jkeny; 15th June 2010 at 12:17 AM.
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Old 15th June 2010, 12:22 AM   #20
jkeny is offline jkeny  Ireland
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I'm sorry again the posting of my method is messed up (does anybody find this frustration with trying to type on a laptop that has a touchpad - my thumb seems to activate this touch pad & cause the cursor to jump somewhere while in the middle of typing causing gibberish as above)

It should read:
Here's my proposed technique:
- measure the voltage out across the output pins when outputting digital silence without any caps in-line
- using a multimeter & potentiometer across the pins adjust the potentiometer until half that voltage is now read
- the reading of the potentiometer is approx the impedance of the output stage

Does the output stage being balanced cause any problem here?
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