Details of the 75ohm impedance of S/PDIF for short distances.

Garbz · 7th August 2005, 05:52 AM

I'm designing a DAC which is almost finished now, based on the CS8416 receiver. The DAC will also have a small USB->S/PDIF receiver onboard for interface with a computer, and I'm wondering how I can link these two together and isolate them using an ISO150.

The S/PDIF interface calls for a 75ohm impednace accross the cable, but in this case there is no cable. There is simply an output, and an input to the ISO150, and the output of the ISO150 via a 0.1uf film cap to the RX pin of the receiver.

Will this work or does some sort of resistor divider still need to be used to match impednaces?

Guido Tent · 7th August 2005, 12:00 PM

Quote:

Originally posted by Garbz
I'm designing a DAC which is almost finished now, based on the CS8416 receiver. The DAC will also have a small USB->S/PDIF receiver onboard for interface with a computer, and I'm wondering how I can link these two together and isolate them using an ISO150.

The S/PDIF interface calls for a 75ohm impednace accross the cable, but in this case there is no cable. There is simply an output, and an input to the ISO150, and the output of the ISO150 via a 0.1uf film cap to the RX pin of the receiver.

Will this work or does some sort of resistor divider still need to be used to match impednaces?

Hi

if we talk cm's here, you can ommit the 75 ohm parallel termination, and use some tens of ohms in series. That way you achive some series termination which works fine.

What would be the bennefit of the iso150 ?

cheers

Garbz · 7th August 2005, 01:17 PM

we are talking cms here. How would the few ohms in series be impliemnted, i.e. where do i put the resistors before the iso150 and receiver or after the usb chip and iso150?

I was thinking of using the iso150 to isolate the ground channel from the computer which comes via the usb port, and the dac in much the same way that digital and analogue grounds are separated in DACs. However i'm not fully set on this idea yet, I may end up using a 1:1 pulse transformer which would not require powersupply rails.

Jocko Homo · 7th August 2005, 03:59 PM

You really aren't builing an interface, in the sense that you need to worry about impedance. I would be more concerned that the enclosed loop the signal makes is very tight, to prevent it from radiating.

I would put the series resistor on the transmit end.

Jocko

Garbz · 7th August 2005, 10:51 PM

Ok will give that a try when assembly time comes. Thanks to both of you.

7th August 2005, 05:52 AM	#1
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	Details of the 75ohm impedance of S/PDIF for short distances. I'm designing a DAC which is almost finished now, based on the CS8416 receiver. The DAC will also have a small USB->S/PDIF receiver onboard for interface with a computer, and I'm wondering how I can link these two together and isolate them using an ISO150. The S/PDIF interface calls for a 75ohm impednace accross the cable, but in this case there is no cable. There is simply an output, and an input to the ISO150, and the output of the ISO150 via a 0.1uf film cap to the RX pin of the receiver. Will this work or does some sort of resistor divider still need to be used to match impednaces?

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7th August 2005, 01:17 PM	#3
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	we are talking cms here. How would the few ohms in series be impliemnted, i.e. where do i put the resistors before the iso150 and receiver or after the usb chip and iso150? I was thinking of using the iso150 to isolate the ground channel from the computer which comes via the usb port, and the dac in much the same way that digital and analogue grounds are separated in DACs. However i'm not fully set on this idea yet, I may end up using a 1:1 pulse transformer which would not require powersupply rails.

7th August 2005, 03:59 PM	#4
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	You really aren't builing an interface, in the sense that you need to worry about impedance. I would be more concerned that the enclosed loop the signal makes is very tight, to prevent it from radiating. I would put the series resistor on the transmit end. Jocko

7th August 2005, 10:51 PM	#5
Garbz diyAudio Member Join Date: Dec 2003 Location: Brisbane	Ok will give that a try when assembly time comes. Thanks to both of you.

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