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DIT4096: Input and Output Impedance of the COAX SPDIF for DIT4096 and DIR9001

Wayne Chen (ex FAE)
Genius 15950 points
Part Number: DIT4096
Other Parts Discussed in Thread: DIR9001,

Hi Sirs,

1. How do we define the output termination resistors for 75 ohm COAX SPDIF application for the DIT4906? We see 75ohm resistor on R001 of the DIR9001 EVM, but some customers do not add the termination, 110 ohm and 300ohm resistor on DIT4096 reference design. 

2. Any impact with DIR9001 EVM without a transformer at COAX input?

DIT4906 Reference Design with 110 for XLR and 350 termination for BNC 



DIR9001 Reference Design with 75ohm Termination

 

Thank you and Best regards,

Wayne Chen
10/12/2020

  • 0
    TI__Mastermind 23050 points

    Hi Wayne,

    The termination resistors should be chosen to match the impedance of line you are driving. 75 Ohms and 110 Ohms are common values for balanced transmission. In the diagram for the recommended 75 Ohm termination case, a 300 Ohm resistor is used because of the 2:1 turns ratio of the transformer. A load on the secondary side of the transformer 'appears' as a reflected load on the primary side and is scaled by the square of the turns ratio. So a 75 Ohm cable on the secondary side will look like a 300 Ohm load on the primary side, which is why this termination was recommended. If you used a transformer with a turns ratio of 1, then you would terminate with a 75 Ohm resistor.

    There are a number of reasons to use transformers in digital audio applications. They are used to provide balanced outputs, improve CMRR, reduce conducted EMI, and isolate the ground of the receive side from the source side. The impact of not using one will depend on the system design and requirements. Scientific conversion has a paper that goes into more detail about using transformers in digital audio systems. You can find it here: