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PCM2705 and S-PDIF coaxial output

Other Parts Discussed in Thread: PCM2705, SN75176B, PCM2706


I'm planning to use the PCM2705 to output an S-PDIF signal via coax cable (with an RCA connector on board). Should I use any precautions, like resistors and other terminators? How should I do it?

Thanks in advance!

Kind regards, Samuel Lourenço

  • You'll have to add a coax line driver circuit. Examples are here:

  • Hi Samuel,

    Digital output is in TTL level and it is not much powerful. I used SN75176B (RS485 transceiver) which is very cheap and you can use them for both S/PDIF outputs like coaxial 75ohm and symmetric AES 110ohm. I used voltage divider on their output for smaller voltage according to standard and match output impedance. Also you can use pulse transformer on the output for galvanic isolation. You can inspire on my page with PCM2706 based USB DAC:

  • Thanks Pavel,

    Unfortunately I have trouble finding the impedance matching transformer. I also thought about going optical, but the parts are also difficult to find. Eventually, I gave up on that idea. For a very compact USB DAC, I wan't to keep parts to a minimum (I'm doing a pen).

    Your idea though, is to consider for another project [[Just a board to generate the signal to test another DAC, that will be built around the CS8416 and the CS4354. This other DAC will have an interface to mute the serial data case another signal than audio is detected by the S-PDIF receiver. It will use very fast 74AC gates, worst case causing a 20ns delay (the clock cycle is 80ns at 192Kbps)]]. As for the S-PDIF generator, I was actually thinking about using an 74AC not gate driving two mosfets (half-bridge), then the attenuator to reduce it to 1.5V, but the SN75176 might be fast enough and a better idea.

    By the way, I had come across your site before, and indeed I consider it to be a reference.

    Kind regards, Samuel Lourenço

  • Hi Samuel,

    for S/PDIF isolation is usually used 1:1 Pulse transformer. I succesfully used these three types: S22083, PE-65612, DA102C. For example DA102C is relatively cheap and is available on the Farnell in Europe. Optical parts are also available there. Specially parts from Sharp. Optical has limitation of Bandwidth. I have better experience with isolated electrical S/PDIF connection than optical.

    Best regards, Pavel

  • Thanks Pavel! I thought I would need something different, but I think I can get those trafos on Mouser.

    Your help was really valuable.

    Kind regards, Samuel Lourenço