http://www.andrewkilpatrick.org/blog/?page_id=215 S/PDIF circuits Here are a few S/PDIF circuits you can incorporate into your own designs. I haven’t included drawings for TOSLINK devices because they are quite simple, and the datasheets provide enough information to use them easily. These schematics are very basic but usable circuits. These circuits were originally found on other sites, but after having been successful using them, I decided to post my own drawings of them. # The general idea with the the TTL to coax circuit is as follows: * Buffer the TTL input with U1a. This causes it to invert, and provides a high input impedance for the output from your TTL S/PDIF connection. * U1b, c, and d provide lots of current output to drive the output line. * C1 provides some AC coupling. A S/PDIF signal is supposed to wiggle around ground as an AC signal of about +/-0.5V. C1 removes the DC and lets the signal float around ground. S/PDIF is a signal in which each bit causes a transistion in the signal. AC coupling only works for a signal like this. * R1 and R2 form a voltage divider to bring the voltage level down. The output from the logic gates will be 5 volts. But S/PDIF coax wants about 1V. (peak to peak) Also, it wants somewhere close to a 75 ohm source impedance. Perhaps these values should be slightly lower to match the line impedance better. But this is what I used and it worked for me. # The coax to TTL circuit works as follows: * R4 terminates the input with 75 ohms. * C2 couples the signal, removing any DC components. * R3 and R5 make U2a work as an amplifier with a controlled amount of gain. * U2b buffers the signal to the output. * When a small signal is applied to the coax input, it jiggles the input of U2a enough to case a full 5V transition on the output. # Unused logic inputs? You should always terminate unused inputs on logic chips. Usually this means tying them to the positive supply. (+5V) If you don’t do this they will pick up nearby signals and noise and waggle the circuits inside the chip. This wastes power and can cause interference with other parts of your circuit. Don’t connect unused outputs to anything.