Damping Resistor(s) for MCLK Distribution

     The picture below shows a ADAU1401 eval board sending TDM audio to a ADAU1701 on a re-purposed prototype board.  Four twisted pairs run the MCLK, BCLK, LRCLK, and TDM data between the boards.

     The received sound was erratic until I spliced a 150 ohm resistor into the yellow MCLK wire near its source end.  The 1401's MCLK output has a lo-Z buffer which rings with the unterminated wire, as shown below, left.  It looks much better with the resistor at right:

     Thus it's a good idea to damp the MCLK signal between chips, especially when pcb traces or wires are long,

    •  Super User 
    on Nov 23, 2014 8:19 AM

         On a recent prototype board with distributed clocks (of course no fly-wires) --100 ohm damping made the waveforms sloppy, 10 ohms did not prevent the overshoots, but 47 ohms was just right.

    •  Analog Employees 
    on Oct 2, 2015 1:17 AM

    Also note that Bob twisted the ground wire around each clock fly-wire. This is very important to do. It provides a path back for parasitic currents and provides an impedance for the transmission line. I like to think of this as it gives the magnetic field something to push off rather than flapping in the breeze.

    Dave T