Is there any resistor value limit at the MCLK input of ADAU1761? Why does the eval kit use 49.9ohms? Why can't use other value or is there any rational in between the RC value and master clock settling time?
Thanks and BR,
The 12.288 MHz MCLK waveform has sharp square edges, likely including harmonics in excess of 100 MHz. The eval kit's PCB trace running from the 12.288 MHz oscillator to the -1761's MCLK input has enough stray inductance and capacitance to "ring" at these frequencies. As a result, "spikes" could appear on the MCLK waveform by the time it reaches the -1761. A misshapen MCLK could cause intermittent operation, especially if the ringing reaches excessive peaks.
The 49 ohm series resistor "damps" the MCLK line, minimizing the "spikes." The resistor is placed near the source end of the trace. Typical values range from 22 to 100 ohms depending on PCB trace geometry. The resistor serves to more or less match the trace's "transmission-line" impedance -- thus, reasonable values have only a moderate effect on the clock's rise time. Although design programs that figure trace impedance may be used, it's sufficient to choose a good value by experiment with your prototype PCB. For an example use of damping resistors, see: https://ez.analog.com/message/130241#130241
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