Jitter is created by the quality of transmission of S/PDIF over Coax in DAC-SQ3.
@amirm's statement here is very important to understanding the quality of the signal that is being delivered to the devices using coax cable. The SPDIF spec calls for 75 ohm impedance for the signal chain. While the cable—probably—is very close to the nominal value, the RCA connector is nowhere near the correct impedance value.
RCA connectors—by virtue of their inner conductor to outer conductor diameter ratio and dielectric—are closer to 40 - 50 ohms. There are some companies that claim to make compression fittings in RCA style that are 'true' 75 ohm impedance, but that's simply not possible given the dimensions of the RCA connector. There will always be an impedance mismatch at both ends of the cable.
In many cases the output signal level is also nowhere near what the spec calls for and may exceed the spec's level by several times. The level mismatch is probably intentionally designed in to accomodate losses for longer than spec runs of out of spec cables—think about those cheap video cables with the yellow RCA connectors on them that used to come bundled with every VCR and DVD player with component outputs. So, despite all the cable arguments that surround analog audio, here in the 'digital domain', cables/connectors matter if you want low jitter.
A cable impedance caculator that has the math. You have to guess the dielectric constant of your connectors, but the basic premise still stands.