Evo ovde tekst. Sve jedno kakvo napajnje ima PC - jer se EMI stvara i na samoj Matičnoj. Na samom USB BUS-u - onda interakcija sa svim ostalim uređajima zakačenim na USB BUS
Usb kabl mora da se napravi da obija EMI - ili u samom DAC da postoji mehanizam koji ga odbija.
ZNAČI FERITNA JEZGRA (i ne samo to)
Another potential source of emissions for LS devices is the high frequency energy, generated by othercomponents on the LS device, that can couple from the device’s PC traces and components to the cable.Particular care should be taken that the device’s clock does not couple to the USB cable. It is important tominimize the amount of high frequency energy that the clock circuit generates. This can be done bylimiting the highest frequency to 4x the data rate or 6 Mhz. This frequency permits 4x oversampling for astraightforward DPLL implementation.It is also important to limit the edge rates on all signals that go off chip. Due to the larger capacitance andlarger loop area that these signals (as opposed to intra-chip signals) experience, their dI/dt contributesmore to emissions. For example, a low current output buffer driving an external crystal or resonatorthrough 1.0KΩ produces less noise than a similar frequency oscillator, which is usually designed to drivehigh capacitance loads with fast edge rates. For this reason, a crystal or ceramic resonator is preferable toan oscillator.The efficiency with which a current loop couples to the cable increases with loop area and with itsproximity to the D+ and D- lines. Therefore, D+/D- and other signal path loop areas should beminimized, particularly clock lines. Coupling can also be minimized by maintaining a high qualityground plane. Noise will then couple back into the ground plane rather than to the cables.Power supply noise reduction techniques need to be addressed also. Standard low noise practices such asgood VBUS and ground bypassing should be observed; this means using both bulk capacitance and multiplelow ESL capacitors. Note that the USB spec places an upper limit on the amount of bulk capacitance of10 μf. Power supply noise conduction into the USB cable power conductors can be reduced by the use offerrite beads on both the VBUS and Gnd leads. Due to the large difference between the low speed signalbandwidth (< 4x the bit rate) and the lower limit of class B emissions range (30 Mhz), ferrite beads mayalso be used on the D+ and D- lines, if necessary. However, the frequency response through the ferritebeads on D+ or D-must not roll off by more than 2.0 dB at 6.0 Mhz.
https://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf