I'd lilke to know if I should use a switching power supply to power an ADI DDS.
Generally, it is best not to use a switching supply to power a DDS. The main reason is that switching supplies usually use magnetics (inductors and/or transformers) to store energy instead of wasting it as heat in a series pass element (like a transistor). That is why they are almost always more efficient than linear supplies. Improved efficiency comes at a price, however, because the pulsating magnetic fields associated with a switching supply along with the large current transients associated with the supply's switching transistors, can cause unwanted spurious spectral content to couple into the DDS output signal. These spurious signals can be very difficult to isolate and eliminate. Using a linear supply (series or shunt) eliminates the power supply as a source of spurious coupling.
Switching regulators have been proven to be effective in powering even the most noise sensitive design.
With proper layout and measurment techniques the power supply. A DDS is basically an high speed DAC with custom digital logic integrated and possibly a clock multiplier.
This CN show how the performcance of the DAC is not degraded by a switcvhing regulator.
Similar designs have been demonstrated with other nise sensitive high speed circuits
A DDS is often followed by a differential amplifier, amplifiers have well controlled PSRR in the 1MHz and below region, just like DDSs, DACs and ADCs, so typically care must be taken in controlling noise above 1MHz. Conducted noise is easily eliminated with LC fileters (as shows in the 2 CNs above)
It is true that switching regulators have fast switching transient and their nature needs to be understood. The main technique to contain EMI and GND bouncing is to minimize the lenght of the power path loop (current recirculating loop is formed between CIN, FETs, L, COUT, GND) with a PowerGND dedicated to control the current between the low side FET, Cout and CIN.
This guarantees also that the GND reference for the rest of the ciscuits (DDS, amps, etc) are not referenced directly to the PGND (which it is true it can be very noisy.) But if the AGND is connected to the Output cap away from the recirculating current described above, in this case all conducted switching noises are eliminated.
Often people see switching noises in their measurements because of incorrect measurement techniques. If you measure the output of the DDS with a probe that has a ground lead simply connected to a generic GND test point, then your ground probe acts as an antenna picking up magnetic flux that is not necessarily present at the output of the DDS. The proper technique to measure a signal in the presence of switching regulators is not to use a standard scope probe. Instead use either a coax cable (or a probe with a extemely short GND lead) and connect the GND lead as close as possible to your signal measurement point.
Transmitted EMI that a probe is "picking" up is not actually in the signal. EMI can only couple back to an actual circuit if the layout has long traces that are high impedance. A proper DDS design would use short 50ohm controlled trace which will lot pick up this EMI.
EMI can be further controlled with poroper component choices (shielded inductors), snubbers on the switching node, and in extreme cases shielding.
Typically the most sensitive section of the design is the VCO, because VCO's are controlled by the PLL loop with a digital signal that is directly referenced to the supply in a single ended fashion. DDS's do include VCOs but often they are DLL based and not analog voltage controlled.
In the case of a sustem with an actual Voltage controlled loop then power that specific supply with an LDO, or with a well filtered Switching regulator.
In conclusion, make sure you understand what you are measuring (real signal or just picked-up EMI or incorrect ground reference), use appropriate layout guidelines when designing with switching regulators, use appropriate post filtering where necessary, use LDO is the presence of VCOs and then you can power even DDSs with switching regulators.
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