Does the switching power supply (DC/DC converter) reduce the performance of the ADC?
Engineers generally believe that switching power supplies can degrade the performance of the ADC, so it is usually preferred to use a low dropout (LDO) linear regulator instead of a switching regulator, but this is not entirely true. LDOs have lower ripple and noise specifications, but recent research has shown that efficient switching regulators can also be used in some converter designs, provided the designer has a good understanding of the circuit topology and uses some practical techniques. At the same time take some necessary precautions.
The first is to select the converter and then select the correct switching regulator, not any switching regulator can be used. Check the noise and ripple specifications of the switching regulator and the switching frequency from the data sheet. A typical switching regulator has approximately 10μV rms of noise over a 100kHz bandwidth. Assuming they are all white noise, the noise density in the relevant band is equivalent to 31.6 nVrms/rt-Hz.
Second, check the converter's power supply rejection (PSR) specifications to understand the key points of converter performance degradation due to power supply noise. Most high speed converters typically have a PSR of 60 dB (1 mV/V) in the first Nyquist zone.
The noise floor is 11.26 nVrms when using a 16-bit ADC with a 2Vpp full-scale input range, a 78dB signal-to-noise ratio, and a 125 MSPS sample rate. Regardless of where the noise originates, it must be below this value so as not to affect the performance of the converter. In the first Nyquist zone fs/2, the converter noise is equal to 89.02 μVrms (11.26 nVrms/rt-Hz) x sqrt (125 MHz/2). Although the switching regulator's noise (31.6 nv/rt-Hz) is twice the noise of this converter, note that the converter has a 60dB PSRR that rejects the switching regulator's noise to 31.6 pV/rt. -Hz (31.6 nV/rt-Hz × 1 mV/V). This noise is much smaller than the converter's noise floor, so the noise of the switching regulator does not degrade the converter performance.
In addition, power supply filtering, grounding, and wiring are also important. Adding a 0.1 μF capacitor to the ADC supply pin reduces the noise to less than the above calculation. It is also possible to add a simple LC filter to the power supply output, but the cascaded filter can suppress the switching regulator noise more. It is well known that each time a filter is added, a noise suppression effect of about 20 dB/10 octave is obtained. The tightly stacked power supply and ground plane (pitch <4 mil) add inherent high frequency decoupling to the PCB design. Finally, good physical partitioning is key, trying to keep sensitive analog circuits away from switching circuits.
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