Rarely Asked Question Page
Rob Reeder ,
Senior Systems Engineer
Will a switching power supply (dc-to-dc converter) really degrade the A/D converter's performance?
Engineers commonly feel that a switching power supply can degrade the performance of an A/D converter, leading them to choose a low dropout (LDO) linear regulator over a switcher, but this perception isn't entirely true. LDOs have lower ripple and noise specifications, but recent studies show that efficient switchers can be employed in some converter designs provided the designer understands the topology, applies some practical know how, and takes the required precautions.
First choose the converter and then choose the right switcher. Not just any switcher will do. From the datasheet, check the switcher's noise and ripple specifications, as well as its switching frequency. A typical switcher might have 10 µV rms noise over a 100-kHz bandwidth. Assuming the noise is white, this is equivalent to a noise density of 31.6 nVrms/rt-Hz over the band of interest.
Next, check the converter's power supply rejection (PSR) specification to get an understanding of where the converter’s performance will degrade due to noise on the supply. 60 dB (1 mV/V) is typical for most high-speed converters over the first Nyquist zone.
Using a 16-bit ADC with 2-Vpp full-scale input range, 78-dB SNR, and 125-MSPS sampling rate, the noise floor is 11.26 n V rms. The noise from any source must be kept below this in order to prevent it from being seen by the converter. In the first Nyquist zone, fs/2, the converter noise will be 89.02 µV rms (11.26 nVrms/rt-Hz) × sqrt(125 MHz/2). Although the switcher’s noise (31.6 nv/rt-Hz) is more than twice that of the converter, remember to account for the converter’s 60-dB PSRR, which will suppress the switcher’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 switcher's noise will not degrade the converter's performance.
Supply filtering, grounding, and layout is important too. Adding 0.1 µF capacitors to the ADC power supply pins will reduce the noise even lower than that calculated above. A simple LC filter on the power supply output may work, but a cascaded filter will suppress the switcher’s noise even more. Remember that approximately 20 dB/decade is gained for each additional stage. Tightly stacked power and ground planes (<4 mil spacing) can add inherent high-frequency decoupling to the PCB design. Lastly, good physical partitioning is key; keep sensitive analog circuits away from switching circuits. For further design assistance with power supplies and converters, contact your local ADI FAE.
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Senior Systems Engineer
Rob Reeder is a senior system application engineer with Analog Devices Inc. in the Industrial and Instrumentation Segment focusing on Military and Aerospace Applications in Greensboro, N.C. He has published numerous papers on converter interfaces, converter testing and analog signal chain design for a variety of applications. Formerly, Rob was an application engineer for the high-speed converter product line for 8 years. His prior experience also includes test development and analog design engineer for the Multi-Chip Products group at ADI designing analog signal chain modules for space, military, and high-reliability applications for 5 years. Rob received his MSEE and BSEE from Northern Illinois University in DeKalb, Ill., in 1998 and 1996 respectively. When Rob isn’t writing papers late at night or in the lab hacking up circuits, he enjoys hanging around at the gym, mixing techno music, building furniture out of old pallets and most importantly chilling out with his two boys.