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Power Supply Noise Specsmanship. Caveat emptor....
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Posted on July 11, 2016 at 13:02:56 | ||
Posts: 46200
Location: USA Joined: June 22, 2001 Contributor Since: February 2, 2002 |
I'm looking at the wall wart power supply noise spec by an audio equipment manufacturer. It's on their website as well as on the retail box for their $50 SMPS wall wart. They claim: - Noise*: ~1uV *Average audio band noise floor. I assume they are specifying the RMS noise across a 20 Hz - 20 KHz "audio band" but they don't actually tells us. Reputable power supply manufacturers will often specify noise over a 20 Hz - 20 MHz (not KHz) range and tell us so. And they will also provide Pk-Pk noise specs as random noise spikes are easily hidden in the averaged RMS values. I found an interesting application note by Keysight Technologies titled "How to Read Your Power Supply's Datasheet". For those who are not aware, Keysight Technologies was once Agilent and Agilent was the test equipment and instruments spin-off from Hewlett-Packard many years ago. The complete Application Note is linked below. Here's an excerpt that might be of interest with respect to power supply noise: "The output noise specification is labeled in different ways on a data sheet depending on the vendor. Output noise refers to the deviations of the DC output voltage from its average value over a specified bandwidth. It is typically measured in rms (root mean square) and peak-to-peak(p-p). It is important to note that there are two types of noise to consider: normal mode and common mode. If the specification does not explicitly state that it is common mode noise, then it is a specification of normal mode noise. Normal mode noise is the voltage deviation on the positive output terminal with respect to the negative output terminal." "The most important noise specification is the peak-to-peak voltage noise. If the peak-to-peak voltage noise specification indicates that the deviations are large, they could damage or destroy a sensitive device, such as a voltage controlled oscillator (VCO). The rms measurement is not an ideal representation of the noise and should not be considered a good representation of a power supply's noise performance. Fairly high output noise spikes of short duration could be present despite a low rms noise specification since they do not appreciably increase the rms value. " "You should be wary of a power supply vendor that only specifies the rms noise value. Also, make sure you note the bandwidth over which the measurement was made. It should be in the range of 20 Hz to 20 MHz. Poor noise specifications can be hidden by changing the bandwidth over which it is measured." |
RE: Power Supply Noise Specsmanship. Caveat emptor...., posted on July 12, 2016 at 05:34:44 | |
Posts: 2424
Joined: July 11, 2007 |
I think we had that discussion at least half a year back. Just try "search". ----------------------------------------------------------------- blog latest >> The Audio Streaming Series - tuning kit pCP |
Trolling again, posted on July 19, 2016 at 00:54:19 | |
Posts: 13158
Location: Kent Joined: June 1, 2002 |
nt |