In the past when I did this, I used a high resolution digital voltmeter like a Fluke 8505A in sample mode with a simple 10 Hz low pass filter. I took samples for 10 seconds to get down to 0.1 Hz and calculated the standard deviation to get RMS noise. I made a recent post about this in connection with very low noise DC amplifiers.
To measure output ripple, I use a DSO set to line triggering. Averaging removes all noise which is not synchronous with the AC line frequency. A low noise preamplifier with offset capability or a very low cutoff frequency is still needed for lower level noise measurements. I have a Tektronix 7A22 now but its AC coupled low pass cutoff is only specified as better than 2 Hz and its offset capability is somewhat limited. It works great to help with higher frequency bandpass measurements though. A 7A13 has slightly better offset capability but is also higher noise. You said you want to avoid building a custom low noise amplifier however for more sensitive measurements, even a simple operational amplifier based design will help considerably. I always hack together an AC coupled amplifier using a low noise JFET operational amplifier as needed; the quality of the AC coupling capacitor is what limits low frequency performance. Check out Linear Technology application notes 83, 124, and 159 for some ideas and a discussion of the AC coupling capacitor problem: http://cds.linear.com/docs/en/application-note/an83f.pdf http://cds.linear.com/docs/en/application-note/an124f.pdf http://cds.linear.com/docs/en/application-note/an159fa.pdf On Fri, 1 Jul 2016 08:47:18 -0400, you wrote: >I have several supposedly low noise bench power supplies that I want to check >out. They seem to work well but are quite old, so of course one wonders about >the caps, etc. > >Can someone suggest a practical test regime to verify power supply DC noise >performance? Preferably one that doesn't require building up custom low noise >amps? > >Thanks! > >John _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
