DC Supply for EEVblog HVP-70 - Page 1
Forum section: electronics / testgear

author: forrestc
author_rank: Unknown
timestamp: October 03, 2017, 05:08:55 am
content: Bought two of the HVP-70's a while back....I'm loving actually having a couple good differential probes around. Might actually buy a couple more.However, for multiple-day measurements, it appears that I definitely need to add a power supply. The Manual says 6VDC or regulated 9VDC, or a USB power cord, and has a note that says it must be between 4.5V and 10V to operate correctly. Since I have a high power USB 3.0 hub just below my oscilloscope on the bench (to plug test equipment into), the USB route will work best, so I'll probably just grab a couple 3 foot usb to 5.5x2.1mm cables and coil them up wuith the leads in the box when I'm not using them. (less bulk than a AC adapter).My only concern is about the addition of noise from the power source and/or creation of ground loops (causing more noise). Don't see a spec for PSRR.... And not sure how clean the ground is on the USB bus. Do I need to worry about this for most measurements? (Yes, I know, moving to batteries for the most sensitive measurements is probably a good idea).

author: Shock
author_rank: Super Contributor
timestamp: October 03, 2017, 06:53:13 am
content: There is irony in that question for sure, how much noise will I get on my oscilloscope probe from some random usb hub. Hopefully you have a clean floating DC linear bench supply. This would be a good 3 way test, USB vs bench vs battery.Do something like in this video, after checking the noise floors. The ground lead on the probe flying in the breeze is a little comical in this video and exaggerates results. Be interesting to see the results.

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2017, 07:09:08 am
content: Quote from: forrestc on October 03, 2017, 05:08:55 amAll of which implies there's additional input power supply conditioning when externally powered.....wouldn't you think ?External power for differential probes is common, most of the Pintek range are supplied with wallwarts.Try and see if you can find additional noise from the probe when it's externally powered, if there's any discernible I very much doubt it will impact on measurements.

author: forrestc
author_rank: Unknown
timestamp: October 03, 2017, 08:17:17 am
content: Quote from: tautech on October 03, 2017, 07:09:08 amYes, but having dealt with more than one LDO which in itself has horrible PSRR at high frequencies I figured I'd ask just in case...QuoteSo I ended up being done with one project on my bench and could test this out between projects...Oddly, the amount of noise in my testing seems to be *better* with the USB supply than with fresh Alkalines, although it is a bit more spiky. The DC offset seems to be different by about 2mV, regardless of the scale, but the noise is definitely higher amplitude with the batteries, not a lot, but definitely visibly more. A FFT seems to indicate this as well.. noise level on the batteries are higher across the spectrum, again not by much, but some. Not the result I was expecting.I also tested with the linear supplies on my bench, they both seem to be between the batteries and the USB supply.In reality the difference is so minor, I don't think it's going to matter to me what I use.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2017, 08:34:19 am
content: The low noise when using USB or the lab supply might be due to additional grounding if the USB connected part and the supply. So just connecting one side of the supply should have a similar effect when using batteries.It looks like the power supply filtering is really working well.There might also be the option to use rechargeable batteries, as a step in between.

author: forrestc
author_rank: Unknown
timestamp: October 03, 2017, 09:18:30 am
content: Quote from: Kleinstein on October 03, 2017, 08:34:19 amI initially guessed the same, except I don't think this USB hub is grouned except through the USB cable to the computer. The hub's power supply has an ungrounded power cord. Nothing other than the USB cable to the computer would be connected to ground. And that's probably a 10 ft cord, to a big noise radiator. It *could* be that I've tied the grounds for the big noise source in the room with the ground on the scope more directly, but even that is doubtful, since the scope is also tied to the computer with a USB cable.

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2017, 09:31:29 am
content: BTW forrestc, the Pintek wallwarts are linear units not SMPS.

author: Shock
author_rank: Super Contributor
timestamp: October 03, 2017, 11:13:05 am
content: Same as Tektronix.