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My new oscilloscope seems to be reading DC voltages very wrong. Connecting it to a rasperry pi's voltage rail (which is usually spot on 3.3v) gives a reading of about 4.5v: enter image description here The probes are adjusted correctly (the 1kHz square wave has flat tops and bottoms, but is 5.6v rather than the rated 5v). Testing a battery pack, measured with a multimeter to be 5.1v, gives 6.9v: enter image description here I have no idea what's going on!

The self calibration makes little to no difference, and there doesn't seem to be an option for manual calibration.

Here is the user manual (I have the 1022i version): http://files.owon.com.cn/probook/VDS_Series_User_Manual.pdf

Any ideas?

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    \$\begingroup\$ Have you ruled out your multimeter to be defective? whats the battery packs nominal voltage? \$\endgroup\$ – PlasmaHH Sep 10 '18 at 19:47
  • \$\begingroup\$ Where's the link to the user manual? \$\endgroup\$ – Transistor Sep 10 '18 at 19:48
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    \$\begingroup\$ PlasmaHH - That could be the case, but it's very unlikely. As I said, the raspberry pi 3.3v is usually within 2%, and the oscilloscope reading of 4.5v would be enough to fry some pi-compatible sensors. My multimeter, on the other hand, reads 3.306v. The battery pack is 4*1.2V NiHM batteries (in series). \$\endgroup\$ – JoeP Sep 10 '18 at 19:58
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    \$\begingroup\$ Maybe it's set for Metric voltage :-) \$\endgroup\$ – crj11 Sep 10 '18 at 20:15
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I had the same thing Voltage was reading 40-50% high. Solution:- use dedicated USB supply ( was sharing with phone ). I assume the USB was not supplying a clean 5V, so the OWON was struggling with a low or dirty power supply. Running perfectly now within the +/-3% now.

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  • \$\begingroup\$ That seems to make sense. Where do I get an adapter then? Can I cut open the USB cable and simply swap the gnd and v+ wires for a 5v battery pack (ensuring the laptop and battery pack have a common ground, but not a common v+)? \$\endgroup\$ – JoeP Sep 16 '18 at 12:14
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Before you test the oscilloscope on an unknown voltage, you should test it on a known voltage. Lucky for you there is a port that generates a 3.3V square wave on the unit itself. On point 4 in the pic below is where you should attach the probe. You should see a 1kHz square wave with 3.3V amplitude. If you don't see the 3.3V square wave then the unit is probably broken.

If you do see the square wave, then you need to worry about your setup and where your ground is while measuring. The scope has USB for it's ground so depending on what is upstream this could be affecting the voltage reading or creating a ground loop.

enter image description here

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  • \$\begingroup\$ JoeP has a model with 5V calibration signal, and the scope says it is 5.6V. \$\endgroup\$ – JRE Sep 10 '18 at 20:36
  • \$\begingroup\$ As JRE said, my scope has a 5v square wave. Also, it is USB isolated. \$\endgroup\$ – JoeP Sep 10 '18 at 20:49
  • \$\begingroup\$ Time for a new scope, if you just got it, I'd send it back even if you get a replacement. I wasn't particularity impressed with the owon scope I had, better than nothing I suppose. \$\endgroup\$ – laptop2d Sep 10 '18 at 21:49

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