0
\$\begingroup\$

I am trying to measure the voltage between node 2 and 3, which I am guessing should be zero. With a multimeter I read 0V. With an oscilloscope I am reading about Vb/2 if I remember correctly. Why are these measurements different and is there a way to fix them?

The idea is to feed node 2 and 3 into a difference amplifier, so I want to make sure the Vb/2 factor is either fixed or understood to be zero so that it does not saturate the opamp.

circuit

\$\endgroup\$
  • 1
    \$\begingroup\$ You have to use two probes and subtract the node voltages at scope \$\endgroup\$ – Genzo Apr 14 at 16:23
  • 2
    \$\begingroup\$ Presumably because you have the scope ground on node 5 \$\endgroup\$ – evildemonic Apr 14 at 16:28
  • 2
    \$\begingroup\$ Unless you are using differential probes or your scope has isolated channels, the ground clip on each probe is actually all the same connection. \$\endgroup\$ – DKNguyen Apr 14 at 16:32
  • 1
    \$\begingroup\$ I wouldn't say that measurements are misleading unless your circuit defies the laws of physics. You're either measuring incorrectly or you don't know entirely what you're trying to measure. As @Toor mentioned, you'll need a diff. probe to get the solution you're expecting. \$\endgroup\$ – KingDuken Apr 14 at 16:45
  • 1
    \$\begingroup\$ Some scopes let you SUBTRACT the voltages on two channels. \$\endgroup\$ – analogsystemsrf Apr 14 at 17:10
1
\$\begingroup\$

Since nobody has explicitly laid out what is probably happening here, I will attempt to do so. Most likely you are observing the effects of common grounding. The oscilloscope probe grounds are connected more or less directly to the earth ground of the AC mains power input to the oscilloscope. This connection is inside the oscilloscope and you can't disconnect it.

This means that both oscilloscope grounds MUST be connected to the same thing (because they are internally connected to each other). You can't connect one of them to Vb and the other one across Vr, because doing so effectively shorts those two terminals together (whichever two are on the oscilloscope ground).

There is also the possibility that Vb itself (or some other instrument or plug or cable you have not shown us) is internally earth grounded.

The reason the volt meter works is that volt meters, being battery powered and handheld, have no earth ground connection. Even mains powered benchtop voltmeters will probably work because their measurement terminals are generally isolated.

You don't necessarily have to get a differential probe. You can just decide where you want to ground your oscilloscope probes, and reference all channels to that same ground.

Everybody has to learn this at some point so don't feel bad.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.