# Getting negative voltage reading when negative voltage does not exist. Why?

I am beginner trying to make sense of some weird voltage readings I am getting. Please take a minute to look at the circuit below:

If I take my oscilloscope and capture the voltage activity across resistor R1 at the moment the switch is closed I get the following:

Can you see the issue here? I would have expected the oscilloscope trace to start up at 4 volts and then drop down to 0 volts after the switch is closed. However, instead of that, the trace starts up at 4 volts and drops all the way down to -2 volts! How is this possible when my power supply range goes from 0 volts to 8 volts? The minimum voltage should be 0 volts and not -2 volts right?

So the question is why is this happening? Is the voltage really dipping down to -2 volts or is this some kind of oscilloscope artifact happening here?

In case it helps, the oscilloscope that I am using a Rigol DS1104Z oscilloscope with 100 MHz bandwidth.

Thanks.

• Are you doing this on a breadboard by any chance? Aug 15, 2015 at 19:59
• @Tom Carpenter: Yes, this was tested on a breadboard using 3" jumper cables to connect the components together.
– T555
Aug 15, 2015 at 20:04

There's a bit of ringing at about 70MHz. This is not unexpected unless your layout is really tight. A small amount of inductance in the wiring can cause this, in conjunction with stray capacitance.

For example, suppose you were using a solderless breadboard with 5pF of stray capacitance (WAG) plus the probe capacitance of maybe 15pF. The inductance required to resonate at 70MHz would be only 260nH which could be created by a loop of wires.

• Quick question. Where can I add a physical capacitor and / or an inductor to the original circuit so that I can exasperate this issue? I am asking this because I would like to understand the issue of stray capacitance an inductance a little better since it appears to always come back and haunt me. Thanks.
– T555
Aug 15, 2015 at 20:12
• Add a physical inductor in series with the switch and a cap in parallel with the probe input. Good on you for wanting to understand this stuff. Aug 15, 2015 at 20:29
• P.S. try twisting long wire pairs together in order to reduce the inductance. Aug 15, 2015 at 21:10
• Also consider where you place the gnd lead from your oscilloscope. If you are using a bench power supply then it is likely both the scope and psu will have a common ground. If you then measure across R1 the common grounds will cause you issues.
– BenG
Aug 17, 2015 at 0:43
• @Spehro Pefhany: I added the inductor and capacitor as you instructed and I was able to see how things got a lot worse than before, very interesting. One other quick question if you don't mind In your post you wrote "WAG". What is "WAG"? I tried searching for the term using different context but was unable to find information about it. I did saw a couple of other posts from you where you refer to the same term but I was not able to conclude exactly what that stands for. Thanks.
– T555
Aug 24, 2015 at 17:11

Your scope trace looks exactly as I would expect. Nothing ever really looks like a perfect square wave in a practical circuit. You are very zoomed in on the time scale, so you are probably seeing ringing due to parasitic inductance and capacitance in the test apparatus.

There are components in the practical circuit that aren't modeled in the ideal circuit which are constituting this situation.