Today we will be discussing how ground potential differences will effect oscilloscope measurements. In particular, the issue of using single ended scopes, where you will always have an offset error in your readings.
To illustrate the problem, we will start from a very simple measurement and work closer to reality, intentionally making things erroneous by pushing what would happen, but still staying within the limits of reality.
The scenario used includes a PC providing 5 volts from a USB hub. We will measure that 5v using a single ended scope (like a common Tektronix desk scope). For simplicity and illustration, we will use an ideal 5v source so all distortions we see on the simulated scope traces will be from external factors.
Envision plugging the Tek scope into a 120v wall outlet, and our PC into a different wall outlet upstream from the Tek, but on the same 120v branch circuit. For simplicity, we will use 120v RMS DC instead of 120v RMS AC as our wall outlet power. We don’t want to model rectifiers in the Tek or PC; but the effect on the scope traces using DC instead of AC will be similar; similar enough that it will make clear what is going on.
We will pick currents and wire sizes that are realistic, but intentionally in a direction to best illustrate measurement distortions. For our simulations, we will say the PC draws a nominal current of 3As and the oscilloscope draws a nominal current of 1A. These are modeled as current sources in the simulation.
Our simulated oscilloscope is at the OSC (probe) in the schematics. And as expected, the measured value on our oscilloscope is a nice steady 5v.
But this really isn’t reality. In reality there is some resistance in our probe wires. Typically probe wires are small; let’s assume 24 AWG and a probe length of 3 feet.
Given standard wire gauge resistances, we will have a resistance of about 77mΩs. (3*25.67/1000 ~ 77mΩ).
Let’s redraw our circuit including the 77mΩ probe wire resistance.
And again we get a nice steady 5v measurement on our oscilloscope.
But this isn’t reality. In reality there is some resistance in our power cord that plugs the PC and the oscilloscope into the wall. We will assume the power cords are 18 AWG and are 6 feet long for both. Using our chart, the power cords will present about 38mΩs of resistance (6*6.385/1000 = ~38mΩ).
Let’s redraw our circuit including the 38mΩ power cord resistance.
But now we see a 40mV offset error in our oscilloscope measurement.
If we look at the Probe ground current, we see that there is 500mA (1/2amp) of current going through our little tiny 24 AWG probe ground wire. There is almost no current going through the positive probe wire.
What is happening, the oscilloscope and the PC have different internal ground potentials because of the IR drop in their respective power cords. We can see there is a 38.25mV different (our 0.5a * 77mΩ = ~38mV)
But again, this isn’t reality. In reality there is some resistance in the wall branch circuit between the PC and the oscilloscope. Now a 15 amp wall branch circuit will typically use 14 AWG wire. Let’s assume that our PC is up wind of our oscilloscope by 10 feet between 2 wall outlets. Given our chart, that would yield a wall resistance of about 25mΩ (10*2.525/1000 = ~25mΩ).
Stay tuned for the next installment of this series, where we continue the testing!
This blog post is courtesy of Keith Vogel.