{"id":27205,"date":"2019-10-21T08:13:43","date_gmt":"2019-10-21T15:13:43","guid":{"rendered":"https:\/\/blog.digilentinc.com\/?p=27205"},"modified":"2020-12-21T11:19:27","modified_gmt":"2020-12-21T19:19:27","slug":"scope-noise-due-to-ground-issues-part-one","status":"publish","type":"post","link":"https:\/\/digilent.com\/blog\/scope-noise-due-to-ground-issues-part-one\/","title":{"rendered":"An Examination of Oscilloscope Noise Due to Ground Issues"},"content":{"rendered":"\n

Ground potential differences can wreak havoc on precise oscilloscope measurements, especially with single-ended scopes- where you will always have an offset error in your readings.<\/p>\n\n\n\n

To illustrate the problem, we will start from a very simple measurement and work our way into more complex, realistic scenarios, intentionally making things erroneous by pushing what would happen.<\/p>\n\n\n\n

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 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.<\/p>\n\n\n\n

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\u2019t 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.<\/p>\n\n\n\n

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.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But this really isn\u2019t reality. In reality there is some resistance in our probe wires. Typically probe wires are small; let\u2019s assume 24 AWG and a probe length of 3 feet.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Given standard wire gauge resistances, we will have a resistance of about 77m\u03a9s. (3*25.67\/1000 ~ 77m\u03a9).<\/p>\n\n\n\n

Let\u2019s redraw our circuit including the 77m\u03a9 probe wire resistance.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

And again we get a nice steady 5v measurement on our oscilloscope.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But this isn\u2019t 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 38m\u03a9s of resistance (6*6.385\/1000 = ~38m\u03a9).<\/p>\n\n\n\n

Let\u2019s redraw our circuit including the 38m\u03a9 power cord resistance.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But now we see a 40mV offset error in our oscilloscope measurement.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

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 * 77m\u03a9 = ~38mV)<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But again, this isn\u2019t 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\u2019s assume that our PC is upwind of our oscilloscope by 10 feet between 2 wall outlets. Given our chart, that would yield a wall resistance of about 25m\u03a9 (10*2.525\/1000 = ~25m\u03a9).S<\/em><\/p>\n\n\n\n

To continue, let\u2019s update our schematic to include the 25m\u03a9 wall resistance between our instruments.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

We now measure 5.07v on the oscilloscope, a 70mV offset error. Our 24 AWG Probe ground wire is passing 848mA, and the difference between the PC and oscilloscope internal grounds is now 65mV.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But this still really isn\u2019t reality. In reality, there is probably something else plugged into our wall branch circuit besides just the PC and oscilloscope. Our wall circuit can provide 15 amps from the branch circuit breaker. Our PC takes 3A, or oscilloscope takes 1A, let\u2019s add something big upstream on the branch circuit, say a 7A appliance.<\/p>\n\n\n\n

Let\u2019s add that to our circuit.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Now we measure 5.14mV on our oscilloscope, an offset error of 140mV! Our 24 AWG probe ground wire is passing 1.83 amps! That probe ground wire might be getting warm!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But this really isn\u2019t reality. In reality there is probably some switching noise going on in the PC and oscilloscope. Let\u2019s simulate that with a 500mA 100kHz square wave on the PC, and 250mA 500kHz triangle wave on the oscilloscope.<\/p>\n\n\n\n

Let\u2019s add that to our circuit.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

We see a 34.6mV ripple on our oscilloscope measurement!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But let\u2019s not stop just yet! Let\u2019s add upstream on our wall branch circuit a noisy appliance. Now typically it would probably be at 60Hz, but that is going to be hard to show, so we are going to put a 3A 200kHz sine wave appliance on the circuit. We picked 200kHz so we can easily see the effects on our simulated oscilloscope trace. In reality, it probably would be a much lower frequency appliance, but the effect would still be there.<\/p>\n\n\n\n

For fun, let\u2019s add that to our wall branch circuit.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Now we have nearly 100mV of ripple measured on the oscilloscope and a really ugly looking waveform!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

So here we are, using our nice expensive Tektronix\u2019s oscilloscope, attempting to measure a nice steady 5v source from our PC; and what do we see? We see a messy signal offset by 140mV!<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Our nice 5v output doesn\u2019t look so nice anymore, but it really isn\u2019t real, it is a scope measurement distortion due to differentiating ground references.<\/p>\n\n\n\n

Now let\u2019s take a look at what we have on my desk; a real world condition. Let\u2019s measure my PC USB ground against our Tektronix\u2019s ground (ground to ground) with my Agilent DMM (which has an isolated ground). We see a 95mV of ground differential between the PC and the Tek. This is due to the IR drop in the power cords and in the wiring in the wall, the branch circuit.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

But, beware; even though this shows 95mV of difference, as soon as I hook up the probe ground wire, the grounds will equalize via the probe\u2019s ground wire and the difference will be greatly reduced. But, there will be an IR drop across the probe ground wire and that is what will show up as an offset error on the scope trace. For example, when we hook up the probe ground to my PC, my offset error drops to 5.5mV; and in reality, we typically see anywhere from 5-15mV of offset error on my Tek. <\/p>\n\n\n\n

Using a differential probe (ground to ground), we can see hundreds of mVs of noise (probably switching noise). This will all show up as additional noise on any scope trace.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

So how do we fix this? How do we see what is really there?<\/p>\n\n\n\n