I hope to impart on you a bit of wisdom I have learned from my years of travel and talking with well over 1,000 oscilloscope users. If you do yourself the favor of reading through this you will have gained enough insight to not shoot yourself in the foot like so many of the scope users I have visited. They are not to be judged, they were doing the best they knew how at the time, and we all make mistakes or could do better—I know this to be true for me. Once I pointed out the mistake and the solution to these users they usually all had the same reaction—“oh, that makes sense”, followed by the classic palm slap to the forehead.
These users I speak of all made the same mistake. They spent valuable time and money selecting the best oscilloscope to buy or use for their measurement task. Then they connected a high quality probe to their scope. In some cases the probe was the nice passive probe that came with their scope, other times they had sprung for a snazzy active probe (smart move going for the active probe upgrade, more on this in another blog post). Then, and this is the crux of the matter, they put a bunch of long, dangly connection accessories onto the end of the probe. Maybe it was something innocent looking like a nice convenient long ground lead or one of those super helpful looking long red input wires that make it easy to connect the probe to a grabber that they could clip onto a part on their board. In the end, the result was the same, the signal on screen “looked bad” or the device they were testing started to misbehave. This is usually when they grabbed me and said “Hey, you designed this probe. It’s not working right”.
The Weakest Link
What these users were experiencing was what I like to call the “weakest link” phenomenon. There are three links in the typical oscilloscope measurement chain—the scope, the probe and the physical connection to the target. You can have the best scope and probe that money can buy but if you put some crazy long wires on the end of the probe to make the connection to the target easier you have limited the performance of the measurement system to be equal to the performance of those crazy long wires. The connection accessories are the weakest link. They will limit the measurement bandwidth and they can excessively load your target.
Think of those long connection accessories as inductors that are being placed in series with the probe. If they are connected to the signal pin of the probe they are going to limit the bandwidth of the signal that can pass through to the probe because an inductor’s impedance increases proportional to frequency. Additionally, since there is an impedance mismatch between the long inductive connection accessory and the probe input, the signal traveling up the wire will create a reflection that will show up on the scope. If that nice long ground wire is connected to the probe similar results will follow. The long inductive ground creates a higher impedance path for the ground return currents flowing on the shield of the cable. This will also limit the bandwidth of the probe. Additionally, the impedance resulting from the inductance of the long ground wire can create a voltage potential between the ground on the target and the ground point at the tip of the probe resulting in measurement error and poor common mode rejection. If all that isn’t bad enough, those nice long connection accessories act as an antenna and can pick up noise from your surroundings and couple that noise into your measurement. Finally, there is loading. These long wires that are touching your circuit are now part of your circuit and their inductance and capacitance can change the way your circuit behaves. We call this probe loading.
Shorter is Better
At this point I can almost hear you saying “if those connection accessories are ‘bad’ why do you include them with your probes?” We include those accessories for convenience. The idea is that you use those accessories for qualitative measurements, things like “is the clock toggling”, is there “data on the bus”, “is the 5V up”. They make it easy to poke around your circuit quickly to check for functionality. If you want to make quantitative measurements like rise-time, over-shoot, noise levels, et cetera, then we intend for you to remove the convenience accessories and use the shortest connection possible. That’s it, that’s the punch-line, shorter is better.
Consider this example. I take my fancy 2 GHz active probe and I configure it three different ways, long wires connected to grabbers, long wires only and short input pin and ground contact. Notice how the bandwidth increases as the length of accessory in front of the probe gets shorter. By the way, we try to make it easy for you and we publish these bandwidth limitations in the product manuals.
Notice too how the probe loading (how the physical presence of the probe changes the way your circuit functions) decreases as the length of the connection accessory decreases. In this example the original circuit is producing a rising edge with a rise-time of 1.1 ns (the green trace). Connecting the probe to the circuit using the long wires and grabbers loads the circuit and the rise-time changes to 1.7 ns. When I remove the grabbers and just use the long wires the rise-time gets better, 1.5 ns, though you can see the connection accessories are still affecting the circuit. Finally, I remove all the wires and go with the shortest connections for this probe and the circuit rise-time is back to its original 1.1 ns.
I Hope This Was Helpful
Don’t feel bad if you’ve been making the mistake of using long connection accessories when making important measurements. You’re in good company, a lot of oscilloscope users have made this mistake and to be honest, I have too. Just remember, it’s ok to use those long, convenient connection accessories for a quick peek but if the signal looks strange or you are not getting the answer you expect, you’d do best take them off and go with the shortest connection possible. Shorter is better.
See all of the Keysight Oscilloscope probes.