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This article explains how the inductance of passive probe ground leads can dramatically reduce effective bandwidth, distort measurements, and introduce resonance and frequency-response errors.
When you open the box containing your shiny new oscilloscope, one of the items you'll likely find inside is a set of basic 10:1 passive probes (Figure 1). Those probes have a ground lead that you'll likely want to use when you make measurements. Your probe has a bandwidth specification that's in the 500 MHz range; that spec was obtained at the factory with a specialized test jig having a specific ground inductance and source impedance. Now, the way in which you connect your ground lead can have a big impact on the real-world bandwidth and response of the probe.
Figure 1: Teledyne LeCroy's PP108, a representative passive probe
A glance at the probe's datasheet will show you its input resistance and capacitance. For a probe with a 500-MHz bandwidth, typical values are C = 9.5 pF and R = 10 Mohms. But one spec that's missing from the datasheet is ground lead inductance; that's because there are numerous ways in which people choose to ground their oscilloscope's probes.
The Effect of Ground Lead Loop Inductance
The most common way to ground probes is by using that long ground lead with the alligator clip at its end. However, this could create a ground loop inductance larger than 200 nH, which can greatly compromise the probe's performance. The equivalent circuit model for a probe grounded in this way is shown in Figure 2.
Figure 2: Equivalent circuit model for a passive probe grounded using a long ground lead
The input resistance of 10 Mohms is large enough to be ignored in most cases. What we want to look at is Iground and the effect that different inductance values will have on the response of this circuit. The voltage across the capacitor is the value measured by the oscilloscope; the voltage represented by the AC source is the signal under test. To get an idea of how these two values will differ, let's look at the transfer function of this series RLC circuit, or the ratio of the voltage across the capacitor, Vc, to the voltage source, Vs. We obtain the transfer function by dividing the impedance of the capacitor by the sum of all of the impedances.
Using the definitions shown here,
We obtain the transfer function:
What is the Lowest Inductance Passive Probe Ground Lead?
By hand-picking optimal values for R and L, we can get the probe's 500-MHz bandwidth. And one can get close to this performance by using Teledyne LeCroy's ground blade or ground spring probe accessories, which may have an inductance of 10-20 nH. It's also important to consider the ground-loop issue and connect to a close-proximity, low-inductance ground point on the DUT. Using copper foil can help with achieving a nearby ground connection and to minimize the length and inductance of the ground connection (Figure 3).
Figure 3: Using copper foil to help minimize the length and inductance of ground connections
In most cases, though, the ground connection people use is the long alligator-clip lead. What effect does this have on the bandwidth and frequency response of the probe? Well, for starters, if we use this lead, the ground loop of Figure 2 will be at least 10 inches long. If we take 20 nH/inch as a rule of thumb, we can calculate the inductance at about 200 nH. Using a Teledyne LeCroy ArbStudio function generator with a source impedance of 50 ohms, plugging these two values into the transfer function yields this plot (Figure 4):
Figure 4: Bode plot of passive probe response using high-inductance long alligator clip ground lead
We can see that the added inductance has moved the -3dB bandwidth of the probe from over 500 MHz to about 175 MHz. Not only that, but the probe's resonant frequency was pushed well within its bandwidth to about 115 MHz. At 65 MHz we can still see an error of 40%. In fact, to keep the error under 10%, we need to limit the frequency to 35 MHz.
Eric Bogatin produced a variety of webinars that were later grouped into the Eric Bogatin's Design and Test Basics webinar series. One of these webinars was A Simple Technique To Measure The Bandwidth Of Your Probe-Scope System was presented by Eric Bogatin of Teledyne LeCroy and Matt Diessner of NoiseCom (Wireless Telecom Group). Click the link above to view the series and register for this particular webinar, or use the button below to do the same. You can also preview the webinar content by downloading the webinar transcript file.
Steve is an Applications Engineer who most recently developed a remotely-accessible Test Engineering Lab. Steve’s 30-year Teledyne LeCroy career includes technical, marketing and sales roles. Steve enjoys bicycling, alpine skiing and camping.
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