Isolated Oscilloscope Inputs vs. Isolated Oscilloscope Probes

Some users in high-voltage test environments seek measuring instruments with isolated inputs because they want the safety and convenience of isolation without having to spend money on an isolated oscilloscope probe, like the Teledyne LeCroy DL-ISO or the Tektronix IsoVu®. While that's understandable, isolated inputs built into the instrument channel may be convenient, but they don't necessarily give you good performance, certainly not as good as  you would get from a high quality, high-voltage isolated probe.

Figure 1. Cascaded H-bridge signals captured using an isolated input (left) and an isolated probe (right).

Choosing a High-voltage Oscilloscope Probe for SiC/GaN Power Semiconductor Device Measurements

Figure 1: Wide-bandgap (GaN) power semiconductor device
waveforms captured using two, different probe topologies.
Click on any image to expand.

In our last post, we introduced you to a new tool on the Teledyne LeCroy website: The High-voltage Probe Selection Guide. To demonstrate the benefits of the guide, let’s explore further what must be considered when choosing an HV oscilloscope probe for power semiconductor device measurements.

Why are power semiconductor device measurements challenging?

How to Choose the Best High-voltage Oscilloscope Probe in 5 Minutes

Figure 1: The High-voltage Probe Selection Guide
color codes better or worse probe selections based on
your answers to three, simple questions.
Click any image to enlarge.

Probing high-voltage (HV) circuits for analysis with an oscilloscope presents unique challenges due to the potential for injury or equipment damage, as well as the demands of the materials used in HV semiconductors. HV floating measurements are extremely dangerous and difficult to make. Conventional passive probes are not the answer, but isolated and high-voltage differential probes are options. Yet, with many possible choices in these categories, how can you decide which is actually the best HV oscilloscope probe for your application?

Teledyne LeCroy offers this new, easy way to help you select a high-voltage oscilloscope probe based on your specific application—the High-voltage Probe Selection Guide—available on the Teledyne LeCroy website at: teledynelecroy.com/powerprobes

How to Test the CMRR of Differential Probes

Figure 1: CMRR plots for two attenuation
settings of an HVD3106A differential probe.

While recently we told you not to connect two probes to the same place at the same time, there is a case where connecting two tips of a differential probe to the same place at the same time is useful, and that is when testing the probe’s common mode rejection ratio (CMRR). CMRR is frequency dependent, so part of developing “situational awareness” of your test environment is to know how your probe behaves with different signals at different frequencies. 

Although CMRR as a function of frequency is a principal specification for differential probes, manufacturer's CMRR plots are the result of testing with a narrowband source under strictly controlled laboratory conditions. In real-world applications of probes to broadband sources, you can expect a different result. This quick test will inform you how different.