Don't Attach Multiple Probes to the Same Place at the Same Time!

Figure 1. Response of two different probes to an
upper-side gate drive measurement.

Along with using single-ended passive probes for high-voltage measurements, another probing "no no" to avoid is attaching multiple probes to the same place at the same time.

You are probably aware that all measuring instruments, including oscilloscopes, are subject to conditions of observability.  As we discussed in a recent post on The Impact of the Interconnect, the very act of connecting the oscilloscope to the circuit with a particular probe affects the measurement in a particular way. Probes affect the circuit by applying additional resistance and capacitance loads in parallel with the circuit at the test point. Moreover, the probes themselves limit the fidelity of the measurement due to limits of bandwidth, slew rate and common mode response. So, it is always a good idea when making a measurement to compare how different probes/interconnects will affect the measurement…but don’t try to do it all at once.  

Probing Techniques and Tradeoffs (Part V): Probe Loading

Figure 1: A probe's impedance varies
with frequency

Earlier in this series of posts, we alluded to the topic of probe loading, which is an outcome of the fact that to make a measurement, an oscilloscope probe must "steal" some energy from the circuit or device under test. Thus, the probe's tip must have a finite impedance across the frequency range of interest.

Probing Techniques and Tradeoffs (Part II)

Figure 1: A snapshot of available probes from
Teledyne LeCroy

Our first post in this series concentrated on connectivity and various means by which one might apply an oscilloscope probe to a circuit or device under test. Now, we'll look at an "ideal" probe vs. a real-world probe, and then begin a discussion of probe specifications.

Probing Techniques and Tradeoffs (Part I)

Figure 1: Probes are the signal's gateway to the oscilloscope

As any oscilloscope user (hopefully) knows, probing is perhaps the most critical element of getting good measurement results (Figure 1). We must understand our probes' specifications to ensure that we obtain the best possible signal fidelity, and thereby accurately characterize our signal under test. In this series of posts, we'll take you through probing tradeoffs and techniques and help you choose the right probe for the task at hand.

Testing the DDR Memory Interface's Physical Layer (Part IV)

Figure 1: Probes are a key element of the total signal
acquisition system

In this multipart survey of testing the DDR interface's physical layer, we've looked at the basics of the interface itself, a high-level overview of the testing, how to access DDR signals, and read/write burst separation. In this installment, we'll cover preparation for the actual testing.

Video: The Many Varieties of Oscilloscope Probes

Got a minute (OK, a minute and a half)? Take a look at this quick tutorial video that takes you through the four basic types of probes and what they're used for:

If this little thumbnail sketch whetted your appetite for more info on oscilloscope probes, we've got you covered with a series of popular blog posts on the topic:

The Realities of Oscilloscope Probes

Back to Basics: Probes (Part I)

Back to Basics: Probes (Part II)

Back to Basics: Probes (Part III)

Back to Basics: Probes (Part IV)

Understanding Probe Calibration Methods

Understanding Probe Calibration Methods (Part II)

The Effects of Passive Probe Ground Leads

Understanding Probe Calibration Methods (Part II)

Figure 1: Experimental setup for comparing a
source-referred signal to an unloaded signal

As many engineers and techs know, there's ample room for confusion when it comes to a proper understanding of oscilloscope probe loading. In an earlier post, we covered probe calibration methods and tried to sort out terms such as calibration, correction, compensation, and de-embedding.