Archive for the ‘Probes’ category

My probe is [insert superlative] than your probe

June 8, 2010

Like most bragging right comparisons, when it comes to highly touted test and measurement instrument specifications it is important to understand the trade-offs involved with achieving a headline-worthy spec. This will help you to truly understand how good the spec really is, and whether it has value for your application. Let’s look at three common specs and see how they don’t tell the complete story.

To read the rest of this article please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Proper Practice for Preventing Probing Pitfalls

May 13, 2010

When visiting engineering labs world-wide, I am often asked to characterize or debug a system. Before I do any of that, I always take a very close look at the connection to the device under test. In about 25% of my characterization sessions, the gotcha I run into is improper connection to the device under test. Here are some of the most common, along with some tips for avoiding them.

To read the rest of this article please visit my Scope Guru on Signal Integrity Blog, on EDN’s site.

Making Floating Measurements

January 26, 2010

Question: How do I know where to hook up my probes if I don’t know where ground is?

Answer: Differential probes allow you to make floating measurements. For switching loss measurements, you’ll want to look at the voltage across the switching device.  For example, with a MOSFET, you would connect your differential probe to the drain and the source to measure the voltage drop across the MOSFET.

Temperature Impact on Passive Probes

November 25, 2009

Question: What can happen when I exceed the operating temperature range of a passive probe?

Answer:  Employing a passive probe outside of its rated operating range may cause any or all of the following:

  • Safety regulations violation, destruction of the probe or the circuit under test, or measurement inaccuracy could all result.
  • Burnout of the probe head resistor could occur at a combination of high temperature and high voltage due to power dissipation heating plus ambient temperature.  A burnout could also potentially cause a fire.
  • Parts could come apart or break at high or low temperatures that are outside of the rated operating temperature range.  This is due to the temperature in combination with physical stress (such as tension, bending, compression, or probe motion/vibration), and the characteristics of the adhesives and materials used in the probe.
  • Even if there is no mechanical stress on the probe, multiple repeated temperature swings over the non-operating temperature range could cause parts to break, and adhesives to fail.
  • If the probe comes apart, the probe ground could contact a non-grounded part of the circuit, causing damage or fire.

Measurement inaccuracy could occur due to high or low temperature, causing a change in the resistor value, or capacitor value of the parts in the probe head; or a change in the dielectric constant or dielectric loss of the coax cable.

More on Measuring Small Signals

November 12, 2009

Question: I’m looking at a small differential signal.  Would you still suggest your active probes?

Answer: If you’re looking at a small differential signal, or even a ground-referenced signal in a noisy environment, a differential probe can be the best choice for accurate measurements. A differential probe measures the voltage difference at the probe tip, eliminating common-mode signals from the measurement. Because the oscilloscope only “sees” the difference signal, the signal-to-noise ratio and the measurement accuracies are improved.

Measuring Small Signals

November 12, 2009

Question: Recently I’ve been trying to measure a small signal.  But, my results aren’t really repeatable. Any suggestions?

Answer: I would start by looking at the probe. Most oscilloscopes come with standard passive probes. These probes work well for many applications, but tend to have high input capacitance, which can load the circuit under test. This loading can increase rise- and fall-times and decrease bandwidth. Probably the best solution would be to use an active probe instead. An active probe puts an amplifier out at the probe tip, reducing the input capacitance by an order of magnitude or more, while increasing the measurement system bandwidth.  One helpful resource might be this primer, the ABCs of Probes, which outlines key considerations when probing.