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Avoiding scope errors (Part 2)

Posted: 04 Oct 2013 ?? ?Print Version ?Bookmark and Share

Keywords:probe? coaxial cable? high Q? capacitance? bandwidth?

Another mistake to avoid is influencing the measurement with the probe. The top panel in figure 4 shows the measurement from our first example, and the results are approximately the same as reported above. In the lower panel, a 500MHz, 9-pF probe was connected to the output as well as the 50-Ohm connection. The 9-pF probe tip capacitance results in a 55 per cent error in the 50-Ohm measurement. One of the more common probe influence issues is the measurement of the switching frequency of a pulse width modulation (PWM) converter via the oscillator ramp pin. The capacitance of the probe can easily alter the switching frequency.

Figure 4: The addition of a scope probe on the signal we are measuring alters the result of the measurement. The 500MHz probe (lower trace) is also poorly calibrated. Often this occurs when more than one piece of test equipment is connected to a single high-speed signal.

Recommendations
For all high-fidelity measurements up to 100MHz, remove the ground clip from the measurements. Above 100MHz, consider an active probe.

A typical oscilloscope probe presents 10-15 pF of capacitance. Many circuits cannot tolerate this, and, at the least, this capacitance can cause significant ringing. In most cases, an active probe is required.

Sometimes the best probe is no probe at all, and a coaxial cable is perfect. This solution also maximises the sensitivity of the measurement. However, keep in mind that a 50-Ohm coaxial cable must be terminated into 50-Ohm.

About the author
Steve Sandler is the founder of AEi Systems.

To download the PDF version of this article, click here.


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