What is probe compensation?
Oscilloscope probe compensation matches an individual probe to an individual oscilloscope channel. The compensation process requires a square wave signal generator, probe, oscilloscope, and a tuning tool. The process requires the technician to monitor the square wave response on the oscilloscope, while adjusting the probe’s trimmer capacitor for minimum distortion. The trimmer capacitor is shown in Figure 1 with the results shown in Video 1.
This engineering brief demonstrates how to perform the compensation process on the Digilent Analog Discovery Pro (ADP2230) and the Analog Discovery 3. The process is generally applicable to all oscilloscopes. However, emphasis is placed on using the Discovery’s signal generator in place of the conventional probe compensation generator featured on the face of the conventional oscilloscope. Some experience with the Digilent products is assumed such as basic use and configuration of the function generator and oscilloscope.
Figure 1: Digilent Analog Discovery Pro (ADP2230) probe compensation via adjustments of the trimmer capacitor. Note that the probe is set to 10x.
Video 1: Screen capture of probe being compensated. The Digilent Waveform oscilloscope is in the upper frame while the 1 kHz square wave generator is in the lower.
Tech Tip: Many oscilloscope probes feature a 1X and 10X switch as shown in figure 1. The compensation procedure is applicable to the 10X mode and NOT the lower impedance 1X mode. A common error is to make wild unfruitful adjustments to the trimmer capacitor while the probe is in the 1X position.
Procedure
The compensation process consists of a few simple steps:
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Set the probe(s) to 10X. This is clearly shown in Figures 1 and 2 for the ADP2230 and AD3, respectively.
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Physically connect the probe(s) to the waveform generator as shown in Figures 1 and 2. A banana to BNC binding post was used. See this article.
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Configure the Waveforms signal generator to produce a 1 V square wave.
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Configure the Waveforms oscilloscope to monitor the waveform. Figure 3 presents the waveform of a poorly compensated probe.
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Adjust the trimmer capacitor for minimum square wave distortion. Figure 4 and Video 1 present a properly compensated probe.
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Keep the probes in place. For example, never swap probes as the compensation may be different between channels. Repeated compensation adjustment can lead to short probe life.
Tech Tip: The 10X stipulation is mentioned multiple times as it is a significant and ongoing source of user error. As the saying goes, if I had a nickel for every time the probe was set wrong, I’d be a millionaire – I’ve done it, and my students have done it. This problem is so bad that some equipment manufacturers have abandoned the switch altogether. It’s something to consider when you purchase new probes. Also, some oscilloscopes are designed to detect and automatically respond to the probe’s multiplier.
Figure 2: Digilent Analog Discovery 3 probe compensation setup. Note that the probe is set to 10x. Also note that the breakout board and probes are offered as part of the AD3 Pro Bundle.
Figure 3: Response of a poorly compensated probe. Note the square wave distortion immediately following a transition.
Figure 4: Response of a properly compensated probe. There is no observable distortion in the square wave.
Tech Tip: The trimmer capacitor is delicate and easily damaged. For long probe life, limit your compensation adjustments by physically associating a specific probe to a specific oscilloscope channel. If the probes stay in place, you will need to periodically check but rarely adjust compensation.
Parting thoughts
Oscilloscope probe compensation is required to match a specific probe to the attributes of a specific oscilloscope channel. When properly compensated, the 10X probe provides the lowest possible signal distortion. This procedure is applicable to all oscilloscopes, the only difference with the Digilent Discovery products is the need to use the internal function generator as the square wave reference. The waveforms software makes this very easy. It’s even easier the second time, provided you saved the workspace with a memorable name such as “compensation.”
Do you have any questions about the Digilent instrument? If so, please leave a note in the comments section.
Did I mention the 10X stipulation?
Check that switch position!
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.