Did you know that your signal's amplitude and rise time could appear totally
incorrect and you may not even realize it?
Forgetting about probe calibration is a common pitfall that is easy to correct
and will save you from making critical measurement errors.
Hi, I'm Melissa. In this episode of Probing Pitfalls you'll learn about what
problems uncalibrated probes can cause. We'll also look at how to solve these
problems very easily with the oscilloscopes built-in probe calibration.
Hooked up to my oscilloscope, I have two probes measuring my signal. This is what
my signal looks like with an uncalibrated probe. At first glance this
looks okay. If I didn't know any better I'd proceed with my measurements. On channel
3, in blue, this is the same measurement with a calibrated probe. Now you can
see how far off my measurement would have been with the uncalibrated probe.
Before we dive into probe calibration, let's take a look at the path your
signal takes when you connect it to the oscilloscope so that we understand what
is causing the skew. The original signal you see here is a representation of the
signal that is actually in your DUT before you probe it. When you probe the
DUT the probe may add distortions to your signal. These distortions are caused
by things like probe capacitance or inductance. The signal on the probe tip
is then attenuated or amplified and sent to the oscilloscope input. And finally,
the oscilloscope converts this analog signal to a digitized version using
digital signal processing. Skew and amplitude changes can occur during this
process which can cause inaccurate measurements. Skew on a signal is
basically a timing difference between rising edges. If a signal is skewed
this means the rising edge shown on the scope is delayed compared to when the
rising edge truly occurs in your device. Along with these pesky timing
differences, amplitude differences can occur as well, meaning your vertical
measurements like peak-to-peak voltage will be incorrect. Not seeing a true
representation of your DUT's signal on the oscilloscope
can make it difficult if not impossible for you to debug and complete your
design work. The good news is that both vertical error and skew
can be easily fixed simply by calibrating your probe. Probe calibration
is easily forgotten but it's an important step to ensure you make the
most accurate measurement possible. The probe and the oscilloscope together are a
measurement system and that creates the signal path we discussed. Each probe's
output and each oscilloscopes input characteristics will differ due to
slight component variations. This is comparable to when you measure to one
ohm resistors. Neither of them will be exactly one ohm but they are within a
tolerated range to meet their specification. You can take some of this
variability out of the system by calibrating your probe to the
oscilloscope input. This helps prevent major amplitude
changes and skew errors. Keysight oscilloscopes can recognize Keysight
active probes and give you a message on the screen
if your probe is not calibrated to your scope. Let's take a look at what
happens if you use uncalibrated probes. In yellow you can see a 757 Hertz clock
signal before probing. This is being fed straight into the scope using a special
deskew device. Now I'm probing the signal with the uncalibrated probe which we can
see on channel 2 in green. The generator output on channel 1 is right around 520 mVpp
520 mVpp and the probe signal on channel 2 is 550 mVpp
In addition the skew from channel 1 to channel 2 is massive, around
320 microseconds which means the rise times don't line up at all. If this were
a digital communication line you might interpret this as misaligned data and
clock lines which is a design problem, but in reality it's just your probe
causing this error. Or if you're making a power measurement, having your current
and voltage signals significantly skewed like this could leave you with
dramatically incorrect power calculations. Uncalibrated probes can
lead to a lot of confusion in many test scenarios leading you to mistakenly
think your design is flawed. Let's fix this issue by calibrating your probe. On
my Infiniium oscilloscope, I can go into the channel menu and see the probe
calibration button and we can walk through the amplitude and skew
calibrations in less than 5 seconds each. The scope prompts you when the
calibration is done. Now if we make the same measurements we did before we can
see the results are much more accurate. The amplitude is improved by about 25
millivolts peak to peak and the skew has been corrected. Both rise times line up
so well that we can't even see channel 1. Now we are seeing the accurate
representation of the signal. The amplitude calibration may not seem as
drastic as the skew in this case but when you're making critical measurements
a 10 millivolt difference can be a really big deal. So the signal path from
your DUT to your probe and through your oscilloscope to the screen can cause
errors in your measurements if the probe is not calibrated correctly and it only
takes a few seconds to calibrate your probe. That's time well-spent to save
hours of head-scratching as you try to figure out what's wrong with your device.
Always calibrate your probe with the oscilloscope channel you are using for
the best fidelity in your measurement. Check out the link in the description to
download application notes that can help you learn how to probe more effectively
for your specific tests. Be sure to subscribe to our YouTube channel to see
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