Oscilloscopes are typically used to measure voltage. What you may not know is
that you can just as easily measure current with a scope. Hi, I'm Ally. To
measure current with an oscilloscope you have a few options. You can use the Ohm's
Law method to measure the voltage drop across a shunt resistor, you can use this
voltage and your known resistance to calculate current. This is a nice quick
way to measure current, but it requires you to do some math in your head to
interpret the waveform and this can get pretty confusing if you have to document
your waveform and share it with others and depending on the shunt resistor you
use it can also introduce extra uncertainty into your measurements. For
more accurate measurements, use a current probe. With a current probe you'll get
the measurement in amps right on your scope screen, no calculations required.
There are three situations where you'll need to measure current: if you're
working with really high currents, really low currents, or something in between and
each one of these situations calls for a different measurement strategy. Let's
start with how to measure high current. And when I say high current, I mean high
current. As in, hundreds or thousands of amps. You'll often run into large
currents like this when measuring the startup of a device or the switching of
a switch mode power supply. A Rogowski coil current probe can measure high
currents because it uses an air core instead of a traditional metal core and
with a metal core we'd have to worry about too high of a current saturating
it but with air we don't have to worry about that. When current is detected the
Rogowski coil produces a voltage that is proportional to the rate of change or
derivative of the current that's enclosed by that coil loop. The voltage
is then integrated and this allows the probe to provide an output voltage that is
proportional to the input current that you're measuring.
Rogowski coil probes have been getting a lot of attention with engineers because
they're really easy to use the probe head is just a flexible loop that you
can bend and loop around any component that you want to test. Now let's talk
about how to measure currents in that in-between range, somewhere between 10
milliamps and 30 amps. For these measurements most engineers are using a
clamp on style current probe, also known as a magnetic core current probe. These
clamp on style current probes are often used to measure current consumption of
things like high frequency digital circuits, ICs and power supplies. When
you're doing this type of testing it's often important to get
accurate, low-level measurements which means you need a probe with high
sensitivity and low noise. One thing to note is that clamp on probes are really
easy to use. All you have to do is clamp it around the wire and you're ready to
start measuring. There's no extra components or accessories necessary. And
these clamp on style current probes have a hybrid ac/dc measurement technology
which means that they have both a Hall effect sensor element for measuring
low-frequency DC contents and a current transformer for measuring those AC
contents, so you're able to account for the DC offset of your signal instead of
the probe locking it out. Let's look at an example I have a couple different
clamp on style current probes plugged into channel one and two. Let's just take
a look at the baseline noise of these two probes. On channel one I have an
older Keysight probe that has a ten to one conversion factor. This means that
we're only able to go down to 10 milliamps per division on the vertical
setting and this really isn't good enough for most low level current
measurements but with the probe on channel 2 we can actually go down to one
milliamp per division. With this high sensitivity we can measure low level
currents with much more accuracy. I've also set up peak to peak measurements so
we can compare the two probes baseline noise and you'll notice that the probe on
channel two is giving us about five to six times less noise in the probe on
channel one. With this increased sensitivity and lower noise we're going
to be able to make more accurate measurements, so when you go to pick a
clamp-on style current probe make sure you choose one with an appropriate noise
level for the measurements you need to make. This is especially important if you
care about signal detail. So let's look at an example if you're working with
something like this CAN bus that I have on screen here you can start to see how
the noise level of your current probe can really affect your measurements. With
the older current probe on channel one, we can barely make out the edge
crossings of this CAN bus. With a newer probe on channel 2 we can clearly define
all of the bits and make more accurate measurements. Generally clamp on current
probes don't have much bandwidth. With the current probe we're using on channel 2
the N7026A oscilloscope current probe
has the highest bandwidth
available at 150 megahertz. The third type of current measurement is
low current and when I say low current I mean in the micro amp to amp range.
Measuring low level current like this is often necessary when testing things like
battery powered devices, charging devices, or memory chips. Engineers are always
looking for ways to maximize battery life of their devices and to do this you
need to be able to accurately test your device's low current states so you can
optimize power consumption. To get this level of sensitivity use a high
sensitivity current probe that allows you to measure these low signals with
very low noise, while maintaining a high enough dynamic range that covers the
input signal. So if we look at an example on channel 2 I have that clamp on
current probe we are looking at before and on channel 3 a high sensitivity
current probe. The true signal that we're measuring is 6 milliamps and we're
getting almost exactly that with the high sensitivity current probe because
there's barely any noise from the probe riding on the signal. These high
sensitivity current probes are optimized to measure current flow within
a DUT to characterize sub circuits and this allows you to see both large
signals and small details on fast and wide dynamic range current waveforms and
often when you're doing current testing you need to be able to make low level
current measurements while still being able to analyze those larger currents
and to do this you have two options: first you can double probe your signal
but this will double probe loading and give you an inaccurate view of your
signal. The other option is to use a two channel high sensitivity current probe
like this one. Making more accurate current measurement starts with the
right probe, for more information on how to pick a probe for your measurements
check out the selection guide that's linked below and for even more probing
resources, make sure you download the probe training kit linked below and
subscribe to our YouTube channel for more videos like this. I'm Ally with
Keysight technologies and I'll see you next time.