Oscilloscopes are typically used to measure voltage,
but 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.
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.
Each one of these situations calls for a different measurement strategy.
Let's start with how to measure high current.
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 switched-mode power supply.
A Rogowski coil current probe can measure high currents
because it uses an air core instead of a traditional metal core.
With a metal core, we'd have to worry about too high of a current saturating it.
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's 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 mA and 30 A.
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 a wire, and you're ready to start measuring.
There are no extra components or accessories necessary.
These clamp-on-style current probes have a hybrid AC/DC measurement technology
which means that they have both a Hall Effect sensory element
for measuring low-frequency DC contents
and a current transformer for measuring those AC contents.
You're able to account for the DC offset of your signal instead of the probe blocking it out.
Let's look at an example.
I have a couple different clamp-on-style current probes plugged into channel 1 and 2.
Let's just take a look at the baseline noise of these two probes.
On channel 1, I have an older Keysight probe that has a 10:1 conversion factor.
This means that we're only able to go down to 10 mA per division on the vertical setting.
This really isn't good enough for most low-level current measurements,
but with the probe on channel 2, we can go down to 1 mA 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.
You'll notice that the probe on channel 2 is giving us
about 5-6 times less noise than the probe on channel 1.
With this increased sensitivity and lower noise,
we're going to be able to make more accurate measurements.
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.
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 1,
we can barely make out the edge crossings of this CAN bus.
With the 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,
but the current probe we're using on channel 2, the N7026A,
has the highest bandwidth available at 150 MHz.
The third type of current measurement is low current,
and when I say low current, I mean in the microamp 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.
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.
If we look at an example, on channel 2,
I have that clamp-on current probe we were looking at before,
and on channel 3, a high-sensitivity current probe.
The true signal that we're measuring is 6 mA.
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 subcircuits.
This allows you to see both large signals and small details
on fast and wide dynamic range current waveforms.
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.
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 measurements 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.
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.