Selecting the wrong probe tip or ground lead will reduce your probe’s bandwidth and create
signal distortion due to loading.
To maximize the bandwidth and minimize the loading effects, learn about these steps that
will guide you away from the pitfall of using the wrong probe accessories.
Hi, I’m Erin. in this episode of Probing Pitfalls, you’ll learn about some of the
common accessories for single ended active probes and what effect they have on your signals.
Engineers often think accessories are perfect because they make probe to DUT connections
easier, but they don’t realize how much those accessories may be limiting their measurements.
When looking at accessories, it’s important to think about the impacts of: bandwidth,
signal fidelity, and loading.
You learned about the pitfalls of not having enough bandwidth and how to go about finding
the correct amount in our first episode.
Well, all of that hard work finding the perfect probe bandwidth could quickly go to waste
if you don’t consider the impact from your accessories.
Connection bandwidth is really important to think about.
The shorter the connection is between the probe tip and your device under test (DUT),
the higher the bandwidth of the system will be.
The longer the leads, the higher the inductance that is felt on your circuit.
This inductance forms an LC circuit with the probe tip capacitance and any other parasitic capacitances
The result of this will be lower amplitude, slower rise times, and rounder edges, which
will make all of your measurements inaccurate.
To take a closer look at the bandwidth impact and signal fidelity using accessories, let’s
analyze this 125 MHz square wave.
The yellow trace shows the true signal that is being fed right into the scope from the generator.
Note that the true rise time of this signal is 104 ps.
We can see what the signal looks like in green using an active probe with no accessories attached.
This signal has the same rise time as channel 1 and we can see a lot of signal detail since
we have the shortest connection possible and enough bandwidth.
Now, if I attach the probe using a Y lead adapter, we can see that there is already
quite a bit of loading on the true signal on channel one, just from connecting the Y
lead adapter to the DUT.
This loading is going to greatly affect the rise time.
Looking at the result of the probe with the Y lead on channel 3, we can see the edges
are a bit more rounded because there isn’t enough bandwidth to capture the critical harmonics.
And we can also see that the rise time measurement is only 2.5 ns because of the loading caused
by the accessory.
Lastly, on channel 4 we have the longest connection using the Y leads and the flex nose grabbers.
You can see the signal is completely deformed because we do not have enough bandwidth at
all with this connection, and the loading is even worse.
This visualization of the response using different accessories helps to understand how this component
can change the rise time of the response you are seeing on screen along with the bandwidth
of your system.
Looking back at channel two, the probe using the shortest connection, one other observation
you may be questioning is why this signal using the ground blade has more ringing than
what we saw with the Y Lead Adapter.
For our example here, I purposely selected a signal with a really fast rise time simply
to demonstrate and explain this ringing.
Ringing will occur when either the capacitance or inductance at the probe tip are too high.
This formula helps to understand this a little better.
You can use this to see what the resonant frequency is as the capacitance and inductance vary.
The lower the capacitance and inductance are, the higher the resonant frequency will be.
Typically, we will want the resonant frequency to be higher than the frequency of the probe
and circuit you’re working with.
In this case, the capacitance and inductance are too high, which means the resonant frequency
is lower than our bandwidth, and therefore causing this ringing on our signal.
So, you can see It is really important to ensure you are using the right probe tips
and accessories for your measurements.
Each accessory option has its own trade-offs and impacts on bandwidth, signal fidelity,
and loading.
Understanding some common mistakes and best practices helps you select the accessories
you need.
To avoid the pitfalls of added inductance by longer lead accessories, remember these
two tips: Use the shortest possible accessory leads to see the best representation of your
circuit’s performance and signal fidelity.
Use longer accessory leads to get quick general measurement such as troubleshooting and when
you do not care about sacrificed bandwidth.
To learn how to avoid other probing pitfalls, check out this link.
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