Have you ever been fighting noise on your measurements and can’t tell where it’s
coming from?
There are four main contributors to this.
Learn about how to decrease the impact they have on your signals.
Hi, I’m Erin
Noise can be a pain to deal with, but it’s important to try to minimize it in order to
make the most accurate measurements.
The main contributors to noise in your measurements are your device under test, your probe, or
your oscilloscope.
In this episode of Probing Pitfalls, we’re going to learn about how noise impacts can
affect your measurements, what the noise sources are in your probe and your scope, and some
best practices that will help decrease the noise coming from those components.
The first thing we’ll look at are the consequences of system noise.
The “system” I am referring to is the probe and the oscilloscope being used together.
The sources of noise in this system can come from the probes amplifier
and the oscilloscope's attenuator.
Let’s start out by looking at the noise floor of an oscilloscope with no probe attached.
We can see pretty steady noise on the screen.
Now, if I attach a probe to the scope, you can see the noise increases significantly.
It’s worth noting this phenomenon will happen with ALL probes.
This happens because you are basically connecting an unterminated antenna if we aren’t probing
on anything.
So, it’s picking up noise from around the room.
This noise level will be reduced when your probe is well grounded, but it is worth noting
the increased noise level just by adding a probe.
Follow the advice covered in this video to decrease the noise in your measurements.
Not to jump too far ahead, but most of what I am going to talk about revolves around the
analog to digital converter, or ADC.
ADC’s only accept a fixed voltage range.
So, your signals typically have to be adjusted to fit into the ADCs range.
There are two tools for this: the first is the probe attenuation.
Probe attenuation is like a rough tuning.
You can use it to get your signal in the right ballpark range.
The oscilloscope’s front-end acts as fine tuning.
It attenuates and amplifies the signal to get it precisely to the right levels.
To learn more about that, check out the Oscilloscope Front End Design speaker series on this channel.
So, basically, the signal is attenuated to make it smaller so that it can fit within
the range of the oscilloscope’s ADC.
The attenuation ratio of a probe dictates how large or small of a signal you can measure
accurately with that probe.
For example, a 1:1 attenuation ratio will leave your signal as is.
A 10:1 probe will scale your signal 10 times smaller before it enters the oscilloscope.
And a 100:1 will scale you signal 100 times smaller.
If you don’t select the correct attenuation ratio for your measurements, you can introduce
noise to your system that will throw off your tests.
Higher attenuation ratios lead to more noise because the oscilloscope’s
front-end amplifier noise gets multiplied.
As an example, think about a 1V signal on a 10:1 probe.
This signal gets attenuated to be 10 times smaller, 100 mV, and then it is passed to the ADC.
However, the noise doesn’t actually get completely attenuated.
Therefore, after the signal goes through the attenuator and amplifier, the signal to noise
ratio may be even higher than it was before.
When that signal and noise goes through the ADC and is displayed on screen, it displays
the signal back to its original size, 1V.
So, this means that after that signal went through the ADC, it was multiplied by 10 in
order for it to appear the right size on the screen.
When that signal is multiplied by 10, the noise riding on it is also being multiplied by 10.
So, you can see how using a higher attenuation ratio will end up adding more noise to your system
This can be easily fixed by knowing how to select the correct attenuation ratio.
Let’s look at this 1:1/10:1 switchable passive probe as an example.
I have two of these probes connected to my scope and I am measuring the same signal with
both, at 1Vpp.
On channel 1, I have the probe set to 1:1, and channel 2, I have it set to 10:1
If you are working with higher voltages, you will need to use a higher attenuation ratio,
such as 10:1.
This means that when you connect your probe to your device, the representation you see
on the oscilloscope screen will be 10 times smaller than the signal that is actually coming
out of your device.
If you are working with higher voltages, shrinking it down 10 times allows it to fit within the
range of the ADC and be properly processed.
If you are working with small voltages, you will want to use something like a 1:1 probe.
Of course, this may differ in some cases depending on loading and bandwidth.
But this makes it so you are inputting a properly sized signal to the ADC, giving you a more
accurate view of your small signal.
If you use a probe with too high of an attenuation ratio, this will introduce noise to your system,
and this is actually because of how that attenuated signal is processed through the ADC.
For this signal I am working with on the scope, the 1:1 attenuation ratio is perfect for this size.
The 10:1 attenuation ratio is excessively high for this, so if I do a peak to peak measurement
on both, you can see on the 1:1 signal it is about 200 mVpp, where as the 10:1 probe is
is about 260 mVpp!
You can even visually see more noise on this signal with the 10:1 probe.
In addition, you must make your signal as large as possible on screen in order to fully
utilize the ADC and reduce noise.
This will reduce the amount of gain needed by the oscilloscope.
You can see when my signal is scaled to be small on the screen, the peak to peak measurement
is about 300 mVpp, but when I make it as large as I can on screen, the peak to peak measurement
is actually around 200mVpp.
Attenuation ratio can vary from 1:1, all the way up to 1000:1 for really high voltages.
There are pros and cons to the various attenuation ratios, but again, it is all dependent on
the size of your signals.
Using a higher attenuation ratio will bring out more of the noise from the oscilloscopes
amplifier, and using a lower attenuation ratio means less noise, but much lower useful voltage
range.
Most oscilloscopes can detect your probes attenuation ratio and will automatically adjust
the vertical scale accordingly.
You can follow these guidelines and limitations of various common attenuation ratios to help
determine what you need for your measurements.
One easy way to estimate the amount of your probe noise is to check the attenuation ratio
and the probe noise level from the probe’s data sheet or manual.
Many probe manufacturers characterize the probe’s noise as equivalent input noise
(EIN) and will be listed in volts rms.
With this knowledge you can much more easily select a probe with the correct attenuation
ratio for the application you’re working on.
It all just depends on how big of a signal you are working with, which you will typically
have a general idea of.
To wrap up all that information you just learned: your system noise can be exaggerated by probe
and oscilloscope noise levels.
Selecting the correct probe attenuation ratio AND bandwidth for your application will lower
the amount of system noise added.
As a result, your measured signal will be a cleaner representation of what is actually
on your DUT.
To learn more about probing more effectively, be sure to check out the link in the description!
Be sure to subscribe to our YouTube channel to see more videos like this, and follow us
on Facebook, Instagram, and Twitter.