Hi I'm Melissa and welcome to Scopes University.
Many engineers want to know
what makes one oscilloscope better than another and how to tell if an
oscilloscope makes high quality measurements. This becomes even more
important when you go up in bandwidth because you might be catching higher
frequency noise. By looking at the data sheets you might think that the big
characteristics of the ADC is the best tell of the quality of the oscilloscopes
measurements, but it doesn't tell you the whole story. You want an oscilloscope
that shows you the best representation possible of the signal that's actually
occurring on your device. You don't want an oscilloscope that will inject noise
and jitter into the waveform that could lead to reduced margins and failing
specifications. So today, let's decipher three different specifications and learn
which ones translate into meaningful information about the quality of
measurements your oscilloscope is able to make. ADC bits, system enob, and noise
floor. The place that many people start their evaluation of an oscilloscope is
with the characteristics of the Scopes ADC. There are scopes now with 8, 10, and 12
bit ADC s so what does that actually tell you about the oscilloscope? Just as
the name suggests the analog to digital converter takes your analog signal and
quantize it into digital information. So if I have an 8-bit ADC that means that I
can get 256 quantization levels. In terms we can relate to, this is the
vertical resolution of the ADC. Here on screen, I've got a 1.8 volt peak-to-peak
signal which means I want to set my vertical scale such that I have about
2 volts full screen. With 256 quantization levels at this vertical
setting the best resolution I could get is 7.8 millivolts. The S-series has a
10-bit ADC, so with the same vertical settings, I can now get 1.9 millivolts of
resolution. Just like pixels on the screen, the finer
the resolution, the clearer the representation will be. As we can see in
the map that we just went through, two additional bits can potentially give you
four times the resolution. I say potentially four times more resolution
because there's a lot more to consider. For starters there's no such thing as an
ideal analog to digital converter. This means that just the process of
quantizing that signal can induce noise and jitter. The better the ADC the less
noise will be injected therefore if you want to know the applicable specs of ADC
you want to look for the effective bits of the ADC to account for any noise added
in by the analog to digital conversion. That will get you one step
closer to understanding the quality of measurements made by the oscilloscope.
But the ADC is just one part buried inside the oscilloscope. What about all
the other stuff between your signal and the analog to digital converter? The
front end of an oscilloscope consists of attenuators, preamplifiers, signal path
routing, and possibly filters, before it gets to the ADC. Each of these components
could add noise and impact your measurement. If the front end of the
oscilloscope isn't designed well, it doesn't matter how many effective bits
you have on the ADC. A poorly designed front end will inject a lot of noise and
a lot of jitter into the measurement providing you with an inaccurate
representation of your signal. All the noise from the front end will be
injected into your signal wasting the ADCs resolution on quantizing noise.
That's why I recommend looking at the system's effective number of bits system.
System enob will account for all the parts that I discussed in the front end and
the ADC and tell you the true vertical resolution achievable on your oscilloscope.
Now that we know the importance of looking at the entire system, a good test
of the quality of measurements the oscilloscope is able to make is to check
the noise floor. Here's a quick way to measure the noise on your oscilloscope.
First let's disconnect all inputs. Remember I said we want to check
multiple vertical sensitivities. Let's start with 50 millivolts per division
since that's an industry standard for measuring noise, then we'll go to analyze,
select histogram. We'll choose vertical since we're
looking at noise, select the default window, and turn the histogram on. You can
see that this 8 gigahertz scope at 50 millivolts per division gives me 1.3
millivolts of noise. I also want to look at my noise results as I move position
on screen. I want to make sure the noise doesn't change based on the waveforms
position. If I'm comparing to another oscilloscope I want
to make sure the bandwidths match. For example if I want to compare to a two
gigahertz oscilloscope, I can apply a hardware filter to make an
apples-to-apples comparison. Now configured it as a two gigahertz
oscilloscope at 50 millivolts per division
I get 590 micro volts of noise. When you do this on your own make sure to change
your volts per division settings. Because we're taking the measurement on the
oscilloscope itself and not depending on separated component specs provided by a
vendor, we know that we're accounting to the entire system of the oscilloscope.
Even though ADC bits have become a major banner spec now you're smarter
than the marketing folks. You can determine on your own which oscilloscope
is going to provide you with the highest quality measurement. When you have a well
designed oscilloscope you won't lose important details to noise, you'll have a
true representation of your signal, and you don't have to worry about failing a
margin spec due to induced noise and jitter, Thanks for joining me today and
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