Not understanding the difference between the number of ADC bits and the effective
number of bits can cause you to get into a load of trouble by not getting the
accuracy and measurement quality that you expect and need.
Hi and welcome to our new video series Exposing Signal Integrity Myths. I'm Erin
and in this new series we'll explore the key elements of signal integrity and how
you can use that knowledge to make the best measurements possible. The number of
ADC bits in a scope is one of the most talked about specifications. Because of
this, many engineers tend to rely on this as the sole spec that determines an
oscilloscopes quality but boy will they be disappointed. The importance of ADC
bits is often exaggerated while other critical indications of signal integrity
get pushed to the background. The number of ADC bits specified in the datasheet and
the number of those bits that are actually effective in showing you your
measurements on screen are very different. The specification that you
should really be looking for is the system's effective number of bits or
system enob. If you've never heard of system email before I'm not too
surprised. The spec isn't talked about much but I just said that it's the most
important spec in determining measurement quality, in fact it's what IEEE
uses to measure an oscilloscope's signal integrity. So why isn't it talked
about? System enob usually isn't mentioned because it's typically so poor.
Also notice that I keep saying system enob, not just enob, and definitely not
ADC enob. You'll learn more about what the differences are of those soon but before
I dive more into why knob and ADC bits are so different let's take a step back
and think about why we even care about the number of bits to begin with.
The number of this is important because of quantization levels or Q levels. The more
bits there are, the more Q levels there can be, the more Q levels there are, the
higher the accuracy. Because we're able to capture and display more of the true
signal you're working with if there are more levels. So when the ADC converts a
signal from analog to digital, it can only convert at certain levels.
You can imagine Q levels as each step in this animation you're seeing. Say we have
a 10-bit ADC like the S series. With this we can have 1,024 Q levels.
If we look at a 1.8 volt peak-to-peak signal inside our vertical scale so that
we're at 2 volts full screen with a 10-bit ADC, I should theoretically
be getting a sensitivity of 1.95 millivolts per quantization level or in
other words 1.95 millivolts of vertical resolution. However like I said before
just because your scope is spec'd to have a 10 bit ADC, does not mean that all of
those bits will actually be useful in your measurements. You're lucky if you
get 8 bits worth of usable quantization levels and that's why we look at the
effective number of bits. This is the number of bits that are effectively
being used to show you your measurement on screen. So what is it that makes the
actual effective number of bits so much lower than what the ADC is spec'd to?
The number of bits is primarily impacted by noise and distortion. If the
oscilloscope's random noise is larger than each quantization level, so in this
case if the noise is greater than 1.95 millivolts, then there is no way for the
scope to differentiate that noise from the actual signal. Therefore basically
making that bit completely irrelevant. And since there's no earthly way to
build an oscilloscope that doesn't have some noise that means there's always
going to be noise affecting your number of bits and this is what drives the
effective number of bits to be lower than the ADC bits available. This also
means it's really important to analyze the baseline noise of an oscilloscope to
be sure it's really low. Check out that video by following the link on screen or
in the description. Something that I mentioned earlier was the difference
between system enob and ADC enob. Since system enob is typically pretty low,
there are some vendors that will list a ADC enob on their datasheet. Some people
see the term enob and take that for face value but they're being fooled. ADC
enob refers to the effective number of bits in the ADC and only the ADC. However
everyone knows an oscilloscope is made up of much more than an ADC. ADC enob is
not representative of number of bits that are actually being
used in the entire oscilloscope measurement system which is what's
really going to matter when you go to make measurements. The system enob is the
number of bits actually being used to display a signal, make measurements, and
use features to analyze it. When you're evaluating various oscilloscopes based
on their data sheets, be sure the enob is listed out based on
bandwidth and also make sure you're looking at the system enob rather than
ADC enob. Don't be fooled by any impressive ADC specifications. While the number of
ADC bits is still important because it means more quantization levels that
could be irrelevant if the rest of the oscilloscope isn't designed properly.
Understanding enob and what that means to your measurements you end up making
is critical in creating a design that has the accuracy you and your customers
need. I also mentioned it's important to look at the noise floor of your scope to
determine the overall quality. Learn more about vertical noise and some of the
other important signal integrity specifications by checking out the rest
of this video series. Do you have a topic you want to learn about but you can't
find a video for it? Le t us know in the comments we always want to be creating
new content that will actually be beneficial for you in your daily testing.
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