Analog to digital: Counting the bits

08/13/2010


Dear Control Engineering: I was reading a release about an analog-to-digital converter that says it’s 12-bit. What does that mean exactly? Is 12-bit better than eight-bit?

 

Bit counts are all about trying to convert analog measurements to digital. Let’s say you’re trying to measure the diameter of a coin with a ruler. You put the ruler on the coin and notice that it’s slightly more than 11/16 in. The actual is somewhere between 11/16 and 3/4 in. You can eyeball the measurement and interpolate in your own mind. Machines aren’t as good at that as you are.

Getting a machine to make that measurement requires converting analog to digital. And since digital deals with high and low (on or off, 1 or 0, etc.) you have to break the reading into discrete segments. For a machine, an analog measurement might be given as a voltage, such as 0 to 10 V. To digitize the measurement, you can use a comparator that turns from off to on when the voltage reaches 5 V. Using that, you have just created a one-bit A to D converter. If you apply this to your ruler, let’s say anything below 1/2 in. is <5 V. Anything over 1/2 in. is >5 V. Unfortunately, this isn’t very precise. But if you’re clever, you realize that if you add a second comparator, you can effectively double the number of marks on the ruler. You now have a two-bit device which gives you marks at quarters. Adding another comparator makes a three-bit device and gives you eighths. Every time you add another bit, you get twice as many divisions. So your ruler that has sixteenths is equivalent to four-bits.

If you keep extending the math, a 12-bit converter gives you 4,096 units. So relating back to the A-to-D converter you mentioned initially, this means that whatever range of measurement you’re dealing with is divided into 4,096 individual units. If you’re using that over one inch, it means each increment is 0.00024414 in. That’s pretty precise and certainly capable of giving you reliable readings to three decimal places. The same applies regardless of what you’re measuring: pressure, temperature, size, flow, level, weight, or whatever the application, the total range span will be divided the same. So for a given range, 12-bit conversion with 4,096 units allows you to be more precise than eight-bit with only 256 units.

Digital communication methods also pay attention to bit counts. The earliest was the telegraph, which is a one-bit device: dot or dash. Getting something faster, such as the teletype, required a higher bit count to allow each character to have its own code. Teletypes use six-bit which allow for 64 different characters. This was fine for a while, but moving to ASCII codes requires 128, or seven-bit. Early word processers changed to eight-bit to provide 256 characters.

Digital sound reproduction also uses bit counts. A standard audio CD uses 16-bit reproduction. That means that there are 65,536 increments running at 44.1 kHz, so if you are trying to digitize the wave form of one second of music, you have a mosaic that’s 65,536 by 44,100 squares. Poorer quality sound reproduction is only 12-bit, and you would probably be able to hear the difference. Some audiophiles consider 16-bit to be too crude and insist that 24-bit (with about 16.8 million increments) running at 96 kHz is necessary for really accurate sound.

Ultimately, if you’re trying to determine what bit count you need for a specific application, you have to ask how precise the measurement has to be. If high precision over a wide range is necessary, say for robotics or a coordinate measuring machine, 12-bit may not cut it. On the other hand, if you’re trying to measure pressure between 0 and 100 psi, and ±5 psi is close enough, even eight-bit resolution is overkill.

Peter Welander, pwelander@cfemedia.com
Control Engineering

Visit the Control Engineering Process Control Channel.



The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Power system design for high-performance buildings; mitigating arc flash hazards
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me