Back to Basics: Data conversion details

Bit counts matter when converting analog data to digital.

09/03/2010


An analog-to-digital converter is an electronic or device that changes a continuous quantity into a digital number proportional to the magnitude of the input. When evaluating the specifications of analog-to-digital converters, it's useful to understand the basics of bit rates and frequency as they relate to data acquisition.

For bit rate, imagine measuring 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—but machines aren’t as good at that as you are. Getting a machine to make that measurement requires converting analog input to digital input.

Since digital data 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. Applying this to the ruler used in the example above, you can say anything below 1/2 in. is less than 5 V, and anything over 1/2 in. is greater than 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 a ruler that has sixteenths is equivalent to a four-bit device.

The specifications for an A-to-D converter will say that it's an 8-bit or 12-bit device. By extending the math, you find that a 12-bit converter gives you 4,096 units; that means, whatever range of measurement you’re dealing with is divided into 4,096 individual units. In the ruler example, each inch would be divided into 0.00024414 in. increments. The same math applies regardless of what you’re measuring: pressure, temperature, size, flow, level, weight, etc. 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 conversion, which provides only 256 units.

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.

If bit rate is the vertical or y-axis on a graph, frequency is the horizontal or x-axis. Bit rate says how many divisions you have, and frequency says how often you make a mark. Say you need to draw a series of parallel lines 1/8 in. Apart. . If you have a ruler and a pencil with a sharp point, that’s no problem. But if all you have is a chisel point marker that makes a thick line 1/4-in. wide, you will have a hard time. Your drawing instrument has to be finer than the distance between the lines.

It’s the same when converting analog data to digital. Just as you must have a sufficient bit rate to duplicate subtleties in the analog waveform, your frequency also has to be up to the task. Digital frequency should be at least double the highest analog frequency that you have to convert.

Do the math

  • To digitize a measurement, you can use a comparator that turns from off to on at a certain increment.
  • If you add a second comparator, you effectively double the number of marks. This is a two-bit device.
  • Every time you add another comparator, you get twice as many divisions, so, a 12-bit converter gives you 4,096 units of measurement accuracy.

This “Back to Basics” is based on an “Ask Control Engineering” blog post by Peter Welander, Control Engineering; edited by Renee R. Bassett, a technical writer for Control Engineering. Reach her at renee.bassett@gmail.com.

Tutorials Channel at www.controleng.com/tutorials.

Ask Control Engineering blog at www.controleng.com/blogs.

Analog Devices, manufacturer of ICs for analog and digital signal processing applications, also offers online tutorials. www.analog.com/training



No comments
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.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

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.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me