Download PDF

Reach Out and Touch with the Personal Data Assistant

By Bonnie C. Baker,
Senior Applications Engineer, Data Acquisition Products Division,
Burr-Brown Corporation

It has been said that all you have to do is pick up a phone to reach out and touch someone. A simple analog conversation can clarify the most confusing of circumstances with immediate gratification. There are times, how ever, when a phone conversation doesn't always work. Maybe the written word is the key, in which case a likely alternative to voice communication would be a fax. International business agreements that can be hung up in the postal system for days or weeks are now completed in a manner of minutes. Even though the written word can be effective sometimes it can be incomplete -- it doesn't always give the whole picture.

The Other Part of the Complete Picture

For instance, imagine that you want to troubleshoot your latest circuit creation. Even a face-to-face conversation in the lunch room requires a picture. In this type of setting you would never think of bringing a copy of your drawing, more likely "what's for lunch." The drawing used in the this type of conversation is usually scratched out on the cafeteria or restaurant napkin. If your whole design team is there, this style of communication is perfectly adequate. The hard part is saving your conversation in electronic form and e-mailing it to your design partner. Enter the-touch screen technology. The touch screen brings back some of that warm fuzzy feeling that may have been lost in the phone conversation or the written word. For one, the touch screen is user friendly. Touch-screen interfaces can also be real-time. Coupled with the computer trends of more memory and computing power in a smaller space, this combination is suddenly expanding the capability of the remote user. These types of devices are finding their place in a multitude of applications, including restaurants, hospitals, and manufacturing facilities.

Bring the Analog into Focus

Now, imagine drawing your picture on the screen, real-time, and in the analog domain (by hand). Then take the picture and send it immediately to other interested parties. Although, the implementation of the touch screen interface appearance is purely digital, the human interface is purely analog. Conversion of the analog human touch to digital code is effectively and completely done today with a big "A" to little "d" converter (A/d).

Touch Screen Technologies Offer a Lot of Alternatives

There is an abundance of technologies used to build touch screens. Two of the many types are capacitive and resistive. The analog capacitive screen has a resistive coating that lies below a protective layer of glass. It also has electrodes that are positioned in each of the four corners of the screen. These electrodes are used to establish an ac field in the coating; the controller monitors the current flow through each electrode, calculating the ratio of the current flow from each electrode.

The resistive screen consists of two sets of transparent conductive opposing layers. Each layer is coated with a resistive material and is separated by a matrix of insulated spacer dots. The resistive membranes respond to the pressure that is applied with a finger or stylus to the screen's plastic membrane by changing resistance value. The resistive screen has good resolution, is generally more resilient to surface damage and, best of all, fairly inexpensive.

Implementation of the Analog Side of the A/d Conversion

In the case of the resistive screen, an analog interface is required prior to the A/D converter. This is done by exciting the screen's resistive membranes with a dc current. The effective resistance of the screen can then be sensed in terms of its x-coordinate and then its y-coordinate. In this manner, the change in resistance of the screen quickly identifies where the screen is being touched. The tricky part of the analog interface circuit is to create the current sources that are switchable and ratiometric to the A/D converter's measurement. The ratiometric relationship between the converter's measurement and the current sources effectively reduces dc and temperature errors in the system. If the burden of this intricate analog problem is placed on the integrated circuit designer rather than the board level designer, the end result is usually a more accurate, more compact solution.

As you may have guessed, this type of analog front-end has been attached to the analog section of a high-performance converter. But that's not all. The front-end of high-end converters are being modified with numerous types of analog enhancements. For instance, thermocouple-ready, bridge-ready, power-supply monitor, photo-diode ready, RTD-ready and multi-channel, to name a few.

Ring My Bell

Want to learn more about the above A/d converters with enhanced analog interfaces? Give me a call. Or better yet, send me a fax. But, by all means, stay in touch. As you know, a picture could be worth a thousand words.

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

		Click here to get your listing up.