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Data Acquisition Facts (not just FAQs)
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There have been lots of tutorials written on topics like Signal Conditioning, Filtering, Resolution, Accuracy, Sensitivity, Real-time, and allied topics.  I don't want to spend my life reinterpreting what has already been so well covered.  So, what I am going to do on this page is to provide an "index" of online resources for each topic that I think may be useful.  Let me know what you know, so that I can pass it on.

A good source for general data acquisition information is the Developer Zone at National Instruments.

bulletSignal Conditioning           

Whenever an input exceeds the common mode input range of an A/D, the signal must be conditioned.   Although often A/D inputs are protected to voltages greater than the common mode input range (perhaps +/-30 volts typically), isolation modules provide the ability to measure differential voltages that have common mode voltages as high as 1500 Vrms (or higher) above system ground. The isolation modules are also protected from inputs of 240 Vrms (or more).  In the event that a signal’s "differential" voltage exceeds the input range of the A/D, the A/D and computer are protected from damage.   Isolation also is an important safety feature, and is vital in any system that comes in direct contact with human or animal subjects.

Conditioning also covers the amplification or attenuation that might be required to raise very low-level signals or reduce high-level signals to approximate the dynamic range, or span, that the A/D provides.  Amplification can improve Signal to Noise Ratio (SNR), although such improvement usually is accomplished by judicious filtering out of frequencies that are outside the range of frequencies of interest.

           Designing a Data Acquisition and Control System  (CyberResearch tutorial)

          Just How Important Is Signal Conditioning? (Evaluation Engineering article)

         Signal Conditioning (National Instruments tutorial)

         Signal Conditioning Tutorial (Analog Devices tutorial)

            9 Key ADC Specifications You Must Know (ECN Magazine article)

            Everything That You Ever Wanted to Know about Data Acquisition (United Electronics Industries)

bulletSignal Filtering 

          Windowing Functions Improve FFT Results, Part1 & Part2   (Test and Measurement World articles)              

          How to Pick the best Antialiasing Filter for your Signal by Chao-Sun Pang (Evaluation Engineering magazine article, 5/1999)

          Filter your data in software  by W. Haussmann.  (Test and Measurement World, 4/1/2002)Digital filters remove unwanted frequency components from digitized signals. Learn how digital filters work (with source code and other useful information).

          Filtering? Before or after?  By Bonnie Baker (EDN, 2/20/2003).  << Click this link for the complete text.

Have you ever needed a lowpass or highpass filter in your circuit and wondered where you should place it in the signal path? Before the introduction of controllers and processors, engineers used analog circuits to implement all filters. With this type of design, you needed to think ahead and work up a "pencil design" before going to the breadboard. If you cut corners, you would probably end up disassembling the circuit and rebuilding it, hoping to get it right the next time. Then came the digital filter. This type of filter implementation can duplicate the frequency response of any analog filter in the digital domain. A major advantage of digital filters is that you can painlessly adjust them with firmware. This scenario sounds too good to be true, and it is.

There are times when you should build the filter with analog hardware and times when it is appropriate to implement it with a controller or processor in firmware...

bulletResolution, Accuracy, Sensitivity

Resolution in data acquisition specifies the smallest measurement that can be detected by a data acquisition system, and is usually specified in bits. The resolution is determined by the analog-to-digital (A/D) converter. The conversion formula for converting resolution bits to actual resolution is shown below.

Resolution = one part in 2^(# of bits)

To calculate the resolution in volts (or any other physical unit that is represented by the applied voltage), take the full-scale input range and divide it by the number of bits. If you are using a 12-bit A/D, and the voltage range (span) covers 5-volts, the overall resolution would be as follows:

Resolution = one part in 4096

Resolution in respect to volts = 10 volts/4096
= .00244V or 2.44 mV per bit.

So in this instance, the lowest detectable increment would be 2.44 mV. Generally, 16-bit A/Ds are more expensive than their lower resolution counterparts. So, it only makes sense to match the required resolution to your actual system requirements.

        Resolution vs. Accuracy vs. Sensitivity (Evaluation Engineering article)

          Designing a Data Acquisition and Control System (CyberResearch tutorial)

bulletReal-time

         Here is what I have to say about this critical subject.

        Understanding Real-Time for Measurement & Automation (Evaluation Engineering article)

bulletIsolation Techniques

"Isolation techniques for high-resolution data-acquisition systems" is an article by Helen Stapleton and Albert O'Grady was published in the February1, 2001 issue of EDN Magazine.  Reprints of this article are available from the publisher.

bulletPC Sound Card

A fairly sophisticated data acquisition system that is built into most PCs is the PC sound card.  Depending on your needs, it may be just the ticket for a low-cost system.  Check out the information on both hardware and software on The MathWorks and Physics freeware.  Another source is FreeView Sound at www.inesinc.com/sound.htm. For more details, click here.

bulletGeneral Topics

         Glossary (National Instruments tutorial)

         Data Acquisition (National Instruments tutorial)

         The Five Most Common Industrial Measurement Problems - Which Do You Need to Solve? (Analog Devices tutorial)

           Five PC-Based Production Measurement Traps (Evaluation Engineering article)

           What Bus Architecture Is Right for You? (Evaluation Engineering article)       

            Internet-Enabled Data Acquisition using VB6. Parts 1 and 2 (Desktop Engineering article)

            Introduction to Thermocouples (Control Engineering Article)

            Going the distance, a tutorial on distance sensors (Machine Design, May 21, 2004)

            Resistance Temperature Detectors (Control Engineering Article, November, 2005)

            Basic Temperature Measurement (IOTech Tech Tip, January 2007)

 

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Last modified: 11/25/09
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