Tutorial: Isolating signals to kill crosstalk

When signals make the jump from one cable to another, it degrades accuracy. Here's advice on how to minimize the effect.

04/16/2009


In April 2007, we discussed the causes and cures of ground loops in analog instrumentation wiring. A different problem that can cause similar headaches is crosstalk, where the contents of one channel jump onto another to cause reading errors.

A common problem that instrumentation engineers encounter involves a phenomenon where the contents of one data acquisition channel are superimposed on another. This condition, known as crosstalk, can cause measurement errors, ranging from subtle to major, that may go undetected. In its most exaggerated form, a nearly exact duplicate of one channel appears on an adjacent channel to which nothing is connected.

With the PC-based instrumentation revolution came active use of multiplexers and their
promise of low cost per channel. Getting a per-channel cost of $30 to $40 saved money, but along the way, a hallmark of traditional instrumentation has been dropped: an isolation amplifier for each channel. The system under test is connected directly to the multiplexer's inputs; however, the multiplexer is not always an ideal signal processing device. Its inputs have capacitance that stores a charge that is directly proportional in magnitude to the sample rate and the impedance of the signal source. This inherent characteristic causes crosstalk.

Consider an application where the multiplexer’s input is connected directly to the output of an isolation amplifier. In this situation, the impedance the multiplexer sees is stable and low, with 10 ? being a typical value. Crosstalk is greatly minimized or eliminated altogether since the impedance of the source is low enough to bleed off the charge on the multiplexer's input capacitance before the analog-to-digital converter (ADC) reports a value.

Even under this nearly ideal impedance situation, a high sample rate can boost crosstalk by minimizing the capacitive discharge time on the multiplexer's channels. In effect, the capacitance has less time to bleed off its charge before the analog-to-digital conversion takes place, resulting in crosstalk where none existed before.

As source impedance and sample rate increase, the probability of crosstalk increases as well. To prevent this from happening, keep these points in mind:

  • Minimize the source impedance of the signal source. Use isolation amplifiers to keep it below 100 ohms—although, at very high sample rates, even this value may be too high.

  • When source impedance cannot be controlled, an isolation amplifier is needed between the signal source and the data acquisition card multiplexer. An instrument with a built-in isolation amplifier is needed on each channel to provide protection from stray signal paths.

These strategies become even more important as the sample rate increases. The best and most predictable results are obtained when an instrument is used that has a fixed scan interval; this helps control any high sample rate bursts.

Thanks to Roger Lockhart, Dataq Instruments , and Dataforth Corporation .

Download a more extensive and detailed discussion of crosstalk and common mode voltage problems from Dataforth.

—Edited by Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly eNewsletter
Register here to select your choice of free eNewsletters .





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.
Electric motor power measurement and analysis: Understand the basics to drive greater efficiency; Selecting the right control chart; Linear position sensors gain acceptance
Protecting standby generators for mission critical facilities; Selecting energy-efficient transformers; Integrating power monitoring systems; Mitigating harmonics in electrical systems

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.