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Issue 11: Differential v Single-Ended Inputs; Employment Sites

CONTENTS

Windmill News | Single-Ended or Differential Inputs | Single-Ended | Differential | Pseudo-Differential | Making Your Choice | Employment Sites

WINDMILL NEWS - 35% OFF WINDMILL BLACKBOARD

Throughout July we're offering Blackboard for just 95 pounds when purchased from our on-line shop - a saving of 50 pounds on its normal price! This offer is only available over the internet.

Blackboard lets you

• Log information typed on the keyboard (such as batch numbers) alongside real-world data, and
• Allows programs that support DDE (like Excel) to intercept the incoming data and pre-process it before logging.

Blackboard works with Windmill 4.3, a suite of software for logging, charting and output control. Our Monitor newsletter subscribers can download Windmill for free.

CONNECTING YOUR SIGNALS: DIFFERENTIAL OR SINGLE-ENDED INPUTS?

When monitoring analogue signals such as temperature, strain or vibration, you often have a choice of single-ended or differential connections. What is the difference between the two and which should you use?

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Single-Ended Inputs

With single-ended inputs you connect one wire from each signal source to the data acquisition interface. The measurement is the difference between the signal and the ground or earth at the interface. (The interface might be a card that plugs into your PC, or an external unit connected to your PC.) This method relies on

1. the signal source being grounded (earthed), and
2. the signal source's ground and the interface's ground having the same value.

Differences in Ground Levels
We think of the ground as a constant 0V, but in reality the ground, or earth, is at a different level in different places. The closer together the places, the more likely the ground level will be the same. Make a connection between two grounds and the difference in levels can drive large currents, known as earth or ground loops. This can lead to errors when using single-ended inputs.

Noise Errors
Single-ended inputs are sensitive to noise errors. Noise (unwanted signal contamination) is added because signal wires act as aerials, picking up environmental electrical activity. With single-ended inputs you have no way of distinguishing between the signal and the noise. The ground and noise problems can be solved by differential inputs.

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Differential Inputs

With differential inputs, two signal wires run from each signal source to the data acquisition interface. One goes to a + input and one to a - input. Two high-impedance amplifiers monitor the voltage between the input and the interface ground. The outputs of the two amplifiers are then subtracted by a third amplifier to give the difference between the + and - inputs, meaning that any voltage common to both wires is removed.

This can solve both of the problems caused by single-ended connections. It means that differences in grounds are irrelevant (as long as they aren't too large for the amplifier to handle). It also reduces noise - twisting wires together will ensure that any noise picked up will be the same for each wire.

Floating Signals
A common problem when using differential inputs is neglecting any connection to ground. For example, battery-powered instruments and thermocouples have no connection to a building's ground. You could connect a battery, for instance, between the interface's + and - inputs. The 2 input amplifiers will try to monitor the voltages + to earth and - to ground. However, as there is no connection between the battery and ground, these voltages to ground could be any value and may be too large for the amplifier to handle.

For these "floating" signal sources you should provide a reference. The interface will generally have a socket labelled 0V, REF or GND. Run a wire from, say, the - wire to this ground socket, either directly or via a resistor. (If your signal source is itself grounded don't make a connection to the interface's ground socket.)

Amplifier Ability and Operating Range
The three amplifiers used for differential inputs are collectively known as an "instrumentation amplifier". Ideally, as previously described, any voltage common to both wires (common mode voltage) is cancelled. In practice the two input amplifiers are not perfectly matched so a fraction of the common mode voltage may appear. How closely the instrumentation amplifier approaches the ideal is expressed as the common mode rejection ratio (cmrr). This is the reciprocal of the fraction let through and is usually given in decibels. The higher the rejection ratio the better.

Another specification to look for is the common mode range. This is the maximum contamination voltage with which the amplifier can cope. If the difference in ground levels between your interface and signal source exceeds this value, your measurement will be inaccurate. (Your hardware operating range may be given as higher than the common mode range, but the operating range just guarantees that your hardware won't be damaged, not that it will work properly.)

Less Signals with Differential Inputs?
An obvious disadvantage of differential inputs is that you need twice as many wires, so you can connect only half the number of signals, compared to single-ended inputs. Should you decide that single-ended inputs are OK for you - if you have short signal wires, close together signal sources, and signals larger than around 100 mV for example - you can use differential inputs in single-ended mode. To do this short one of the signal wires (usually the - input) to the interface ground input. Differential inputs, therefore, give you the option of either mode.

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Pseudo-Differential Inputs

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Summary: Making your Choice Between Single-Ended and Differential

1. Signal leads over a few metres in length?
   Choose differential to reduce noise.
2. Small signals under around 100 mV?
   Choose differential to reduce ground and noise errors.
3. Signals with different grounds to each other, as happens when signals are remote from one another?
   Choose differential to remove ground errors.
4. Sensors with high resistance such as strain gauges?
   Choose differential to remove common mode voltage. High resistance gives greater pick-up and thus higher common mode voltage.
5. Need twice as many inputs, and have none of the above problems?
   Choose single-ended.

For data acquisition hardware offering differential and single-ended inputs, see our on-line shop.

To subscribe to this newsletter (Monitor - ISSN 1472-0221) and regularly receive data acquisition tips, tutorials and special offers, fill in your e-mail address here.

EMPLOYMENT: SEEKING AND OFFERING

The web is an ideal medium to match job seekers and employers, evidenced by the plethora of employment sites now available. In this piece we've picked out recruitment sites featuring engineering and technology, indicating the area of the world they service. We've used the term CV, standing for curriculum vitae, rather than the term resume.

The sites fall into 3 broad categories: magazines, newspapers and recruiters. Magazines make the job adverts on their pages available on their web sites. Newspaper Search Sites let you search a number of newspapers' job adverts. The Recruiters specialise in placing candidates. Many will e-mail suitable jobs to you and post your CV for prospective employers to view.

Engineering Central

Job adverts for chemical, civil, electrical, industrial, manufacturing and mechanical engineers. There is also a special job bank for entry level engineers. (America only.) http://www.engcen.com/

Asia Net - Worldwide

A very specialised site for candidates speaking English and either Japanese, Chinese or Korean, with technical or business skills. You can get job updates via e-mail, filtered by language and skills according to your CV. Employers pay a fixed fee for their job openings to be sent to suitable candidates.
http://www.asia-net.com/

Overseas Jobs Express - Worldwide

Gives vacancies that are open to non-citizens of your target country. They issue a printed newspaper with around 1500 jobs per issue, but also display hundreds of jobs on-line. Recruiters and employers can place classified job ads here free of charge. Lots of categories including many engineering disciplines.
http://www.overseasjobs.com/

Newspaper Searches - Worldwide

The first 6 sites let you search a range of newspaper job adverts. The last site gives you a list of newspapers in your chosen region so you can visit the newspapers' sites themselves to search for jobs.
http://www.market.fairfax.com.au/jobs/mkt.html (Australia)
http://www.jobnetnz.co.nz/cgi-bin/jobnet/jobnet.w (New Zealand)
http://www.globecareers.com/ (Canada)
http://www.jobhunter.co.uk/ (UK)
http://new.careerpath.com/ (US)
http://www.mediainfo.com/emedia/ (Worldwide)

Top Jobs on the Net - Australia, Thailand, US, UK, Ireland, Switzerland and Poland

Top Jobs aim to provide high quality recruitment solutions within the management, professional and technical fields. Their site is easy to use - you first select a country, then a top level category such as engineering, then, optionally, a sub-category and location. They will also e-mail you jobs matching your criteria.
http://www.topjobs.net/

Eures Job Search - EU and EEA

Eures stands for European Public Employment Service. It covers all 18 countries in the European Union and European Economic Area and brings together jobs advertised by the national employment agencies. You can search by occupation and by country.
http://europa.eu.int/eures/cgi/en/jv_search

Employment Guide's Career Web - Worldwide

Not a specialist site but you can simultaneously search many engineering and technical categories. Can post CV and receive job openings by e-mail.
http://www.cweb.com/jobmatch/

Executive Recruitment Services PLC - UK, Europe, US

This company is split into divisions, each division specialising in one discipline. The divisions include technical (defence, process control), communication and environmental. As with other sites you can post your CV, be e-mailed suitable job postings and search by your own detailed criteria. A high quality site from an established recruitment agency.
http://www.ers.co.uk/ers

Monster.com - North America, Europe and Australia

This is solely an internet based service. Employers pay a fixed fee to post a job and to be e-mailed details of candidates fitting their requirements. Job seekers can search jobs and post their CV for free.
http://www.monster.co.uk/ (UK)
http://www.monster.com/ (US)
http://www.monster.ca/ (Canada)
http://www.monsterboard.nl/ (Netherlands)
http://www.monster.be/ (Belgium)
http://www.monsterboard.com.au/ (Australia)
http://www.monster.fr/ (France)

PhysicsWeb - Worldwide but with bias towards UK

Lists all the job and studentships advertised in Physics World magazine. Details appear before the magazine publication date and new vacancies can be e-mailed to you. The search could be improved as you can search for academic or commercial, but can't specify an area.
http://physicsweb.org/jobs/

JobServe - 90% UK - 10% Rest of World

This site compiles vacancies from over 1700 recruitment agencies specialising in IT. It adds around 6000 new jobs every day. You can search by permanency, skills and location. They will e-mail you details of jobs fitting your skills and preferences. JobServe also sends European agencies a list of professionals actively looking for work. The agencies can then request a copy of your CV.
http://www.jobserve.com/

Beechwood Recruitment - UK + Rest of World

Specialise in technical vacancies. Their search engine is not the easiest to use (you don't get any results unless you enter an "additional term"). Employers pay a subscription to access the database of candidates and submit job requirements.
http://www.beechwoodrecruit.com/

Bulldog Engineering Recruitment and Management Services - UK

Operating since 1988, Bulldog's database holds over 12000 applicants and 2500 companies. More importantly, their consultants are all fully qualified engineers. Job seekers need to register with them and provide a great deal of information, but the serious about job hunting can be confident that their details are sent only to companies exactly matching their requirements. Employers using Bulldog pay a percentage of the annual salary of anyone they choose to employ.
http://www.bulldog.co.uk/recruitment/

dotJobs - UK

This UK site has the latest jobs from Electronics Times, The Engineer, Food Manufacture, Packaging Magazine & Printing World. You can search by location, job description and magazine. The search engine has a reasonable, but slightly idiosyncratic, understanding of UK locations. Prospective employees can register to receive jobs by e-mail. An easy-to-use site that lets you quickly find relevant jobs in the UK.
http://www.dotjobs.co.uk/

National Academy of Sciences - US + Worldwide

Registrants come from the disciplines of: Engineering; Interdisciplinary ; Life Sciences; Medical Sciences; Physical Sciences and Mathematics; Professional; and Social Sciences. Aimed at students and recent graduates.
http://www4.nas.edu/osep/cpc.nsf

TechJobBank - US + Worldwide

Specialises in technical jobs. You can submit your CV and search by state, country or continent.
http://www.techjobbank.com/

Engineering Solutions Intl - US

Specialise in placing engineers and scientists. You can submit your CV and they will contact you before they pass it to any potential employer. They offer free advertising for 30 days to employers and recruiters. You can browse, but not search, their database. http://www.engineeringsolutions.com/

American Institute of Physics Job Opportunities - US

Online searchable jobs database, which is updated daily. There are hundreds of jobs posted each month.
http://www.aip.org/careersvc/index.html

Engineering Job Databases - US

Lists American recruitment agencies and headhunters, together with which branch of engineering they specialise in and the areas they cover, with links to their sites. Can submit brief details of your CV for employers and recruiters to view.
http://www.engineeringjobs.com/

Do you have a comment or suggestion for this newsletter? Why not email the editor - Jill - at studholme@zetnet.co.uk

Copyright 1999 Windmill Software Ltd. All rights reserved. This newsletter may be distributed in its entirety. However, individual sections may not be reproduced without the prior written agreement of Windmill Software Ltd.

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