Windmill Software Ltd
Data Acquisition Intelligence

30 June 2011

The Newsletter for PC-Based Data Acquisition and Control
Issue 155           June 2011
--------------------ISSN 1472-0221----------------------

Welcome to Monitor, the newsletter for data acquisition 
and control.  

This month we are pleased to offer you our 751 data 
acquisition and control unit for half its normal price. 
For more details read on below or see

Don't forget that you can download free copy of our
ComDebug data logging software.  This lets you 
log data from instruments connected to the PC's COM 
port using RS232, RS285 or Modbus.

Finally check out our new Twitter account at where we'll be 
posting data acquisition news and updates.

We hope you find the newsletter useful, but should you 
wish to remove yourself from our mailing please go to 
Monitor Newsletter

* Windmill News: Over 50% off Multi-Function Unit
* Differential or Single-Ended Inputs?
* Excel Corner: Reading Big Files into Excel
  into Excel
* DAQ News Round-up

Windmill News: Multi-Function Unit Reduced in Price

This month we are very pleased to offer you a discount 
on the price of our 751 data acquisition and control 
unit, reducing it from £595 to £295 (336 Euro or 
480 USD).  The 751 unit can capture data from 16 sensors 
and probes of various types. It can also switch digital 
outputs and count pulses.

The package includes the Windmill 7 logging, charting 
and control software and free technical support for 

You can connect up to eight 751s to one PC, giving 
128 analogue inputs, 256 digital inputs & outputs and 
8 counters.

The 751 has been used to measure temperature, 
CO2 concentration, humidity, photosynthetically active 
radiation, pH, strain, pressure and other signals, 
although extra hardware is needed for some of 
these measurements.

Designed for maximum versatility, you have a choice of 
four analogue input ranges, selected through 
Windmill software individually for each channel. 
Alternatively, choose automatic ranging and let the 
software match the input signal as closely as possible. 

You can also use the Windmill software to select the 
resolution of the A-D converter, from 12- to 18-bits. 
Choose high throughput, high resolution or 
something in-between

For accurate measurements the 751 uses 
differential inputs, which reduce noise and 
ground errors.  See the next article for a 
discussion of the differences between differential 
and single-ended inputs.

At regular intervals Windmill uses a stable on-board 
reference voltage to recalibrate the 751. 

One of our most popular hardware packages, the 
751 is an excellent multi-purpose unit.

For more information on the 751 data acquisition and 
control unit, see, 
e-mail [email protected] or 
telephone +44 (0)161 833 2190.

Differential or Single-Ended Inputs?

When monitoring analogue signals such as pressure or 
vibration, you can choose single-ended or differential 
connections. What is the difference between the two and 
which should you use?


Single-Ended Inputs

Single-ended inputs give you twice as many analogue 
channels for your money - so why not always use them?  
The reason is that they might give inaccurate measurements.

With single-ended inputs you connect one wire from 
each signal source to the data acquisition unit. The 
measurement is the difference between the signal and 
the ground or earth at the unit. This method relies on

1. the signal source being grounded (earthed) and
2. the signal source's ground and the unit'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 may 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.

When to Choose Single-Ended Inputs?

  Where you have short signal wires, close-together signal 
  sources and signals larger than around 100 mV, then 
  single-ended inputs are a cheaper option than 
  differential inputs.  You could choose our 750 data 
  acquisition unit for example: details at  
  This is still at the low price of £150 (240 USD / 170 Euro).


Differential Inputs

With differential inputs, two signal wires run from 
each signal source to the data acquisition unit. One 
goes to a + input and one to a - input. Two high-
impedance amplifiers monitor the voltage between 
the input and the unit's 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 - if you 
twist the wires then 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 data 
  acquisition unit's + and - inputs. The two 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 data acquisition units 
  like the 751 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 you 
  don't need to 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 data acquisition unit and signal 
  source exceeds this value, your measurement 
  will be inaccurate. Be aware that 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?

  A disadvantage of differential inputs is that 
  you need twice as many wires and differential 
  units tend to be more costly.  Our 751 provides 
  16 differential inputs and is on promotion for 
  £295 (470 USD / 330 Euro).  More details at

  With some data acquisition interfaces you can 
  choose whether to use the inputs in differential 
  or single-ended mode, for example the 
  Microlink 3050 module 


Pseudo-Differential Inputs

Some manufacturers offer pseudo-differential inputs. 
These may be used when the signal sources are close 
together and share a common ground. Pseudo-differential 
is similar to single-ended but the signal source's 
ground is isolated from the data acquisition 
interface's. A wire runs from this ground to the 
interface. By subtracting the interface ground 
from the signal ground, differences are removed 
from the measurement. However, this method is no 
use for reducing noise.


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 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 units offering single-ended and 
differential inputs see

Excel Corner: Reading Big Files into Excel

Some versions of Excel have a limit of 65536 rows in a 
data sheet. When you are collecting data over a long 
period of time, this may not be sufficient. With Windmill 
you can set Logger to automatically close one data file 
and open another one to prevent files getting too big. 

Alternatively you can write a macro in Excel to read data 
from the log file and open a new worksheet to hold the 
data when 65536 rows are reached.

An example of a macro, which was written by John Morton 
of Poole Hospital and very kindly shared with us, is below.

Thanks John. 


Importing more than 65,000 rows of data into EXCEL

Reading data from a file located in "c:\windmill\data.wl" 
the macro loops until it reads EOF End of file message.
Data is entered on sheet 0 until the pointer goes above limit 
65,000 (worksheets actually have 65536 rows). The pointer is reset 
to 1, and a new worksheet is added to the workbook and this 
worksheet starts to receive data from file. Maximum number of 
worksheets is 255, so you could read in 255 * 65536 (16,711,680) 
lines of data from a file.
ScreenUpdating is turned off to prevent screen flicker during import.

Sub newImport()
' newImport Macro
' Macro created 24/11/2008 by Morton
myfile = "c:\windmill\data.wl"

Open myfile For Input As #1
Application.ScreenUpdating = False
pointer = 1
sheetNo = 1
Do While Not EOF(1)

    Line Input #1, myLine
    Cells(pointer, 1) = myLine
    pointer = pointer + 1
    If pointer > 500 Then
        Worksheets.Add after:=Sheets(sheetNo)
        pointer = 1
        sheetNo = sheetNo + 1
        If sheetNo > 255 Then MsgBox "Maximum number of worksheets have been added  - STOP !"
    End If
Application.ScreenUpdating = True
Close #1

End Sub


Any Excel questions, or tips, please get in touch

Further Reading: 
Using Excel for Data Acquisition

DAQ News Roundup

Welcome to our roundup of the data acquisition and 
control news.  If you would like to receive more 
timely DAQ news updates then grab our RSS newsfeed 
at  Read for notes
on how to display the live news on your own web site.

Final Version of Industrial Control Systems Security Guide Published
   The National Institute of Standards and 
   Technology has issued the final version of its 
   Guide to Industrial Control Systems (ICS) Security, 
   intended to help managers of critical infrastructures 
   to secure their systems while addressing their unique 
   performance, reliability, and safety requirements.
   Source: NIST

'Orca ears' inspire researchers to develop ultrasensitive undersea microphone
   Researchers have developed a microphone that can be 
   used at any depth in the ocean, even under crushing 
   pressure, and is sensitive to a wide range of sounds, 
   from a whisper in a library to an explosion of TNT. 
   They modeled their device after the extraordinarily 
   acute hearing of orcas.
   Source: Stanford University News

New Sensor To Measure Structural Stresses Can Fix Itself
   Researchers have designed a sensor that can measure 
   strain in structural materials and is capable of 
   fixing itself when broken: an important advance 
   for collecting data to help make informed decisions 
   about structural safety in the wake of earthquakes, 
   explosions or other unexpected events.
   Source: North Carolina State University

Industrial Automation Electronics Equipment Market Set to Surpass Pre-recession levels in 2011
   Following initial reports of a strong first 
   quarter for industrial automation electronics 
   equipment (IAEE), IMS Research has upgraded its 
   forecast to 12.9% market growth in 2011. The global 
   IAEE market is now projected to be worth $97 billion 
   in 2011, according to a new report, “The World Market 
   for Industrial Automation Electronics Equipment”.
   Source: IMS Research

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see

We are happy for you to copy and distribute this 
newsletter, and use extracts from it on your own web site 
or publication, providing the above notice is 
included and a link back to our website is in place.

An archive of previous issues is at
and an index of articles at

Windmill Software Ltd, PO Box 58, North District Office,
Manchester, M8 8QR, UK
Telephone: +44 (0)161 833 2782
Facsimile: +44 (0)161 833 2190
E-mail: [email protected]


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