Windmill Software Ltd
Data Acquisition Intelligence

analogue output control system
July 2001

------------------------Monitor-------------------------
The Newsletter for PC-Based Data Acquisition and Control
Issue 36          www.windmill.co.uk           July 2001
--------------------ISSN 1472-0221----------------------

Hello and thank you for subscribing to Monitor. Should 
you wish to cancel your free subscription please visit 
http://www.windmill.co.uk/newsletter.html

Our main article this month covers analogue output 
signals, whilst the Excel Corner gives tips on analysing 
only the most recently collected data. 

CONTENTS
==========
* Windmill News: Windmill results in 90% efficiency 
  improvement
* Control and waves: Generating analogue signals
* Excel corner

________________________________________________________

Windmill News:
Windmill Results in 90% Efficiency Improvement
________________________________________________________

The free Windmill software has resulted in a 90% 
efficiency improvement for a company using a Quadra 
Check 2000 optical measuring system. You can now 
download Excel spreadsheet and DDE examples for the 
Quadra-Chek, plus the Windmill setup files, from our 
Setup File Library.
http://www.windmill.co.uk/serial_driver.html

Other files available from the library include those for 
the Ashtech GPS receiver, Ecom gas analyser, Furuno 
sonar, Mettler Toledo pH transmitter, Molyteck data 
logger, OMRON plc, Parallax BASIC Stamp and Sartorius 
balance. If you haven't yet downloaded the free Windmill 
4.3 software with the LabIML serial driver, subscribe to 
Monitor for full instructions.
________________________________________________________

Control and Waves: Generating Analogue Signals
________________________________________________________

Analogue output signals tend to fall into one of two 
broad categories: control and waveform generation. When 
they are used as control signals, they are generally 
slowing changing. The computer software might be 
monitoring one or more input signals and using these to 
calculate a control signal. Alternatively, the computer 
may generate an output voltage as a stimulus prior to 
measuring the response.

The second category of analogue output is that of 
waveform synthesis - where the output signal changes 
several million times a second.

In both these categories, a digital-to-analogue 
converter is used to generate the signal. For waveform 
synthesis the analogue output will be a voltage signal. 
For control the output may be a voltage or a current.

Analogue Output Hardware: Understanding the 
Specifications
============================================

Analogue output hardware comes in many guises, such as 
boards that plug into your PC or units which connect to 
the computer over (for example) a USB, Ethernet, GPIB or 
RS232 link. The hardware suppliers quote various 
specifications for these devices, including: Range, 
Resolution, Slew Rate, Settling Time and Linearity. It's 
not always clear, however, what they mean by these 
terms.

                           *

OUTPUT RANGE
The maximum and minimum voltage or current that can be 
generated. The range may be bipolar, covering a range of 
-1 V to + 1 V for example; or unipolar, perhaps covering 
a range of 0 to 10 V. Many systems offer a choice of 
ranges, which you can select through either software or 
hardware. It's best to choose the smallest range 
spanning the output you need, as this optimises the 
resolution.

                           *

RESOLUTION
The resolution is the number of steps into which the 
output range is divided. The resolution is normally 
expressed as bits (n) and the number of steps is 2n-1. 
This equates to 2n values. A converter 
with 12-bit resolution, for example, divides its 
output range into 212 or 4096 values. In 
this case a 0-10 V range will be resolved to 0.25 mV, 
and a 0-100 mV range to 0.025 mV. The output is thus 
not truly analogue but an approximation. How close 
the approximation depends on the resolution.

                           *

SLEW RATE
Slew rate is the maximum rate of change of the output 
signal, measured by the rise in voltage divided by time. 
Slew rate is expressed as volts per microsecond.

                           *

SETTLING TIME
When the D-A converter changes from its minimum output 
level to its maximum, the output signal swings through 
its "full scale". The settling time lets you know how 
long it will take the output to settle to its final 
voltage. It is normally given as the time to settle to a 
percentage of the full-scale voltage or current range, 
following a full-scale change in the voltage or current.

                           *

LINEARITY
Ideally a D-A converter with n-bit resolution will 
convert the input range into equal steps. In practice 
the steps are not exactly equal, which leads to non-
linearity in a plot of D-A output against output signal. 
This is expressed as a percentage of the output's full-
scale range.

                           *

BUFFER AND MEMORY
Waveform synthesis devices need a memory to hold the 
points used to generate the waveform. Memory may be 
expressed as the maximum number of points: 2 M (two 
million) for example. Alternatively the memory may be 
shown in bytes. For example, if you had a 12-bit D-A 
converter each point might occupy two 8-bit bytes and in 
our example the memory would then be 4 Mbytes.

Typically you use software to synthesise the waveform, 
storing the points in the memory. The memory contents 
are then scanned into a D-A converter at the required 
speed - often regulated by hardware for precision. 
Lastly a buffer amplifier drives the actual output.

                           *

For more information on analogue outputs please contact 
Windmill Software at [email protected]
________________________________________________________

Excel Corner
Filtering and Summing Columns: Showing only Recent Data
________________________________________________________

Suppose you've used Windmill to route data from your 
measurement instrument into Excel (see 
http://www.windmill.co.uk/excel/ for details how). 
You took readings every hour for three months and have 
2208 rows of data. However, at the moment you are only 
interested in the readings from the previous week. To 
hide the superfluous data use Excel's AutoFilter.

Select a cell in the date column. From Excel's Data menu 
select Filter, then AutoFilter. The column title becomes 
a drop-down list. Select the down arrow and choose 
Custom. You can now elect to only show information 
gathered on dates greater (or later) than a week ago. 
You can use the AutoFilter not just for dates but for 
any criterion you like. To filter data from two columns, 
such as temperatures over 70oC in the last week, use the 
Advanced Filter option.

So far so good, but what if you were analysing your data 
- perhaps summing your data columns. When you filter the 
data the summed total remains for all the data, not just 
that now displayed. You can correct this and display a 
value which changes according to the rows you have 
chosen to display. To do this, instead of the SUM 
function use the SUBTOTAL function. SUBTOTAL will ignore 
any hidden rows that result from a list being filtered. 
Use SUBTOTAL(9, ref), where ref is the range of data 
cells in the column. 

You can also use SUBTOTAL for Average, Maximum, Minimum, 
Standard Deviation and other Functions. In these cases, 
instead of 9 use these numbers
1   AVERAGE
2   COUNT
3   COUNTA
4   MAX
5   MIN
6   PRODUCT
7   STDEV
8   STDEVP
9   SUM
10  VAR
11  VARP

I hope you've found this tip useful. If you would like 
to share any tips for using Excel in data acquisition 
applications, please send them to [email protected]
________________________________________________________

* Copyright Windmill Software Ltd
* Reprinting permitted with this notice included
* For more articles see http://www.windmill.co.uk

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 
http://www.windmill.co.uk/newsletter.html
and an index of articles at 
http://www.windmill.co.uk/newsletter.html

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]

http://www.windmill.co.uk/

http://www.windmillsoft.com/



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