The Newsletter for PC-Based Data Acquisition and Control
Issue 36 www.windmill.co.uk July 2001
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Our main article this month covers analogue output
signals, whilst the Excel Corner gives tips on analysing
only the most recently collected data.
* Windmill News: Windmill results in 90% efficiency
* Control and waves: Generating analogue signals
* Excel corner
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.
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
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
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
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 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.
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.
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-
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]
Filtering and Summing Columns: Showing only Recent Data
Suppose you've used Windmill to route data from your
measurement instrument into Excel (see
https://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
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 https://www.windmill.co.uk
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