Basic Channel Input Specifications
In a sense, data acquisition hardware is a commodity. All you need
to make a single-channel data acquisition system, as shown in Figure
2.1, is an appropriate sensor, an analog link, an A/D converter,
a digital link, and a computer.
The sensor is a transducer that "feels" the physical
parameter of interest, and puts out an analog (voltage) signal whose
level indicates the value of the physical parameter. That analog
signal travels over the analog link to the A/D converter, which
actually makes the measurement and puts out a digital word indicating
the analog level and, by extension, the physical parameters
value. That digital word then travels through the digital link to
the computer. The computer, finally, absorbs the information and
stores it so that somebody can, later on, do something useful with
Click image to see full size
Figure 2.1: Each channel of a data acquisition system includes
a sensor, an analog signal link, an A/D converter, and a digital
signal link into a computer that receives and archives the data
in a data storage device.
That is the minimum you need for a single-channel data acquisition
system. Other amenities, such as signal conditioning and data buffering,
are icing on the cake to make the whole process go more smoothly,
accurately and reliably.
For a multi-channel data acquisition system, you need only to duplicate
the sensor, analog link, A/D converter, and digital link for each
channel. Since the computer can really accept only one data word
at a time, somewhere along the line you need to add in a scanner
to multiplex the signals together, interleaving them so that they
arrive sequentially rather than in a bunch. The scanner funnels
all your data channels down to one channel feeding the computer.
All data acquisition boards essentially do the same thing: they
provide the A/D converter, the scanner and the digital link. It
is in this sense that DAQ boards are commodities.
The idea of a "commodity," however, also carries the
connotation of sameness: one commodity of a certain type is pretty
much the same as the next one. When looked at in this light, DAQ
boards cease to look like commodities at all.
As a DAQ-system designer, you dont care at all about a lot
of the things closest to the hearts of DAQ-board suppliers. An obvious
example is the boards ability to serve in many different applications.
The board supplier wants all his or her boards to cover as many
applications as possible because it minimizes the number of different
boards that must be designed, manufactured, stocked and so forth.
You, on the other hand, only care about one applicationyours!
Having a board covering many applications is likely to influence
its price to you, and you certainly are interested in minimizing
that price. You dont, however, care how that price gets minimized.
You just want (among other things) the lowest one you can muster.
If the manufacturer lowers the price by making more-general-purpose
boards, so be it.
What you really care about are the boards qualities that
fit it for your particular application. It is up to you to identify
those qualities that are most important for your application and
put values to them. The qualities are the specifications you will
look at and their values are what you want the board to meet.
Begin with an inventory of the physical parameters you want to
measure. Go through your physical system bit by bit and identify
everything you might want to monitor there. That will give you two
things: a total input-channel count and a sense of what type each
In fact, the only thing DAQ-board inputs sense is voltage. Even
an input that nominally senses current really only senses the voltage
that the input current develops across the input impedance. The
input-type specification really just tells you that input channel
has signal-conditioning circuitry that makes it work particularly
conveniently with a certain type of sensor.
Of course, having a DAQ input conveniently set up to work well
with, say, Type K thermocouples can be a great convenience indeed!
The next bit of information you need for each channelthat
is, for each physical parameter you want to monitoris the
range of values it is likely to take on. DAQ-board manufacturers
often specify gain and offset rather than range. Gain and offset
acknowledge the fact that the boards A/D converter works more-or-less
linearly over a limited dynamic range of ADC-input values. To widen
its range, the signal-conditioning electronics ahead of the A/D
converter includes a DC amplifier with programmable gain, and a
DC level shifter to provide programmable offset.
Suppose the A/D converter is designed to take voltages from zero
to 1 volt positive, but youre sensor puts out voltage in the
range of 6.0 to 6.1 volts. The span of those voltages is 0.1 volts,
so to take best advantage of the ADCs dynamic range, youd
best put in a gain of 10, increasing the span to 1 volt.
But, that would give you a totally unacceptable range of 60 to
61 volts! So, you need an offset of -6.0 volts at the amplifier
input (changing the range from 6.0-6.1 volts to 0.0-0.1 volts),
then apply the 10X gain. The signal-conditioning electronics then
translate the 6.0-6.1 volt range of the sensors output to
the 0-1 volt dynamic range of the A/D converters input.
Bandwidth and Sampling Rate
After determining each physical parameters range, you also
need to find out how fast it varies. Actually, all you care about
is the highest Fourier component of interest in the signal waveform.
If, say, your physical parameter is the total weight of a tree whose
growth pattern youre studying, your highest frequency component
could have a period of months. If, on the other hand, youre
looking at the sound produced by a motorcycle exhaust system, the
highest frequency of interest will likely be in the high audio range
(perhaps a harmonic of the self-resonance of some baffle in the
That highest Fourier component of interest is usually called the
signals bandwidth (on the presumption that the lowest component
of interest is at zero Hertz). Although DAQ-board electronics, like
all electronics, have a characteristic bandwidth that must be large
enough to pass all the significant signal-component frequencies,
the sample rate turns out to be a more important board specification.
The theoretical minimum sample rate you should use for any data
acquisition application is two samples per cycle at the maximum
frequency of interest. Many manufacturers recommend at least four
samples per cycle. Ive always suggested shooting for 10 samples
per cycle if you can get them. It is reasonable to assume that any
board manufactured to achieve a given sample rate will have signal-conditioning
electronics with a bandwidth high enough to pass any frequency components
that can be captured at that sample rate.
Between them, the type, range and bandwidth characterize the physical
parameter that a single channel of a data acquisition system will
be monitoring. Those characteristics need to match the DAQ-board
specifications of input type, gain and offset, and sample rate.
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