FREQUENTLY ASKED QUESTIONS
How do I get my captured signals into my PC?
As you would probably suspect, this isn't as simple as merely connecting wires
from your sensors into your PC. Let's assume that you'll be using a data
acquisition board installed in your PC. At a minimum, you'll still need a
termination panel to which you'll connect your sensor wires, and a ribbon cable
to connect the panel to the data acquisition board.
An important part of many systems is signal conditioning. Signal conditioning converts signals generated by your sensors or transducers into voltages that a data acquisition board can digitize. Data acquisition boards typically accept voltages in a range from ± 10mV up to ± 10V. Getting signals into this range can require amplification, current-to-voltage and/or resistance-to-voltage conversion, and voltage division. Additionally, signal conditioning may provide isolation, which protects your computer and data acquisition equipment from spurious and potentially damaging signals.
An extensive variety of industry standard signal conditioning modules are available from Intelligent Instrumentation to convert virtually any signal of interest. They plug directly into special termination panels specifically designed for these modules. The types of signals you're measuring and the types of transducers you're using will determine what signal conditioning you'll need.
How far can my PC be from my signal source?
As you extend the distance between your PC and your signal source, you increase
the level of noise introduced to the signal you are measuring. The answer to
this question then depends on a large number of factors. Signal source type,
signal level, cable type, noise source type(s), noise intensity, distance
between the cable and the noise source(s), noise frequency, signal frequency
range and required accuracy are just some of the variables to consider.
Noise is most likely to affect low-level voltage signals. It can also be very disruptive to high-speed measurements. As a rule, you can generally extend the distance between your PC and your signal source if (1) you're measuring analog current source signals; (2) you're measuring periodic high-level analog voltage source signals; (3) you're using shielded or twisted-pair cable.
The distance between your sensor or transducer and your termination panel will depend on the sensor you're using and its output. For example, some sensors output a 4-20mA current, allowing you to place it a considerable distance from your termination panel.
Generally, the distance between your PC and your termination panel should not exceed six feet in a typical system. If you need to exceed this distance, you can probably do so without worry if you are measuring PC-level voltages at a slow sampling rate in a relatively non-noisy environment. Otherwise, you may want to consider using a two-wire transmitter/receiver configuration. In this set-up, a two-wire transmitter at the signal source end converts your signal to a current. You can then transmit this current across a noisy environment without contaminating your data. The signal is transmitted to a receiver located near your DAS, where it is converted back to a voltage. Be aware that a DC power supply is required to transmit the signal, so the two-wire transmitter/receiver configuration will add additional cost to your system.
Should I use a 12- or 16-bit board? Is
16-bit resolution worth the cost?
The resolution of the analog-to-digital converter (ADC) on your data acquisition
hardware determines how closely the digitized waveform your PC sees resembles
the analog waveform that's going into your PC. Generally, the most common
resolution used on data acquisition boards is 12 bits, or one part in 4,096.
This is more than sufficient for most applications. For applications which
require more information from the measured signal, 16-bit (one part in 65,536)
boards usually provide the necessary resolution. Audio applications needing wide
dynamic range measurements represent typical 16-bit applications.
An important factor when considering 16-bit resolution for your application is the degradation of your measured signal caused by your transducer or signal conditioner. Most common industry-standard signal conditioning modules are only linear to 10 bits, and many transducers are only accurate to 12 or fewer bits. For example, most thermocouples are only rated to 0.1% accuracy, which is equivalent to about 10-bit accuracy. If your measured signal is only accurate to 12 or fewer bits, then using a 16-bit ADC is needless overdesign.
For low-speed applications, you can statistically increase the accuracy of your measurements by taking a number of measurements of the desired signal and then use software to perform averaging.
Do I need to use DMA for my application?
Direct memory access, or DMA, is a popular technique used to enhance system
throughput on an ISA or EISA computer. DMA allows you to store samples directly
into memory without any intervention from the CPU, thus freeing the PC to
perform other tasks. During the process, the data acquisition board communicates
directly with the bus using a special set of hardware handshake signals.
DMA is ideal for a number of data acquisition tasks. Any time you're capturing a stream of data, either to PC memory or to disk, DMA will ensure a more accurate, gap-free transfer. DMA is also necessary for providing accurate hardware timing when performing periodic sampling. This accurate timebase is required for many digital signal processing applications. However, if your application only requires low-speed sampling and/or control, you probably don't need DMA. Most environmental monitoring applications fall into this category.
Incorporating DMA in your application can be tricky if you're writing your own software to control your application. However, if you're using a data acquisition software package that supports DMA, then implementing it is an easy and recommended technique.
Do I need isolation? Or is protection
sufficient?
One important aspect to consider in almost any data acquisition application is
protecting your PC from dangerous voltages which could come in contact with your
signal path. Overvoltage protection up to ± 35V is built in to most data
acquisition boards. Figure 2 (coming soon) illustrates typical overvoltage
protection circuits. This type of solution is probably sufficient for systems
that don't involve power measurements or placing sensors on machinery that could
incur high-voltage transients. However, it won't protect you from some of the
higher process voltages that can exist in control rooms.
You can easily add isolation to your system in the form of isolated signal conditioning modules. Analog modules allow you to safely connect common-mode voltages up to 1500Vrms and up to 240V differential to your system. Digital modules protect your equipment from up to 4000V input to output. As you would expect, though, isolation will add some cost to your system.
Another reason to consider isolation for your system is that isolation also lets you accurately measure signals when a floating ground exists. This relatively common condition exists when there is not a common ground between your PC and your signal source. For example, if you're taking measurements from equipment that is powered off of a different power circuit, there is probably a substantial potential between the equipment and your PC.
How do I reduce noise on analog input signal
measurements?
Anytime you're measuring a signal, it will contain unwanted noise. Whether this
noise is troublesome depends on the signal-to-noise ratio and the specific
application. If the transducer you're using acts as a current source, your
system is inherently less sensitive to external noise sources than if you're
using a voltage-driven device. Otherwise, the noise voltage signal is added
directly to the source voltage signal without attenuation.
Most noise problems can be solved by applying proper grounding and shielding principles. A common mistake is to assume that your ground line and return line are the same. The ground line connects your equipment to earth and does not normally carry current. The return line, on the other hand, is an active part of your circuit and should not have more than one connection to your ground line. Further, the return path should have as low an impedance as possible to reduce the effects of EMI.
Using the proper wiring to interconnect your system is also helpful. Balanced cables are essential to rejecting ground difference potential. Additionally, shielded or twisted-pair wire is also suggested when measuring low-level signals. With twisted-pair cable, EMI is induced equally into both wires, effectively canceling out any error in your measured signal.
You may also have to use filtering to remove noise from your signal. AC power lines and radio frequencies may cause noise in your signal that cannot be removed by grounding and shielding techniques.
Filtering can be implemented in two different ways. A hardware filter consisting of a combination of resistors, capacitors, inductors and amplifiers can remove unwanted noise from your signal prior to the A/D conversion. Figure 3 (coming soon) illustrates examples of some passive one- and two-pass filters. Filtering can also be accomplished with software after the conversion, using digital signal processing to perform averaging. By averaging a series of incoming data points, the signal-to-noise ratio is effectively increased.
Will my DAS have an intrusive effect on the
rest of my system? Is it OK to leave the DAS connected to my system when powered
down?
By its very nature, DAS will have an intrusive effect on the system you're
monitoring or analyzing. In order to measure a signal, the DAS must add a load
to the system. When the DAS is powered down, it still provides a path to ground.
In order to determine if you can safely leave your DAS connected after you've
powered it down, you need to consider the input impedance and capacitive loading
of your data acquisition board (or signal conditioning, if used) during both
power on and power off conditions. The input impedance will be significantly
lower on most data acquisition boards during power off due to built-in
overvoltage protection. Thus, your DAS is likely to have a more intrusive effect
on your system when powered down than when you're actually monitoring the
system. The specific information should be in the specification sheet provided
by your DAS vendor, or it is readily available by calling the vendor.
Why do I need to linearize my thermocouple
signals? How should I do it?
Thermocouples are by far the most common devices used to measure temperature.
Rugged and inexpensive, they are ideal for applications in both industry and
science. Consisting of two wires of dissimilar metals joined together, the
thermocouple produces a voltage dependent on the temperature of the metals (the
Seebeck effect). Different types of thermocouples are available for different
temperature ranges. However, one problem with thermocouples is that the voltage
they produce is not linear with respect to temperature. This is why thermocouple
measurements need to be linearized.
Fortunately, this is easily done through either hardware signal conditioning or software. Signal conditioning modules that perform linearization in addition to isolation and cold-junction compensation are readily available. Also you can use software to perform the linearization. The method you choose is completely arbitrary. It is generally easier to do with hardware signal conditioning; however, software linearization is less expensive and more accurate.
What are the pros and cons of using a visual
software package instead of writing my own software using a high-level language?
Turnkey application packages that perform a specific task, such as strip-chart
recording or oscilloscope simulation, are widely available. Also available are a
number of software packages which let you design your own custom applications
using graphical tools. Most of these packages make it easy to design your own
unique run-time and user interface screens, such as the one created by
Intelligent Instrumentation’s Visual Designer in Figure 4 (coming soon). These
packages are much more flexible than turnkey packages, making them suitable for
a much broader variety of projects.
If you are an experienced programmer, there are some advantages to writing your own code to control your application. This approach will make your program smaller and more specialized, incorporating only the features you need for your application. Program control will probably also be faster, an potentially important consideration if "real-time" data is critical.
However, if your primary concern is cost, investing a little extra for graphical software tools will be worth it in the long run. No matter what your level of programming expertise, using a visual software package, such as Visual Designer, will dramatically reduce your development time and virtually eliminate the sometimes lengthy debugging process. What's more, these packages make it much easier to document, maintain and modify your system. Many vendors will provide free demo disks so you can see if the software will help you; some vendors even provide free evaluation copies, so you can actually design your application before you purchase the software. Intelligent Instrumentation offers both for Visual Designer (an Evaluation Version and a Demo). Also, if you have unique requirements, most vendors provide a toolkit so you can add your own custom functions to their software, such as Visual Designer’s Custom Block ToolKit.