Issue 4: How Analogue Signals are Handled by Data Acquisition Equipment
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When connected to a data acquisition system, an infinitely variable signal is known as an analogue input. This is generally a voltage signal. Thermocouples, strain gauge bridge circuits and gas concentration probes, for example, all produce an analogue voltage. Alternatively the signal may be a milliamp current. In this case the data acquisition hardware will convert the current to a voltage before accepting it. The system then conditions the voltage (amplifies it to a level suitable for the computer for instance) and digitises it with an analogue-to-digital converter. The sampled signal is now in a form the computer can understand and process.
Several different types of analogue-to-digital converter exist, employing different methods to arrive at a digital representation of the voltage signal. They each have advantages and disadvantages.
Reducing Noise: Integrating Converters
When the signal to be measured fluctuates slowly, as in the case of a temperature monitored by a thermocouple, an integrating converter is best. By averaging the signal the converter helps reduce unwanted signal contamination (noise).
Dual Slope Integrating Converter
There are different types of integrating converter - a Dual Slope Integrating Converter allows the input signal to charge a capacitor for a fixed period. The capacitor is then allowed to discharge at a fixed rate and the time to fully discharge the capacitor is measured. This time is a measure of the integrated input voltage.
Charge Balancing Converter
In a Charge Balancing Converter the input signal again charges a capacitor for a fixed period, but in this case the capacitor is simultaneously discharged in units of charge packets. (Meaning that if the capacitor is charged to more than the packet size it will release a packet, if not a packet cannot be released.) This creates a pulse train. By counting the pulses coming out of the capacitor, the system determines the input voltage. (A charge balancing converter is also known as a voltage to frequency converter.)
You can often use software like Windmill to choose the integration time. For countries with a 50 Hz mains supply, for example, choosing an integration time of 20 milliseconds will average over a complete cycle and thus reduce mains frequency interference.
Faster Measurement - Successive Approximation and Flash Converters
Successive Approximation Converter
When you need faster measurements, a "Successive Approximation Converter" is a good choice. This first compares the signal with a reference voltage that is half the converter's input range (5 volts on a 0-10 volt range for example). If the signal is above this level, the converter adds a quarter and compares again (against 7.5 volts in our example). This continues, each step getting nearer the actual voltage. The number of steps determines the resolution of the converter - 12 steps gives 12-bit resolution.
Faster still is a Flash Converter. Like the successive approximation converter it works by comparing the input signal with a reference voltage. A flash converter though, has as many comparators as there are steps in the comparison. So an 8-bit converter has 2 to the power 8, or 256, comparators. Although extremely fast (taking millions of readings a second) a flash converter is more expensive and generally has lower resolution than a successive approximation converter.
A fifth type of converter is a "Sigma-Delta Converter". This digitises the signal with very low resolution (1-bit) and a very high sampling rate (MHz). By over-sampling, and using digital filters, the converter can increase the resolution to as many as 20 or more bits. Sigma-delta converters are especially useful for high resolution conversion of low-frequency signals as well as low- distortion conversion of signals containing audio frequencies. It has good linearity and high accuracy.
An A-D converter (also known as A/D or ADC) is an integral part of any data acquisition system. We've discussed five of the more popular types. Which you choose depends on the speed and accuracy you require. You should also consider the smallest change in voltage you want to detect. This is determined by the range and resolution of the A-D converter - discussed more fully in our previous newsletter.
For more information on A-D Converters see Issue 3 of this newsletter.
(For letters A-H please see our web site Glossary.)
- Symbol for electric current.
- Input/Output. A data acquisition system monitors signals through its inputs, and sends control signals through its outputs.
Institute of Electrical and Electronics Engineers. An American Society that, amongst other things, establishes international standards in the computing, electronic and telecommunications
Also known as GPIB. A communications method used to connect several instruments to a computer for data acquisition and control. Data can be transferred at 200000 bytes per second.
- Interface Management Language.
- Data entering a device from the environment. A signal being monitored by a data acquisition system.
- Any item of electrical or electronic equipment which is designed to carry out a specific function or set of functions. For example an electronic balance, a gas analyser or a chromatograph.
- A positive or negative whole number, or 0.
- A shared boundary. It might be a piece of hardware used between two pieces of equipment, or a software display communicating between the computer system and the people who use it.
- Software enabling a computer to run programs statement by statement.
- An external signal causing the execution of a program to be suspended.
- Integration Time
- The time over which an integrating A-D converter averages the input signal. If chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference.
- A dc-ac converter.
International Organization for Standardization, which is made up of national members. A member is the "most representative of standardisation in its country". For example BSI (British
Standards Institute), DIN (Deutsches Institut für Normung) and ANSI (American National Standards Institute).
- Two circuits are isolated when there is no direct electrical connection between them.
- Isolation to Earth or System
- A high transient voltage at one input may damage not only the input circuit, but the rest of the data acquisition hardware, and, by propagating through the signal conditioning and A-D circuits, eventually damage the computer system as well. You can prevent this type of damage by isolating the input from the earth of the data acquisition and computer hardware.
- Isolation Between Inputs
- A transient at an input can also propagate to other equipment connected to that input. This is prevented by providing isolation between inputs.
- J-Type Thermocouple
- Iron-constantan thermocouple with a temperature range of 0 to 750 oC.
- A unit of stored data. 1K = 2 to the power 10 = 1024. Also stands for a degree on the Kelvin temperature scale.
- A symbol for a thousand, from kilo.
- K-Type Thermocouple
- Chromel-Alumel thermocouple with a temperature range of -200 to 1200 oC.
- Local area network. A data communication system connecting devices in the same vicinity. Data is transferred without the use of public communications. Examples of LANs are ethernet, token ring and Modbus.
- Last in first out. Describes a stack method of data storage.
- Laboratory information management system.
- Ideally an A-D or D-A converter converts the input or output range into equal steps. In practice the steps are not exactly equal. Linearity, or non-linearity, is a measure of how close the steps approach equality.
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