Issue 3: Analogue-to-Digital Converter Specifications
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Data acquisition systems capture real-world signals and convert them into a form a computer can understand. Many signals - such as temperature, pressure, strain, flow and speed - will originally be analogue. These need converting to a digital signal before being transferred to the computer. This is done by an analogue-to-digital (A-D) converter.
This article explains the meaning of some of the A-D specification terms that hardware manufacturers use, namely range, resolution, sample and hold acquisition time, throughput, integration time, linearity, offset errors and re-calibration. Next month's issue will cover the different types of A-D converter.
The input (or gain) range refers to the maximum and minimum voltage that will be digitised by the A-D converter. An input may be bipolar, covering a range of -10 mV to +10 mV for example; or unipolar, perhaps covering a range of 0 to 10 volts. Many systems offer a choice of ranges, and you can select the most appropriate using software like Windmill. It's best to choose the smallest range that encompasses your signal, as this optimises the resolution.
The resolution of the A-D converter is the number of steps into which the input range is divided. The resolution is usually expressed as bits (n) and the number of steps is 2n-1 (which equates to 2n values). A converter with 12-bit resolution, for instance, divides the range into 212, or 4096, values. In this case a 0-10 V range will be resolved to 2.5 mV, and a 0-100 mV range will be resolved to 0.025 mV. Although the resolution increases when you narrow the range, there is no point in trying to resolve signals below the noise level of the system: all you will get is unstable readings. Some A-D converters have a choice of resolutions (offering 12-, 13-, 14-, 15- and 16-bit for example). You can choose the most suitable for your application, balancing speed against accuracy.
A sample and hold circuit freezes an otherwise varying analogue voltage at the moment the sample is required. This voltage is held constant whilst the A-D converter digitises it. The acquisition time is the time between releasing the hold state and the sample circuit settling to the new input voltage. Sample and hold circuits are not used with integrating converters.
Throughput is the maximum rate at which the A-D converter can acquire and transfer data values. Roughly speaking, it will be the inverse of the (conversion time + the acquisition time) of the A-D converter. Thus a converter that takes 10 microseconds to acquire and convert will be able to generate about 100000 samples per second.
Throughput may be slowed down, however, by other factors which prevent data transfer at the full rate. For example, signals which are switched (multiplexed) into the converter may need time to settle - especially highly amplified signals or signals from high impedance sources.
Some A-D converters measure the signal over a period of time - performing integration which helps to reduce noise. Note that the throughput of an integrating converter isn't the inverse of the integration time.
An integrating A-D converter averages the input signal over an integration time. In countries with a 50 Hz mains supply, a 20 millisecond integration time will average over a complete mains cycle - helping to reduce mains frequency interference.
See next month's issue for more details of integrating converters.
Ideally an A-D 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 A-D output against input signal.
Offset is where you get a reading other than zero for a zero condition: every reading will be inaccurate by this amount.
Some A-D converters are able to re-calibrate themselves periodically. They measure a reference voltage and compensate for offset and gain drifts. This is useful for long term monitoring since drifts do not accumulate. If the re-calibrations are set too far apart, there may appear to be small discontinuities in the recorded data as the re-calibrations occur. (Drift occurs because of temperature changes, and because component values change over time. Drift is usually only significant if you are trying to measure low-level signals - a few millivolts - over long periods of time or in difficult environmental conditions.)
The A-D converter is only one part of the data acquisition system. It may be preceded, for example, by signal conditioning such as amplification or filtering. Input signals may be multiplexed (passed in turn) to the A-D converter, or there may be one A-D converter for each signal. These factors modify the converter specifications to provide an overall system specification.
More information on A-D Converters is given in Issue 4 of Monitor.
- An electrical connection with the earth. Known as Ground in USA.
- Electromotive force (emf)
- A potential difference which drives a current round a circuit - a voltage.
- Endurance limit
- In fatigue testing, the number of cycles which may be withstood without failure at a particular level of stress.
- Electronic Industries Association.
- A local area network to which you can connect data acquisition devices.
- E-Type Thermocouple
- Chromel-constantan thermocouple with a temperature range of 0 to 800 oC.
- FIFO buffer
- A first in, first out, store. The first value placed in the buffer (queue) is the first value subsequently read.
- Attenuates components of a signal that are undesired: reduces noise errors in a signal.
- Measured in hertz (cycles per second), rate of repetition of changes.
- Frequency Counter
- Counts digital pulses over a defined gate time. A typical gate time is between 0.1 and 10 seconds.
- Front panel
- The front surface of a unit, generally containing switches and indicator lights.
- Amplification of a circuit.
- General Purpose Interface Bus. Also known as IEEE-488 bus. The GPIB standard was designed to connect several instruments to computers for data acquisition and control. Data can be transferred over GPIB at 200000 bytes per second.
- See earth.
- Hertz (Hz)
- Cycles per second unit of frequency.
- Counting system based on 16.
- Human machine interface (hmi)
- Also known as man machine interface. The communication between the computer system and the people who use it.
Do you have a comment or suggestion for this newsletter? Why not email the editor - Jill - at [email protected]
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