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Maxim > Design Support > Technical Documents > Application Notes > A/D and D/A Conversion/Sampling Circuits > APP 270
Keywords: 4-20mA, 0-10V, +/-10V, 0-5V, +/-5V, multiranging adc, analog to digital, analog digital,
converters, programmable logic controllers, plc, overranging, fault
APPLICATION NOTE 270
Analog-Signal Data Acquisition in Industrial
Automation Systems
Aug 01, 2000
Abstract: Industrial control systems continue to employ standard analog signals for transmitting data
between the process and the control equipment. Robust, 4-to-20mA current-loop signals that are easily
transmitted over several thousand feet, ±5 and ±10V signals are also very common in industrial systems.
This application note showcases Maxim's integrated Data Acquisition System (DAS) solutions. Maxim's
DAS solutions save board space, power, and design time, while requiring minimal external components
to convert standard industrial analog signals.
Despite the availability of the digital field bus in several versions, industrial control systems continue to
employ standard analog signals for transmitting data between the process and the control equipment.
Process transmitters in a chemical plant, for example, convert low-level temperature and pressure
signals into robust, 4-to-20mA current-loop signals that are easily transmitted over several thousand feet.
Speed and position sensors for machine tools and automatic handlers in factory-automation
environments generate unipolar and bipolar voltage signals, which are typically 0V to 5V, 0V to 10V,
±5V, or ±10V. In addition, signals from the popular PT100 temperature-sensing element are often used
directly, without conversion, to a standard range such as 10V or 20mA. As an RTD (resistive thermal
device) made of platinum (Pt), the PT100 exhibits a resistance of 100Ω at 0°C. Its resistance versus
temperature characteristic is linear, and it offers a reasonably high level of output signal (>100mV when
driven with a 1mA current source).
The control function in a process environment is implemented by PLCs (programmable logic controllers),
PCSs (process control systems), or (more recently) by IPCs (industrial personal computers). Because
these devices are digital systems operating with process-specific software, all analog signals must be
converted to digital numbers before a computer can read them.
A/D conversion in a control system is performed by boards or boxes called "analog peripherals." They
connect to the CPU via the system's back-plane bus, or a field bus if mounted remotely (on a machine,
for example). In addition to digital circuits (for communication with the CPU), these peripheral units
include various precision-analog and mixed-signal components. Requirements for a larger number of
channels per board or for smaller packages (to mount on a machine) lead to shortages of space and
power that constitute a major challenge in designing analog peripherals. The following circuits suggest
signal-conditioning techniques and describe an approach for digitizing up to eight channels with a single
chip.
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