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Commissioning
Fig. 148: Schematic representation simultaneous sampling with 4 ADC converters
These ADCs rarely run free in time and sample independently but are normal triggered in some way (the
measurement is triggered) to achieve the most desired effect that the n channels sample simultaneously.
This gives the analog input device the property that all (4) measurements are obtained at the same time.
This gives a temporally consistent view of the machine situation and makes measurement evaluations in the
controller very easy. If the ADC are triggered simultaneously by the sync signal, this is called simultaneous
sampling.
A special added value arises when such devices are synchronized externally, e.g. via EtherCAT
DistributedClocks, and then all analog channels of all devices of a plant operate simultaneously: either really
simultaneously without offset among each other or with the same frequency but with constant, known and
thus compensatable offset among each other.
As shown above, this requires extensive electronics with multiple identical structures. For this reason,
parallel analog devices are usually always simultaneously sampling. Free-running or non-triggered, multiple
ADCs are conceivable (and can then no longer be called "simultaneous"), but are rather uncommon.
Multiplex
Simultaneous sampling is often not required for simple automation tasks. This may be because the simplest
analog electronics are to be used for cost reasons, or the control cycle time is relatively slow compared to
the conversion time in the ADC. Then the advantages of the multiplex concept can be used: Instead of 4
ADC only one ADC is installed, for this a channel switch (from the device manufacturer) must be installed,
which switches through the 4 input channels to the ADC quickly one after the other in the µs range. The
switching process is performed by the device itself and is usually not accessible from the outside.
Fig. 149: Schematic representation of multiplex sampling with an ADC converter
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Version: 3.0
EL3681
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