Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.1 Overview
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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Analog-to-digital conversion
A CPU processes information exclusively in digital format. The analog value is therefore
converted into a bit pattern. Conversion is done using an integrated ADC (analog-to-digital
converter) in the analog input module. For the CPU, this conversion always returns a 16-bit
word for SIMATIC products. The ADC used digitalizes the analog signal and approximates
its value with a stepped curve. Resolution and conversion speed are the most important
ADC parameters.
①
Analog value
②
Digital value
Figure 2-2 Approximation of an analog graph with low- and high-resolution stepped curves
Digital-to-analog conversion
Once the CPU has processed the digital signal, an integrated DAC (digital-to-analog
converter) in the analog output module converts the output signal to an analog current or
voltage value. The resulting value of the output signal corresponds to the output value used
to control the analog actuators. Such actuators include, for example, small servo drives or
proportional valves. You will find more information on wiring actuators in Wiring
loads/actuators (Page 103).