Understanding And Using The Analog Input Module: Accuracy And Repeatability; Accuracy Definitions - Siemens SIMATIC S7-200 System Manual

S7-200 Specifications

Understanding and Using the Analog Input Module: Accuracy and Repeatability

The EM 231 and EM 235 analog input modules are low-cost, high-speed 12 bit
analog input modules. The modules are capable of converting an analog input to
its corresponding digital value in 149 µsec. Conversion of the analog signal input is
performed each time the analog point is accessed by your program. These times
must be added to the basic execution time of the instruction used to access the
analog input.
The EM 231 and EM 235 provide an unprocessed digital value (no linearization or
filtering) that corresponds to the analog voltage or current presented at the
module's input terminals. Since the modules are high-speed modules, they can
follow rapid changes in the analog input signal (including internal and external
noise). Reading-to-reading variations caused by noise for a constant or slowly
changing analog input signal can be minimized by averaging a number of readings.
As the number of readings used in computing the average value increases, a
correspondingly slower response time to changes in the input signal can be
observed.
The specifications for repeatability describe the reading-to-reading variations of the
module for an input signal that is not changing. The repeatability specification
defines the limits within which 99% of the readings will fall. The mean accuracy
specification describes the average value of the error (the difference between the
average value of individual readings and the exact value of the actual analog input
signal). The repeatability is described in Figure A-25 by the bell curve. This figure
shows the 99% repeatability limits, the mean or average value of the individual
readings, and the mean accuracy in a graphical form. Table A-15 gives the
repeatability specifications and the mean accuracy as they relate to each of the
configurable ranges.
Figure A-25
A-48
Average
Value
Repeatability limits
(99% of all readings fall within these limits)

Accuracy Definitions

Signal
Input
Mean (average)
Accuracy
S7-200 Programmable Controller System Manual
A5E00066097-02