II - DCE FEL ČVUT v Praze
II - DCE FEL ČVUT v Praze
II - DCE FEL ČVUT v Praze
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3.1.2 Analog Representation for Measuring Ranges of Analog Inputs<br />
Introduction<br />
How to read the<br />
measured value<br />
tables<br />
Measured value<br />
resolution<br />
I/O Modules with Intrinsically-Safe Signals<br />
C79000-G7076-C152-04<br />
The tables in this section indicate the digitized analog values for the effective<br />
measuring ranges of analog modules.<br />
Tables 3-3 to 3-19 list the digitized analog values for different effective<br />
measuring ranges.<br />
Since the binary representation of analog values is always the same, these<br />
tables contain only a comparison of the measuring ranges with respect to the<br />
relevant units.<br />
Deviating from this, a Sigma-Delta AD-converter is used with the analog<br />
input modules described in the manual. Irrespective of the configurable<br />
integration time, this converter always makes available the maximum<br />
representable 15 Bit +sign. Lower resolution ratings than indicated in the<br />
specifica- tions are due to conversion noise based on the shorter integration<br />
times (2.5, 162 /3, 20 ms). The different integration times change nothing with<br />
regard to numerical representation of the measured values. The number of<br />
stable bits is specified in the technical specifications.<br />
The number of stable bits is the resolution, at which, despite noise, the<br />
”no missing code”-characteristics of the AD-converter are guaranteed.<br />
The bits which are no longer stable at shorter integration times are marked<br />
with ”x” in the following tables.<br />
Table 3-2 Representation of the smallest stable unit of the analog value<br />
Stable bbits ts<br />
Smallest stable unit Analog value<br />
(+ sign) Decimal Hexadecimal High-Byte Low-Byte<br />
9 64 40H Sign 0 0 0 0 0 0 0 0 1 x x x x x x<br />
10 32 20H Sign 0 0 0 0 0 0 0 0 0 1 x x x x x<br />
12 8 8H Sign 0 0 0 0 0 0 0 0 0 0 0 1 x x x<br />
13 4 4H Sign 0 0 0 0 0 0 0 0 0 0 0 0 1 x x<br />
15 1 1H Sign 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1<br />
What can you do<br />
with the<br />
noise-prone bits<br />
SIMATIC S7 Ex Analog Modules<br />
At a constant input voltage, noise causes distribution of the supplied value by<br />
more than 1 digit. In the majority of cases, these ”unsteady” values can be<br />
used as they are. In any case, this is the most effective option when<br />
subsequent processing has integral action characteristics (integrator,<br />
controller, etc.) in any form whatsoever. If this unsteady state is undesirable<br />
(e.g. for display/indication), you can<br />
mask out the ”x” bits<br />
round up to ”stable” bits<br />
filter the successive values<br />
3-3
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