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Entry type: FAQ Entry ID: 26116994, Entry date: 07/28/2014
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What are the operational limits and the basic errors of analog inputs and outputs?

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The most important values for determining the accuracy of analog modules are explained.

Configuration Notes
Accuracy is an important parameter for analog inputs and outputs. It is used to characterize the total error of measured value acquisition. Below are explanations of the most important values for determining the accuracy of analog modules. Operational limits and the basic errors
According to the EN61131 standard, the basic error limit at 25 °C and the operational limit calculated over the entire permitted temperature range for the module must be specified. In practice, the operational limit has priority for selecting and evaluating a module. The "worst case" configuration ensures that the module does not exceed the specified error value for the entire permitted operating range (typical for SM331 is a range of 0 °C to 60 °C). The typical measuring accuracy of the modules is usually better.

Example

  • Measuring range of the analog module: 0 to 10 V
  • Operational limit for the entire temperature range: ±0,5 %
  • Calculation: 10 V · 0.5/100 = 50 mV

This means that each voltage in the entire input range is acquired with a maximum measuring error of ±50 mV. A voltage of 3.50V to be measured can therefore be displayed with a value of between 3.45V and 3.55V. Independent of how the accuracy is specified or according to which formula it is calculated, it always refers to the end value of the measuring range. Even with bipolar measuring ranges, the error specifications always refer to the end value of the measuring range and not to the measuring range span. Thus, with an operational limit of 0.5 % the error voltage is ±50 mV also for the range of ±10 V. The same applies for temperatures.

Example

  • PT100 Standard (measuring range -200°C to 850°C)
  • Operational limit for the entire temperature range: ±0.7% (the specification of the operating error takes into account the complete measuring range, which means that the final temperature value is used for the calculation).
  • Calculation: 850°C · 0.7/100 = 5.95°C
This means that each temperature in the entire input range is acquired with a maximum measuring error of ±5.95 °C. A temperature of 200 °C to be measured can therefore be displayed with a value of between 194.05°C and 205.95°C.

Temperature error
The temperature error is already included in the operational limit (for the entire temperature range). Nevertheless it is needed to determine the operational limit for a specific temperature.

Example

  • Basic accuracy at 25 °C = ±0.1 %

  • Temperature error = ±0.0112 %/K

  • Max. permissible operating temperature = 60 °C

  • Operational limit = Basic accuracy (at 25 °C) + Temperature error · ΔT [ΔT the temperature difference between 25 °C and the maximum operating temperature of the module]

  • 0.1% + 0.0112 %/K × 35 = 0.492% - the operational limit at 60 °C in this case is 0.492%. This case is also the calculated worst case; it is a lot less in a typical case.

Linearity error
The linearity error designated the deviation of the real A/D or D/A conversion from the ideal transfer function. The errors specified in this point are already included in the operational limit. This error is given in the technical data, because this error permits you to qualify the properties of the controls configured. This error also refers to the input area and designates the integral non-linearity of a conversion.

Example

  • Measuring range of the analog module: -10 to 10 V
  • Linearity error ±0.01 %
  • Calculation: 10 V × 0.01 % = ±1 mV - this gives an error of ±1 mV.


Fig. 01

Connection between resolution and accuracy
There must be a specific resolution in order to achieve a specific accuracy (operational limit).

Example

  • Measuring range of the analog module: 0 to 10 V
  • With a resolution of 8 bits, a total of 256 values can be displayed, which is 39 mV or 0.4% referred to the end value of the scale (in the overflow range).
  • With a resolution of 14 bits, a total of 16384 values can be displayed, which is 0.72 mV or 0.007% referred to the end value of the scale (in the overflow range).

The percentage values determined in this way are simultaneously the theoretical "best case" values for the operational limit.

With reference to the example above (measuring range of the analog module: 0 V to 10 V; operational limit for the entire temperature range: ±0.5 %; calculation: 10 V · 0.5/100 = 50 mV; 14-bit resolution), for the operational limit this means that the minimum measuring error is ±0.007% (±0.72mV) and the maximum measuring error ±0.5 %(±50mV).

Note
More information about this is available in these manuals:


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