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Strong variations in the analog input modules

QUESTION:
Why do I receive strongly variating values when using a analog input module?

ANSWER:

You are possibly using a self-powered sensor or two separate power supplies for the CPU plus input module and the sensor. The mass of the two power supplies are not connected to each other. This results in a high and fluctuating common mode voltage which affects the values of the analog input.
Another cause might be a too long or badly insulated connection on the input of the analog module.
Remedy:

Connect the mass M of the analog module with the corresponding mass of the sensor to compensate for these variations.
Attention:
Please make sure beforehand, that this is the only connection between the two power supplies. If another connection exists already, an unwanted compensatory current might occur when adding a mass connection!

Background:

The analog input modules are not potential seperated.
The common mode voltage may not be greater than 12V.
Common mode rejection is 40dB for 60Hz.
Utilze the analog input filter:
In Micro/Win "View > System data module> Tab: Analog input filter".
Choose the analog inputs to be filtered.
Choose the "Number of queries" and the "Dead Time".
The field "Number of queries" contains the number of queries with which the the current average value of several output values is calculated. The value is calculated from the last n values, where n is the value in "Number of queries".
The dead time identifies the maximum of the allowed deviation from the average value. (Further information can be found in entry-ID 8574774)

Keywords:
Test value recording, signal recession

Entry ID:2415890 Date:01/29/2003