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Connecting a voltage output to a 1762-IF4 (Rockwell Automation PLC) Analog input card generates unexplained offset

Posted on 2011-10-04
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Last Modified: 2012-05-12
I have a situation where I am connecting a sensing device to the input of an 1762-IF4 non-isolated input card to a PLC. The output of the device is a voltage output, where the output varies from 1 to 5V based on a corresponding input. The output of the device is faily low impedance (2k ohms).  The card input is a fairly high impedance (200k ohms) which is measured at the differential inputs. The offset can reach up to 200mV, but diminishes once the output is above 50% of scale. The offset is still present even when the analog input module is removed from PLC power.

When the device is powered and not connected to th PLC no offset can be observed.
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Question by:farcuri
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Dr. Klahn earned 2000 total points
ID: 36913843
It sounds like there may be an unexpected current leak through the input.  A miniscule current leak of 1 uA through a 200K input impedance would produce an offset of 0.2 V.

Try shunting the PLC input with a 220K resistor.  If a current leak is the problem, this should cause the unwanted voltage offset to fall to half its value, but should not affect the voltage component due to the relatively low impedance (2K) of the device output.

If this is correct, then shunting the PLC input with a moderate resistance of 20K should cut the unwanted offset by 90%, which might be good enough and would save extended investigation to determine the exact cause.
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