Connecting a voltage output to a 1762-IF4 (Rockwell Automation PLC) Analog input card generates unexplained offset
Posted on 2011-10-04
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.