Subject: Inquiry on Using 4-Wire 4-20 mA Sensors with ControlLogix Analog Card Hello Experts, I would like to know if it is permissible to connect a 4-wire 4-20 mA sensor (with an external power supply loop) to the ControlLogix analog card (1756-IF16) using a single-ended current configuration. The user manual mentions only 2-wire sensor connections (see attached document). Thanks for your assistance!
This information is sourced from the RA knowledge base.
Chopin stated: "This information is sourced from the RA Knowledge Base." Thank you, Chopin! This is incredibly helpful for me.
When there is a voltage disparity between the power supply ground of the field device and the negative return of the analog input (AI), it can lead to a phenomenon known as an 'offset' due to ground loops created by differing ground potentials. This offset impacts all analog inputs, not just the one associated with the ground loop issue. This situation frequently occurs with single-ended analog inputs. To resolve this problem, it's essential to implement a signal isolator with a 4-20mA input and output, effectively eliminating the ground loop and ensuring accurate signal transmission.
Danw mentioned that if there is a voltage difference between the ground of the power supply in the field device and the negative return on the Analog Input (AI), it can result in an 'offset' due to a ground loop created by this difference in ground potentials. This offset can impact all AIs, not just the one linked to the ground loop. This issue often arises with single-ended AIs. When this situation occurs, utilizing a signal isolator (with a 4-20mA input/output) is necessary to eliminate the ground loop. Thank you for your insight, Danw! I’ve encountered a similar issue where all instruments connected to the PLC showed stable and accurate readings, but when I connected a specific device, the readings for all instruments became inaccurate. What causes the potential difference between the two negative connections, and what steps can I take to resolve this issue?
Numerous engineering design guidebooks focus on the critical topic of control system grounding. Ultimately, re-engineering plants to reduce ground loops can be excessively time-consuming and expensive for professionals involved in controls and integration. The most effective solution is to implement a signal isolator. This approach not only streamlines operations but also enhances system reliability while minimizing costs.
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