Can cascading power to FP AI and counters cause signal drift

Question

I am using the following hardware:
FP-1000 > (3) FP-TC-120
|
FP-1001 -> (3)FP-AI-111 -> (2)FP-RLY-420 -> (3)FP-CTR-500
All my AI's are 2-wire 24V loop powered 4-20mA devices (pressure transducers).
Everything is powered by one 50W power supply.
With all sensors at rest, I get 4mA on all channels; when signals are applied to transducers, some channels will drift to 7 or 8mA. Some sensors read only 15-16mA at full span, but work fine on my bench. I am seeing no drop-off in power supply voltage, so I don't think I am over-driving my supply.
Could cascading the power from the network modules to the AI and the counters be causing these problems?
It reminds me a ground loop problem, but all the sensor
s are floating and the only ground is through the AC supply to the Power supply.

Answer 1

Yes, cascading power may be the cause of the problems. FieldPoint IO modules are isolated from the IO stage to the inter-module communication bus. The communication bus ground is linked to the C terminal of the network module. By cascading the power from the V & C terminals of the network module to the V & C terminals of IO modules, the isolation barrier is bypassed and the IO modules no longer have floating commons. Additionally, the RS-232 port of the FP-1000 is not isolated, so it shares it's ground (linked through the C terminal) with the controlling computer. Thus, in your scenario, you have two fixed grounds, the computer and the power supply. Additionally, the input stages of the AI and CTR modules, which are normally floating with respect to the inter-mod
ule communication bus are directly tied to it (and ground) through the power cascading. Thus, as you suspect, you most likely have a ground loop from one or more of the sensors.
Regards,
Aaron