differential inputs with shared negative channels?

[Aaron T.]

Hello,

I am planning a data acquisition system that will acquire readings from 40 DC output displacement probes (Solartron DG DC-DC).  Using a NI PCI-6225 with two SCB-68 connector blocks I can acquire 40 differential inputs.  However, because the probe wiring needs to pass through a connector I am wondering how the number of wires required to connect the probes to the SCB-68 can be minimized.

Is it acceptable to connect the +signal from each probe to a unique positve analog input channel on the SCB-68 and then connect all the -signal outputs from the probes to a single negative analog input channel on the SCB-68?  (Please see the attached file for an illustration of this setup.  It only shows four probes instead of 40.)  The negative analog input channels would then be jumpered together throughout the SCB-68.  It seems as though the instrumentation amplifier would still read a differential signal between pos and neg inputs even though the neg inputs of all channels would be shared.  But, I am not sure if this would provide an advantage over configuring the hardware for NRSE inputs.

Thank you,

- Aaron

[danieleaston]

I'm not sure if this helps but I read: https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider... today about wiring set ups and grounding and found it fairly interesting.

[Jennifer O]

Hi Aaron,

Daniel has provided a link to an excellent document.  The diagrams will help to illustrate how connecting the negative terminals may affect your system.

One important factor to consider is whether your signals are floating or ground referenced.  If the signals are floating, tying them together will bring them to the same potential at the negative pin. If the signals are ground referenced you must ensure that the negative pins are at the same potential to avoid ground loops. Read Avoiding Ground Loops for more information.

Regards,

Jennifer O.
Applications Engineer
National Instruments

[Aaron T.]

Thanks for the replies.  The analog signals I am acquiring from the probes are ground referenced signals and all probes are supplied by the same voltage source.  If the negative signal from the probe is referenced to ground, and the ground is shared amongst all channels... how is it different to have all the negative channels from the probes shared (connected together) than to have grounds for all the sources shared?

[Jennifer O]

If you abstract the scenario and consider each connection point a number (i.e. a voltage).  When your signals are ground referenced, the connection points are fixed numbers.  If your signal is floating it is like a "wildcard" the connection point will adopt the number (or voltage) of whatever it is connected to. 

Scenario 1: Your sensors are floating until they get connected to a power source.  Connecting two floating sensors to the same power source will simply make both sensors adopt the reference of the power supply.  There is no possibility of two different voltages getting connected to the same point.

Scenario 2: Both of your sensors are already connected to a grounded source.  Therefore unless the sensor outputs are floating, both sensors have a set reference.  So long as the sensor output references sit at the same potential (which would generally occur if you are providing the same source) there will be no problem.  However if for some reason the references are at different potentials, then connecting them together creates a ground loop. 

Hope this helps,

Jennifer O.