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Below is the wiring setup I plan to use:

enter image description here

Basically the black rectangle in the middle is this data-acquisition board which has 16 diff ended inputs.

The blue rectangle on the left is a six-axis force transducer which has 6 low level differential ended outputs and an analog ground AGND.

The red rectangles represents single-ended transducers. Since the daq board is diff ended, I plan to daisy chain the grounds of the single-ended inputs to the AI GND as in my drawing.

I don't care about the noise level much for the single ended transducers, but for the force transducers I have to use differential ended outputs.

So basically I need to simultaneously scan and log both diff ended and single-ended inputs by this board. AI GND is not connected to the building earth.

So far I tried hooking up all diff ended inputs with one single-ended transducer and didn't get any peculiar results and especially diff ended data is very clean i.e very low noise level. I also noticed if I also connect the transducers shield to AI GND noise gets even less.

In real I have to input five more single-ended floating sources as shown in my diagram.

Is there any problem in this way of wiring scheme?

Edit:

I decided to use all channels with diff input for better noise immunity instead of wiring the signal grounds to AI GND.

To use differential ended for the floating single ended inputs on the right, I'm planning to use the following red circled scheme:

enter image description here

They recommend it for differential data acquisition for floating sources.

Basically I want to create a balanced system for the floating inputs.

My questions are:

To obtain a balanced system the source output impedance of the floating sources should be equal, transmission lines should be shielded twisted pair(instead of a coax cable). These transducers like this one are single ended and are not fully differential. If they were I would not worry about their balance.

1-) Lets say I made a good shielded twisted pair cable for each transducer with BNC connectors to the DAQ in question, but how about the effect of the source impedance of the floating sources to the system's balance? I mean if the floating source's output impedance is 100 Ohm should I add a series 100 Ohm to the ground terminal of it to balance the wires? Do you measure that in practice or is there a rule of thumb?

2 I encountered two schemes for the diff inputs for floating sources. One of them they tie between 10k to 100k resistors between "each" signal lines to AI GND of the DAQ's amplifier as you see in the red encircled config above And I also see they tie this resistor only between the floating signal's low level input(or signal gnd) and AI GND. If I use resistors between or each wire to AI GND it will load the circuit(between 20k to 200k), but if I use one resistor config. then the balance might be an issue. Which one would be better in my case where the signals are DC-like signals?

3 And finally if I use shielded twisted pair cables should the all shields be wired to AI GND of daq device?

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  • \$\begingroup\$ I wish they would give you the input schematics on those things.. Hard to tell really. \$\endgroup\$ – Trevor_G Jan 15 '18 at 18:32
  • \$\begingroup\$ As you see everything is referenced to AI GND nothing is floating wrt to the daq board, but can there still be common mode range problem? Because earth is not involved in the scheme. \$\endgroup\$ – Genzo Jan 15 '18 at 18:33
  • \$\begingroup\$ What are the sensors? Are they powered? \$\endgroup\$ – Trevor_G Jan 15 '18 at 18:38
  • \$\begingroup\$ @Trevor_G The 6 axis diff force transducer system on the left is here: ati-ia.com/app_content/documents/9620-05-DAQ.pdf And on the right single ended input are there are transducers basicaly around 5V DC outputs temeprature, pressure ect. \$\endgroup\$ – Genzo Jan 15 '18 at 18:41
  • \$\begingroup\$ Did you ask another question on a similar subject at the end of last week? Maybe it was somebody else? \$\endgroup\$ – Andy aka Jan 15 '18 at 18:41
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If you tie the shield to ground at the ADC you will unbalance the impedances of each side of the differential input and be more susceptible to common mode noise, and also create a ground loop. You should rely on the ground from the sensor end.

schematic

simulate this circuit – Schematic created using CircuitLab

Some additional power/decoupling capacitors at the sensor would not hurt if the supply cables are lengthy.

By the way, the BNC model may not be the best choice for 10m.

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  • \$\begingroup\$ Is the ground symbol in your diagram negative terminal of the DC power supply of the sensor? \$\endgroup\$ – Genzo Jan 15 '18 at 19:00
  • \$\begingroup\$ @161776 ya whatever the common side of the signal is... \$\endgroup\$ – Trevor_G Jan 15 '18 at 19:01
  • \$\begingroup\$ But I think they are already connected. I mean I remember the negative terminal of the power supply is connected to the GND of the signal output. If Im not remembering wrong. \$\endgroup\$ – Genzo Jan 15 '18 at 19:02
  • \$\begingroup\$ @161776 ya it likely is. The above is a general representation, you should use the signal-ground pin though. \$\endgroup\$ – Trevor_G Jan 15 '18 at 19:03
  • \$\begingroup\$ I think you mentioned this in case they are isolated? Alright I will check again. But would 100nF cap work? \$\endgroup\$ – Genzo Jan 15 '18 at 19:04

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