0
\$\begingroup\$

There is a single ended floating source connected to a data-acquisition device. The system is categorized as single ended earth grounded. Here is the recommended wiring as I use:

enter image description here

Now imagine the source is unbalanced and causing common mode noise and also imagine there is differential noise from the source as well. Would an active filter take care of both in this case?

Below is the details of wiring in my mind so far:

enter image description here (please left click to view the diagram)

By using the active filter as above would the common mode noise due to unbalanced source be cancelled along with the differential noise?

\$\endgroup\$
2
\$\begingroup\$

You start with a single ended signal from the sensor.

The sensor is grounded on one side, that's OK

The filter is for single-ended signals so that is OK as well.

Then you use a COAX cable which can only be used with single ended signals and that is what you do, so OK as well.

Now it gets a bit more tricky if there are (unwanted) signals flowing through the ground connection. The ground connection (COAX) will have some series resistance (all wires do) so any signal flowing there will add to the signal from the sensor (and there you have the reason for going fully differential).

Then you suddenly treat the signal as if it is differential because you feed it into a differential input amplifier. This is not wrong but also not OK.

But here you can probably get away with it since here the differential amplifier circuit will simply see a "wrong" differential signal since the bottom input is grounded. The amplifier will look at the difference between the inputs and that is equal to your signal. So this will work.

But with this you do lose the benefit of differential signals and that is the suppression of (common mode) disturbances. However, you are using a COAX cable which shields against these disturbances.

Connected like this the differential amplifier cannot even filter out the common mode signal because the signal is not differential!

Also if you want to use a proper differential setup then you have to:

  • add a single to differential converter circuit after the filter

  • replace the COAX cable with a 3-conductor cable (2 signals + 1 ground, the ground and shield could be the same).

In a fully differential solution the only currents flowing through the signal cables are the signals themselves. A current through the ground cable is no issue as it will not add to the signal.

\$\endgroup\$
  • \$\begingroup\$ It is not possible to use for me diff. ended signalling for some reason. Im wondering can we say the signal seen by the diff. amplifier is balanced after the active filter? I want to mitigate the cm noise caused by balancing issues \$\endgroup\$ – atmnt Sep 26 '17 at 13:28
  • \$\begingroup\$ Nope, there is absolutely no balancing or anything even quasi differential going on in this circuit. The output of the filter is not symmetric and neither is the COAX cable. So no, not like this. \$\endgroup\$ – Bimpelrekkie Sep 26 '17 at 13:30
  • \$\begingroup\$ Ok coax is not twisted that is one thing. But what if I add 100 Ohm to GND wire of the coax. would that at least make it more balanced? i saw that in some other question. \$\endgroup\$ – atmnt Sep 26 '17 at 13:32
  • \$\begingroup\$ Nope, I do not see how that would work. A true differential signal needs two identical paths. A single core COAX can never conduct a differential signal. Two coaxes can but then you might make a loop and pick signals up. Hence the twisting, it suppresses that picking up of signals. You can stop looking for such "band aid" solutions (like the 100 ohm) because it simply will not work unless done properly. You have to fundamentally do it the proper way or simply not bother and then accept the penalty (of a single ended solution). \$\endgroup\$ – Bimpelrekkie Sep 26 '17 at 13:35
  • \$\begingroup\$ never tried that conversion. signals source is -10V to +10V range. if i can go for diff. conversion in a way, would this work: analog.com/en/analog-dialogue/articles/… \$\endgroup\$ – atmnt Sep 26 '17 at 13:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.