Hi-Q
Junior Member level 2
Hey,
I have a DAQ board with several analog inputs. The inputs are differential and are connected to the DAQ board by coaxial connectors (in+ shielded with GND and in- shielded with GND).
I have several input amplifier/detector/whatever modules which feed those DAQ inputs, all of them in separate metal cases but put together into a rack. The inputs of the amplifier modules are unbalanced (SMA or 3.5mm rf connectors). The outputs are quasi-differential, meaning that the output signal is actually unbalanced (coaxial connector), but there is a ground reference (sense) signal provided (another coaxial connector), so it seems reasonable to connect these two lines (signal and sense) to the differential input of the DAQ board.
The power supplies of the individual amplifier modules are isolated from each other (at least I plan to do so) in order to avoid ground loops if the inputs of several amplifier modules are connected to a single external module where they then share the same ground (via the external module's case as the rf connectors are obviously all grounded to the module case).
At that point I have a star ground scheme, at least as long as the external module's case is small enough to be seen as the center of a star connection (nice).
Now I have to connect the outputs of the amplifier modules to the inputs of the DAQ board/module.
If I just use two coaxial cables per amplifier/DAQ pair, I will create another star ground point at the DAQ board where the outer conductors of the coaxial connectors are connected.
Unfortunately, something having two star center points is no longer a true star ground scheme. I assume that now I have quite some ground loops, which I wanted to avoid in the first place.
What would be the best way to deal with such a situation?
Should I just connect all the shields (use standard coaxial cables)?
Should I only connect the shields to the connector on one side, and if so, which one (amplifier output or DAQ input) ?
Should I have the shields completely flowing on the other side or should I at least connect the shields of each differential pair on the other side, but not connect it to the outer conductor of the coaxial connectors on that side?
If I would not just connect all shields to ground on both sides, how would I make the one necessary ground connection to the DAQ board?
Would I just connect one of the shields, providing a single ground connection without generating a loop?
Or would it be best to somehow connect the DAQ ground directly to the case of the external module (if this was possible at all) in order to have a true star ground instead of a star with one elongated beam (chain connection of ground from the external module to the DAQ board via one of the aplifier modules)?
Regarding the connections from the amplifier modules to the DAQ board, we are talking about 3 to 18 inches (the DAQ board would be in the same 19" rack as the amplifier modules).
Thanks in advance for all comments.
Regards,
Hi-Q
I have a DAQ board with several analog inputs. The inputs are differential and are connected to the DAQ board by coaxial connectors (in+ shielded with GND and in- shielded with GND).
I have several input amplifier/detector/whatever modules which feed those DAQ inputs, all of them in separate metal cases but put together into a rack. The inputs of the amplifier modules are unbalanced (SMA or 3.5mm rf connectors). The outputs are quasi-differential, meaning that the output signal is actually unbalanced (coaxial connector), but there is a ground reference (sense) signal provided (another coaxial connector), so it seems reasonable to connect these two lines (signal and sense) to the differential input of the DAQ board.
The power supplies of the individual amplifier modules are isolated from each other (at least I plan to do so) in order to avoid ground loops if the inputs of several amplifier modules are connected to a single external module where they then share the same ground (via the external module's case as the rf connectors are obviously all grounded to the module case).
At that point I have a star ground scheme, at least as long as the external module's case is small enough to be seen as the center of a star connection (nice).
Now I have to connect the outputs of the amplifier modules to the inputs of the DAQ board/module.
If I just use two coaxial cables per amplifier/DAQ pair, I will create another star ground point at the DAQ board where the outer conductors of the coaxial connectors are connected.
Unfortunately, something having two star center points is no longer a true star ground scheme. I assume that now I have quite some ground loops, which I wanted to avoid in the first place.
What would be the best way to deal with such a situation?
Should I just connect all the shields (use standard coaxial cables)?
Should I only connect the shields to the connector on one side, and if so, which one (amplifier output or DAQ input) ?
Should I have the shields completely flowing on the other side or should I at least connect the shields of each differential pair on the other side, but not connect it to the outer conductor of the coaxial connectors on that side?
If I would not just connect all shields to ground on both sides, how would I make the one necessary ground connection to the DAQ board?
Would I just connect one of the shields, providing a single ground connection without generating a loop?
Or would it be best to somehow connect the DAQ ground directly to the case of the external module (if this was possible at all) in order to have a true star ground instead of a star with one elongated beam (chain connection of ground from the external module to the DAQ board via one of the aplifier modules)?
Regarding the connections from the amplifier modules to the DAQ board, we are talking about 3 to 18 inches (the DAQ board would be in the same 19" rack as the amplifier modules).
Thanks in advance for all comments.
Regards,
Hi-Q