Ok, I am very bad at sketching.
And I don't exactly know how I would make the power supply thingy. Maybe imagine just a few separate wall plug adapters, each one followed by some serious low pass filtering and a low noise linear regulator (I think the DAQ board is supplied that way)
The DAQ Board (don't know the exact make from my mind) already has the differential inputs with the two coax connectors.
All the outer contacts of the coaxial connectors are connected to ground planes on several layers, which are common for all of the connectors (and all of the complete DAQ board).
The cables between the amplifier boards and the DAQ board are between 3 and 18 inches, depending on where in the rack the modules are placed.
Frequency range is DC to a few MHz.
The DAQ board is connected to computer via an USB chip and between the USB chip and the DAQ board there are isolators.
D1 and D2 are differential input amplifiers on the DAQ board.
The amplifier in the amplifier module is just an example. The real circuit is much more complex, but the sketch shows how the ground of the output signal is referenced to where the shield of the input coax connection is entering the amplifier module.
Please excuse the quality of the two coaxial cables. They seem to have been bent around a lot.
The DAQ board is based on this evaluation board design from AD:
https://wiki.analog.com/resources/eval/ad9684-500ebz
There will be more than one AD9684 on the board, but I think four or six channels and amplifier modules would be dealt with the same way as two of them, because the only trivial case would be a single channel.
The galvanic isolation uses technology comparable to the one you linked to, but it is not done at the USB level because there simply is no isolator chip fast enough for the serial transfer of the required amount of data per time.
Therefore the isolation is done at the parallel signal level. If you look at the evaluation kit, the isolators would be located in between the two boards.
As mentioned before, I plan on using separate isolated supplies for all the modules. Maybe it would be wise to connect the grounds of all the modules by some 10k resistors so they do not float away from each other when not connected to the external module.
The external module is something that may change, just like a DUT within a test environment. It's power supply would also be independent from the other modules.
Yes, a few means from one to up to or under ten.
I wonder if it would be good to connect the shields to ground with small capacitors on the otherwise unconnected side (if I would not connect them directly on that one side) in order to reduce reflections if signal parts would be present that are high enough to make transmission line effects work on them.
Yes, the signals are single ended ground referenced.
The sense line is the one on the lower left of the amplifier modules that shows the center conductor of the coaxial connector going to the right to the ground. So it effectively senses the ground where the amplifier also is referenced to.
This would then be connected to one of the VIN- pins of the AD9684.
I don't see two star points for each opamp. each one has a single star point, just where the positive input joins the ground of the amplifier module's input connector and the sense line (center contact of the lower left coaxial connector).
I agree with that, I think I would also omit the 10 k resistors and connect them directly as close as possible at the opamp board side (not only connection the power supplies ground banana connectors). This avaoids a potential different between the individual opamp boards, which would result in an undesired current flow.
I assume it shares a common ground, where all connector-grounds are connected!?
The shields of which cable?
Those attached to the external module has to be connected at both sides, as you need a return path of the current traveling towards the inner conductor.
The connection by a capacitor is common for the shield of an USB connector, but there is a dedicated GND line too. The connection of the USB shield is pretty controversial and if you are performing a web search on this topic you will find different solutions/suggestions also from connector/semiconductor manufacturers (direct attached to GND; connected by a resistor, capacitor or ferrite bead).
Maybe you could update your sketch, this information is not obvious in your sketch.
I assume there will be a ground plane on your opamp board, which will also be connected to your SMA connectors.
One connector is on the right side, and two on the left side of your opamp board. As each of this SMA connector will be attached to the same ground plane you will have two "star" points (by assuming the two left SMA connectors are close to each other).
I see inconsistencies in the specification that make it hard to answer your questions. You are initially talking about "DC to a few MHz" (post #4) and are finally switching to a 500 MSPS ADC.
The ADC has 2x50 ohm differential input equipped with SMA jacks. A real design would use shielded 100 ohm differential pairs and a differential driver amplifier to achieve wideband common mode rejection.
"Quasi-differential" or pseudo-differential signaling makes no sense for high MHz signals, because it can reject common mode signals only in the low frequency range. For the MHz range, promising methods of common mode suppression are strictly symmetrical differential circuits and common mode chokes. Or do you expect common mode noise to be restricted to the low frequency range?
The isolation point is yet rather vague. There are methods to isolate high MSPS and GSPS data streams, really demanding.
See e.g. the information about Cleverscope CS448 design lined in this thread https://www.edaboard.com/showthread.php?388481-Best-isolated-or-differential-scopes-probes
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