How to connect various grounds

[Dave_Thomson]

   I am making a custom printed circuit board for signal distribution and conditioning.  It has three 68 pin connectors on it coming from 2 different NI cards.  The cards are in the same PXI chassis, so are referenced internally to the same ground.  My question is:  what is the best practice for dealing with the numerous grounds?  Each card has several digital ground pins, analog input grounds, analog output grounds, etc.  In addition, the distribution card will have +24V, +/-15V, and +5 V power coming onto it from an external box.  I have to distribute a mix of things to numerous devices.  For example, a flow controller will get +/-15V power, an analog output setpoint, and an analog input readback.  So it's not simply a matter of keeping an analog input ground associated with a few analog input channels and sending that all to a device.
   Some questions:  connect more than one analog input ground pin from the same connector to an analog input ground plane on the card?  Or only one of those pins?  What about two connectors from the same PXI card?  Should I connect together all the analog input ground pins from both cards to the ground plane, or only those from one card?  Or one from each card?  I can try to keep analog input ground, analog output ground, digital ground, and power ground generally segregated to separate regional ground planes, but they will have to be connected at some point(s).  Should they all come together at a single point somewhere on the card?
   Any ideas or advice would be welcome.

Regards,
   Dave T.

[Otis]

If you are dealing with one of our normal Multifunction DAQ cards, then essentially all of the grounds connect internally at some point.

That being said, we do group those channels before they all touch each other so as to reduce noise problems.  So keep the AI, AO, DIO grounds separate while you can, but you can eventually connect all of those to the same ground plane (in the case of the PXI board, this ground plane will be the chassis ground).

So, from all of the options that you gave I would pick this one:
I can try to keep analog input ground, analog output ground, digital ground, and power ground generally segregated to separate regional ground planes, but they will have to be connected at some point(s).

Hope this helps,

[Buechsenschuetz]

I agree with Otis. You should keep the different GND connections separated in your own circuitry. I.e. you should connect DGND to ground of digital circuit supply on your board, analog input GND to GND of signal conditioning circuitry etc.

It is true that all GNDs are finally tied together on the PXI (or PC) chassis. On PC chassis you also have to deal with the fact that power supply GND is connected to mains earth. So you should NEVER connect any peripheral device to earth in order not to create a ground loop.

It is also good practice to use a separate (or galvanic isolated) power supply for analog circutry and connect it to AGND of the host system in one point only.

Concerning EMC, following this method you create a 'floating ground' which is NOT necessarily connected to earth, and thus sensitive to RF radiation. If you have a floating ground system you may need capacitors between system GND and earth to create a RF ground connection which does not affect DC ground connections.

Anyhow, for precision data acquisition proper grounding sometimes is a game of trial and error, I think NI provides information on (analog signal) grounding schemes on their site.