NIDAQ connecting custom cable

[Mozzi]

For some time, I used the SCB-68 connection box to aquire signals from the device I am bulding.  The device has 4 outputs, single ended, which are in reference to a single ground.  The SCB-68 worked great, but I wanted a more compact solution, so I got one of those 68-pin connectors, slapped it on a PC-board with the correct pinout, and plugged the 68-pin cable from the PCMCIA DAQ to my device.  The result, well.. here's how it goes:

I have channel 1 (AICH0), 2, 3, and 4 on the DAQ, connected to channel 1, 2 etc. on my device.  The negative terminals for these channels (yes, I know they are single ended, but what the heck?) are connected to the ground on my board, as well as to AIGND on the DAQ (again through one of the pins on the 68-pin connector). AISENSE is also connected to the ground plane. 

When only one signal is connected, and all the others are grounded, everything works exactly as expected.

When more than one signal is connected, all the signals superimpose on each other.  I know this is a DAQ issue, because the moment I plug in the ground of my scope probe (not even the signal probe, just the ground) to the ground plane of the PCB, all the signals clean up and act exactly as expected.  Am I missing a shunt resistor somewhere, or is there a setting in the software I need to change?  I've tried both single and differential input modes.  Just to reiterate--the ground on the board is connected to the ground of the PCMCIA card, and when this ground is NOT connected to earth ground via a scope probe, all the signals add to each other and become very noisy.  When this ground is connected to earth ground (or even a really long BNC cable suffices)--then everything is clean.  I've checked coupling capacitors, etc. for leaks--I'm sure this is a DAQ configuration issue, probably I'm missing some passives somewhere.

Thanks!

Mozzi

[Jarrod_B.]

Hello Mozzi,

The fact that when you connect the probe ground to the board, it works, makes it quite obviously a grounding issue.  Take a look at the tutorial Field Wiring and Noise Considerations for Analog Signals.  Pay particular attention to the Table I, which summarizes this different types of terminal configurations.  Your signals may be improperly routed to the DAQ board.  You might want to "Ohm out" each of your traces to make sure you have good connections

On the other hand, the measurement results you've described make it sound like a phenomenon known as ghosting.  Take a look at the KB How Do I Eliminate Ghosting From My Measurements? and the tutorials linked to from it.  This may also provide some insight into the problem.  If it is ghosting causing the problem, you should be able to read from each channel individually and get the correct signal.  Then if you were to scan through all the channels in succession, you would see the ghosting return. 

I would first consider the possible ground issues that might be involved.  I don't think it has anything to do with ghosting, but I just figured I would mention it.  Unfortunately, we will not be able to directly support your custom interface board, so other than the suggestions I've made, there's not much else that I can do to help.  Hopefully the information I've provided will be helpful.

Best regards,