How much is the voltage shift that you are measuring? Is it a +ve or a -ve voltage shift?
The way it looks right now, you might need a bias or a pull-up resistor to offset this shift.
Shorting of AI SENSE to ground is definitely not a good idea as you would not be able to connect NRSE signals any more. I have attached an image file that illustrates proper signal connection for various types of input sources.
If you examine the interface board schematic (IF PCB.pdf), you will see that the tank pressure shunt resistor, R24, is connected to the PCB ground plane. This ground plane is connected to DAQ ground via pins 19, 23, 24, and 33 on connector J1; these pins correspond to DAQ pins AISENSE, AOGND, DGND, and DGND respectively. As you will observe, these pins are tied together on the PCB. You will also note that the PCB ground is connected to the incoming 24VDC minus terminal.
The tester chassis is grounded via the incoming ground line on the 120VAC. This ground reference is also connected to the minus terminals on the tester's DC power supplies, including the 24VDC supply that powers the tank pressure transducer (4-20mA output). The safety ground lugs on the backs of the power supplies are also tied to this reference. At the moment, the safety ground lug on the back of the PXI chassis is unconnected.
The channel definition for tank pressure shows the mode as NRSE.
The KnowledgeBase article you referenced explains how the various grounds inside the E-Series DAQ are connected to a single reference, the bus ground on the PXI backplane. Is this bus ground internally connected to another ground reference?
Thanks for the help.
Message Edited by Otis [DE] on 05-17-2006 05:01 PM
Thanks for the reminder about "the big picture" and basic troubleshooting techniques. It is difficult to tell whether the problem existed since the system was set up, as this work was done at the vendor's facility. The pressure signal problem was masked by an automatic pressure-regulation setup - any shift in the apparent pressure was nulled out by the regulator. It is only when we put a mechanical, lab-grade gauge on the vacuum tank that the problem revealed itself. By that time the members of the team that developed the tester had been dispersed across the country. We could try to contact them for advice, but they are the ones who designed the system in the first place, if you take my meaning.
Since my last post, we have been running tests to determine the precise conditions that precipitate the shift in the measured pressure signal. By operating the various tester devices manually, we were able to determine that switching a certain 120VAC solenoid has a nearly 100% correlation with the shift. This solenoid is one of four AC units on the tester. Each of these solenoids is supplied with 120VAC power on one lead and a PXI relay card switches the remaining lead to neutral. The suspect coil measures 42kohm and should be more like 800ohm. We are investigating whether faulty wiring or a failed coil, or both, are present. It is interesting to note that the solenoid still operates its valve. This tester is unique and thus we have no other for comparison.
After we replace the solenoid valves and inspect/repair the wiring that controls them, we will perform further tests. While I certainly appreciate NI's help in this matter, I realize that your assistance should be focused on the NI equipment and its operation. That being said, I would like some help on determining the proper connection of the various grounds when using an interface board such as ours. As shown in the PCB schematic (IF PCB.pdf attached to an earlier post in this thread), analog inputs, analog outputs, and digital signals are all handled (and conditioned) by the interface board. Should the grounds (AI SENSE, AI GND, AO GND and DGND) each have their own ground planes on the board? Which ones should be tied together? As you can tell, I am not a EE.
Let me know how replacing that solenoid works out.
Regarding the grounds. AIGND, AOGND, and DGND should be separated when coming into the board just to try and make sure that any ground noise is as system isolated as possible. However, ultimately all three of those ground planes will be referenced to the same PCI ground plane, so the three grounds should all be at the same potential. AISense should not really be tied to ground unless you are using a floating voltage source, then you should connect something called a bias resistor (10k-100kOhms) between AISense and AIGND. This allows electrons to flow, but keeps the signal stable. If you are measuring a grounded source then nothing should be connected to AIGND, period.
I hope this clears up some of the grounding questions.
Thanks, your message does clear some of the fog. You mentioned that all three grounds (AI, AO, D) are tied together internally on the DAQ card. I assume that this ground is connected to a PXI chassis ground in some way. How is this internal PXI ground connected to an external ground reference? Is it through the grounding terminal on the AC power plug? I cannot find this stated explicitly in any of the NI documentation. Perhaps it is so obvious that no mention was deemed necessary. Please forgive my ignorance.
I mention this because the PXI chassis is powered via a UPS. We measured continuity between the grounding terminals on the output of the UPS and the AC power plug that supplies the UPS, so we believe that a path to our tester's chassis ground exists. Would it be preferable to drop the grounding via the AC plug (using one of those 2-to-3 prong adapter plugs) and connect the PXI chassis ground lug directly to our tester chassis ground?
Your AI Ground's connected to your AO Ground.Regards,
Your AO Ground's connected to your D Ground.
Your D Ground's connected to the your AI Ground.
And they're all connected to the backplane.
Your backplane's connected to your power supply.
Your power supply's connected to your earth ground.
Your earth ground's connected to your working place.
So don't connect two earth grounds.