voltage buildup in unconnected AI-112 terminals

[RhesusMonkey]

We are registering a slow voltage build-up in an unconnected fieldpoint channel.  Labview registers a ramp from the channel, even though there is nothing connected to the channel.  If we use a voltmeter on the channel, the ramp resets to zero and then starts climbing again.

If we connect the two terminals together, then the channel reads zero; but as soon as we disconnect, the ramp starts again.

We grounded the ground terminal (32) to a known ground - ramping continued.  We grounded only the live wire (screw terminal 16) to a known ground, and this resets to zero, but then ramping starts when we stop the ground.

Sounds like there is voltage coming down the live terminal and overpowering our instrument.

We are experiencing the same problem in a channel that is connected to an instrument, when that instrument is not generating a significant voltage signal.  The instruments generates 0-10 volts which is directed to a FP channel.  But when the instrument is outputting less than, say 1 volt: then the Labview records fictitious values.

Oddly, we do NOT have this problem with another machine, which is running at the same time.

[Matthew_Williams]

That sounds like a build up of static charge.  Can your application tolerate a 100K resistor across the input terminals?

Have you tried swapping input modules to confirm that it is not just a bad module?

Matt

[Jim_M]

Hi Rhesus,

         What do you mean when you say you are not experiencing this issue on another machine?  Another host computer connected to the same FieldPoint?  Another FieldPoint?  Another instrument?

Message Edited by Jim_M on 06-11-2008 09:44 AM

[RhesusMonkey]

Since writing this post, we have disconnected the analysers from the Fieldpoint module and found that the same drift occurs when we read those channels, even with no instrument or wiring connected to it.  So nothing downstream of the fieldpoint is causing the drift; it is happening in the Fieldpoint module I guess.

Yes, I DO have two identical analysers, both connected to the same fieldpoint module.  One of them demonstrates the drift, the other does not.  And neither demonstrates the drift when the analysers are reading a high value, and therefore outputting more than (say...) 1 volt or so...

[Jim_M]

Hi Rhesus,

           You may want to take a look at  this KnowledgeBase document.  Do you have any other FieldPoint hardware that you can try to see if it fixes your issue when the module is reading less than 1 volt?  You say the readings are strange below 1 volt, what exactly do you see?  Have a great day.