NI 9224 open circuit voltage reading is about 1.47 V

[rseabeck]

Hi everyone, can't seem to find anything about this around the web so I figured I'd make a forum post.

I have a cRIO 9045 and a couple NI 9224 AI voltage modules. They work as you'd expect when hooked up to a voltage source and when you short a channel, but things get weird when it's an open circuit. For some reason the open circuit voltage reading is ~1.47 volts and I've never been able to make any sense of it. We have another cRIO with a few NI 9224 modules, and my coworker told me that he's seen the same thing on that one.

I don't think it has anything to do with the DAQmx code, but here's how the channels are configured just in case:

Additionally, here is 18 different AI voltage inputs on our other cRIO reading about 1.47 volts at open circuit to demonstrate that it's repeatable and consistent.

Anyway, thanks for any suggestions. If we have to live with this then it's not the end of the world, but it would improve our error handling capabilities a lot if we can figure this out.

[ZYOng · Accepted solution]

NI does not specify what voltage a device will read when there is no signal connected. See Incorrect Readings on Unconnected or Open Channels of DAQ Device

[rseabeck]

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@ZYOng wrote:

NI does not specify what voltage a device will read when there is no signal connected. See Incorrect Readings on Unconnected or Open Channels of DAQ Device

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Huh, so I guess the solution really is to just live with it. Thanks for the link!

[NIquist]

This is a common issue with all multiplexed signal systems so I'm surprised your searches came up empty.  It's called "ghosting" and there are ways to deal with it.

How Do I Eliminate Ghosting from My Measurements? - NI

Most important though, is the possibility of actually incorrect readings due to cross-talk when jumping quickly between a large voltage and a small voltage on adjacent channels.  If you scan too quickly, the multiplexor and instrumentation amp will still hold a charge from the large signal and you will read it as an error in the small signal channel.  You can adjust the inter-channel timing to fix it or scan "dummy" channels that are grounded between each real channel.

It's all in here:  Getting Started with NI-DAQmx - NI