a simple question

[acewong7]

Hi everyone, I have a really really simple question about the DAQ hardware. I create a task to measure a voltage from an anolog input channel that does not connect with anything. So I thought that the measure result should be zero since this channel connected to nothing. This channel is totally "empty". I mean, one circuit lines connect to the positive and negative terminal of this channel. But they didn't connect with any voltage source. However, the result shows there is a negative 8.5 volts on this channel. Can anyone explains why this happened? thx  

[GerdW]

Hi ace,

- You don't say anything about the configuration of the channel (SE, RSE, DIFF?). Using wrong settings might result in wrong measurements...

- You don't say anything about your DAQ hardware. Using multiplexing DAC will result in "floating" measurements due to loading SH (sample-hold) circuitry up...

[acewong7]

Hi GerdW, the only available terminal configuration for my device NI6356 is differential. And according to this tutorial, http://www.ni.com/white-paper/3344/en, figure 5. I need to wire my pin1(AI0+) and pin2(AI0-) to the signal's positive line, and pin3(AI0 GND) to the negative line of the measured signal. Is this right? But I did in this way to connect AI0 to a function genetator, the result is always zero. Where is the problem? Thanks 

[johnsold]

Many DAQ devices have a capacitor as part of the input circuitry.  When nothing is connected, that capacitor can charge to some strange value (which in your case was -8 V).

I did not look up your device but many differential inputs require a resistor from each input to AI GND to provide a path for required DC bias currents.  Check your manual or try connecting a 1E6 ohm resistor from each differential input to AI GND.

Lynn

[acewong7]

Hi Lynn, you said "try connecting a 1E6 ohm resistor from each differential input to AI GND", can you explain how to do this? for every differential input, it has three pins on my device(+,-, GND). How should I connect a resistor to the AI GND pin (from which termial: + or -? )? Thanks

[johnsold]

Two resistors. One from + to GND and one from - to GND.

Lynn