NI USB-6218 all ai channels show the same measured voltage despite only one channel being connected

[SachinDon]

Hi all, 

I have connected a load resistor (1 Gohm) across channels ao0 and ai0, and I apply a voltage of 1 V at ao0. The problem is that although channels ai0 reads a voltage around 1V (in NRSE, RSE modes) on the NI MAX software, all other ai's read the same even though they arent connected. Please refer the screenshot above.

[Kevin_Price]

This is a pretty normal symptom when using multiplexed devices, often referred to as "ghosting."  When channels are left floating with no path to ground to bleed off the accumulated charge from the channel you *are* measuring, subsequent channels show (most of) the voltage left behind by it.

A very thorough and classic guide to real-life "field wiring" can be found here.

-Kevin P

[santo_13]

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@SachinDon wrote:

Hi all, 

I have connected a load resistor (1 Gohm) across channels ao0 and ai0, and I apply a voltage of 1 V at ao0. The problem is that although channels ai0 reads a voltage around 1V (in NRSE, RSE modes) on the NI MAX software, all other ai's read the same even though they arent connected. Please refer the screenshot above.

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@Kevin has already pointed you in the right direction.

In short, your signal output impedance must be lower than your AI input impedance and any unused AI channel must be connected to AIGND (or at least not floating)