Repeated signal on loose end AI using NI-6353 USB DAQ

[Monkey-D-Ali]

Hello,

I'm having trouble with measurments using a NI-6353 USB. I have a product (smart realy) that needs to be tested on its 4 poles, so I have chosen differential mode to connect my 4 poles to 4 selected AI (To AI+ and AI-). The AI- of all 4 inputs is connected to the 0V of the main power supply, which from its side grounded to the test machine earth.

The issue is that using NI MAX, i was testing the measurement to see the exactitude of the values, and by chance the fourth AI was physically disconnected from the DAQ pins, I found out that even if it's disconncted, when I have signal on the previous AI (AI3 disconnected but the wires have a signal from the product and AI2 connected) which doesn't make any sense. I tried disconnecting AI2 and doing the same thing and I found that the signal follows, so AI2 have the signal of AI1 eventhough it's discconnected.

I tried everything I had in mind, like changing the connection point to the ground, rewiring the inputs, changing the physical channels, but I have the same thing everytime.

I'm really out of thoughts. Your help would be much appreciated.

[crossrulz · Accepted solution]

What you are seeing is called ghosting. The DAQ uses a multiplexer to switch between the channels. If it switches to a channel that is disconnected, there is nothing to drain the hold capacitor on the ADC. This results in seeing the signal from the previously sampled signal.

With that said, if your negative channel is just getting ground, you should be using Referenced Single Ended channels instead of Differential. Regardless of which configuration, you need to make sure AI GND is connected to your ground.

[ZYOng]

Incorrect Readings on Unconnected or Open Channels of DAQ Device

NI does not specify what voltage a device will read when there is no signal connected.

Since an open or unconnected channel is not being actively driven to any specific voltage you cannot expect a specific voltage to appear on the channel. To see zero volts on a given channel, there will need to be a zero voltage signal applied across the + and - terminals of the channel, which is the same as grounding the channel.

[Henrik_Volkers]

What type is your smart relay output? 

Usually relay contacts are low impedance when closed and very high impedance when open, 

An easy way would be to provide a pull -up or -down resistor ( providing some mA current depending on the voltage applied)

to check a closed contact.