NI USB DAQ card ghost readings

[Thelastwil]

Hello,

I have a question concerning the behavior of the NI USB-6218 DAQ card.  Right now I am using Labview to take multiple current readings from different AI channels.  I am using 250 ohm resistors for each channel just as the instructions say to do for current readings.  I had an incident where one end of a resistor was disconnected at the port input.  I expected to see the reading go to zero, but instead, it began to piggback off the readings of another channel giving me a sort of "ghost reading" because there was essentiall no current going through.  The disconnected channel reading was displaying the same current reading of the connected channel even as the values went up or down.  Can anybody explain why the DAQ card would do this? and is there anyway to prevent this from happening?

Thanks.

[crossrulz · Accepted solution]

A DAQ uses a multiplexer to route a signal to the ADC.  Due to having only 1 ADC, you will get this affect if the capacitance has no bleed path.  There is no way to avoid this if you are disconnecting on the DAQ side, leaving the input channel open.

You could try adding in some amplifiers specifically made for current shunts.  I have used TSC103IPT with success.  This amplifier gives you a single ended output.  But I am not sure what will happen with these if you disconnect one side of the resistor from the amplifier.

[Thelastwil]

Okay that makes sense.  Thank you for your response and your suggestion!