What am I likely to measure with one side of an Analog Input floating?

[AlanKnowler]

We will be using a PXI-6133 Differential Analog Input module (hardware yet to be delivered - so I can't try this 'hands-on' yet).

If I apply 5V to one side of the 6133 differential input channel (either + or -) and leave the other side floating (open circuit) what am I likely to measure?

I need to do this as I wish to test for an open circuit condition on the AnIn inputs.

Thanks!

Alan

[johnsold]

Alan,

I do not know that particular device, but generally floating inputs do exactly that - they float - usually wandering around unpredictably.

If you need to test for an open circuit input condition, a better approach is that used on many thermocouple modules.  Put a pull up resistor on the input.  If the signal connection is lost, the pull up causes a known maximum voltage signal to appear at the output.  The value of the resistor and the voltage to which it will be connected depends on the nature of your desired signal source.

Lynn

[AlanKnowler]

Agreed. That's what would normally be implemented, but in this situation the incorporation of pull up/down resistors is not possible, hence my question.

[johnsold]

Alan,

What is the signal source?  How quickly do you need to respond to its absence?

Lynn

[AlanKnowler]

Thanks for your response, signal source is a fixed 5v DC supply.