Voltage Read on open input (USB-6251)

[Eyebolt]

I'm using a USB-6251. One of the ways the AI is being used is to check continuity of pins in a device. The theroy is that the device is sending a set voltage out on a pin, and if the measured voltage is greater than 0 then the pin is considered to be connected. However, what I am finding is that if the measured pin is open or not connected to the board then my USB-6251 reads ~6V with a RSE (AI GND to system GND), 10/-10 reading. What would be causing this and how can I correct it?

[Abhinav T]

Hi Eyebolt,

It is generally very tough to predict the open terminal voltage unless it is being driven to a particular value. Having an open ended connection does not garuntee zero volts on the analog input. Here are a two things I would suggest to make your continuity tests:

1) Try using a pull down resistor to AI GND. If you use a resistor of about 100k you should be able to pull the voltage to zero if there is nothing driving that line.

2) Use a precision current source instead of a voltage source to measure the resistance on that path and check continuity that way - this will require a redesign of your circuit and is a little more involved.

Please let me know if any of the two steps above work for you.

[Eyebolt]

Thanks. I wanted to make sure that there wasn't someting selectable or deslectable (pull downs or pull ups) within the DAQ itself before modifying my hardware.

[Eyebolt]

I inserted 100k Pull Down Resistors and that took care of it for me.

Thanks...