DAQ-6008 Analog Input issues, non-zero voltage at pin when disconnected

[catran]

I'm using the NI-DAQ 6008 to output a series of voltages and then reading a sense resistor using the analog input (RSE). I noticed that my analog input is giving me 1.3 V, when I probe it (relative to the device's ground), when completely disconnected. This alters the reading to give me a different measurement of the sense resistor than expected.

Why is my analog input pin non-zero??? Any help would be appreciated. Thanks!

[crossrulz]

It's floating.  It could, in theory, be anything without something to somehow tie it to ground.  If you have it disconnected, don't read from it.

[catran]

That still doesn't explain things though.

The reading across my resistor using a voltmeter is 2.5V. When I make the same reading using the AI pin, it gives me 1.3 V.

[Dennis_Knutson]

So? A dmm and a multiplexed daq card (with a pretty low input impedance) is not at all the same thing. Your reading is expected and you have been given the solution.

[catran]

Explain?

[crossrulz]

---

catran wrote:

The reading across my resistor using a voltmeter is 2.5V. When I make the same reading using the AI pin, it gives me 1.3 V.

---

Is one side of the resistor actually tied to ground?  If not, that is not a fair comparison.  And I thought we were talking about an open to the Analog Input.  Maybe pictures of your schematic would be helpful.

[johnsold · Accepted solution]

The 1.3 V is expected. The USB-6008 and -6009 have the world's strangest input circuit. The input impedance is about 144000 ohms terminated into 1.4 V. See the User Guide and Specifications document.

Lynn