DAQ, why initial voltage not measure 0V?

[ssefd]

For measuring voltage through the example of continuous voltage input in a DAQ, 0V is initially found to be insignificant.

Even if it is not connected forever, 3~5V is captured instead of 0V.

How should we deal with these issues?

[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. To pull down the signal to zero, you can use a pull-down resistor.

[Bob_Schor]

Following up on @ZYOng excellent suggestion:

To introduce students to LabVIEW and DAQmx, I did a "beginner's Lab Exercise" to build a touch-sensor using a USB-6001 and a 1 Meg-ohm resistor (I think that was the value I chose -- haven't done this recently).  I showed them how to open MAX, open a Test Panel to their plugged-in 6001, and look at AI0 (I think I did RSE, ±10V).  I had them put the resistor between AI0 and Gnd, and also connect a wire to AI0 that they could touch.

Many of the students were first-year EE majors.  Most of them were surprised by what they saw, then figured out what was going on, and how to write LabVIEW code to turn on a Boolean when (and only when) they touched the wire.

Morals --

• The resistor value can be important
• Proper grounding can be important

Bob Schor