Incorrect Readings on Unconnected or Open Channels of DAQ Device

Updated Mar 12, 2019

Reported In

Hardware

  • C Series Voltage Input Module
  • C Series Voltage Input Device
  • PXI Analog Input Module
  • PXI Temperature Input Module
  • Multifunction I/O Device

Issue Details

I have one or more channels on my DAQ device disconnected or not connected to any signal or sensor or the device is disconnected from the computer.
  • Why are all of the channels reading the same value?
  • Why are my channels floating?
  • Why are the open channels reading random values?
  • Why is the open channel reading out of expected range?
  • My open channels look unstable.

Solution

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. See Can I Use Pull-Down Resistors for Analog Inputs? for more details.

Additional Information

There are four fundamental reasons why the voltage across an open channel will change:
  1. Ghosting- The voltage of the previously scanned channel will reflect or ghost onto the open channel. This is caused by the essentially infinite source impedance of the open channel. In other words the impedance between the + and - terminals of the channel is very large. This high impedance dramatically increases the settling time for the channel. For a more detailed description see How Do I Eliminate Ghosting From My Measurements?
  2. Noise and Crosstalk from the Surrounding Environment - Since the channel is open and not being actively driven to any particular voltage it is highly susceptible to noise and other factors from the environment that allow charge to build up on the channel. This can cause the channel to float and often eventually move to one of the rails.
  3. Charge Injection from the Multiplexer - When the DAQ device's multiplexer moves from one channel to the next it can introduce a small charge on each channel. Since the open channel does not have a path for this charge to dissipate the voltage of the channel will be affected. This can also cause the channel to rail.
  4. Internal Bias Resistors - Bias resistors present on some DAQ devices can also pull the voltage of a open channel and cause the voltage to change.

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