Unexpected Voltages, Floating or Crosstalk in Analog Input Channels

Updated Nov 9, 2018

Reported In

Hardware

  • PXI Analog Input Module
  • C Series Universal Analog Input Module
  • Voltage Input Module for SCXI
  • NI-9207
  • NI-9201

Driver

  • NI-DAQmx

Issue Details

I see crosstalk, floating, random spikes, fluctuations, excessive noise, or some other kind of unexpected voltage on my analog input channel. What are some possible reasons for this behavior, and how do I troubleshoot?

Solution

There are a number of reasons that you may be seeing unexpected voltages or crosstalk on an analog channel. Some of the more common reasons are:  
  • High Source Impedance

Impedance is a measure of how circuitry impedes current in a system. It is a combination of resistance, capacitance, and inductance. In an ideal system, an ADC would have infinite impedance. In reality, this isn’t the case. They simply have a very high impedance.

Sometimes though, the source that the ADC is measuring can have a high impedance as well. This high source impedance can cause a noticeable drop in the measured voltage of the ADC. This can lead to inaccurate measurements.

More information on this is available in the Input Impedance section of Specifications Explained: C Series.
 

  • Ghosting

If you are experiencing an unexpected voltage in your digital channel measurement or your unexpected measurements go away when you are only sampling from one channel, this may be due to ghosting between signals. Ghosting can be caused when using a high sample rate on multiplexed devices. See How Do I Eliminate Ghosting from My Measurements? for more information.

 
  • Unconnected or Open Channels
Similar behavior to high source impedance occurs if unconnected channels are included in your scan list. Another common behavior of an unconnected channel is slow drifting or floating to one of the rails caused by charge injection.
 

For more detailed information on troubleshooting and resolving unconnected channel issues see Incorrect Readings on Unconnected or Open Channels. Additional issues can occur when scanning open SCXI channels as explained in Why Do I Experience Incorrect Reading When Sampling Open SCXI Channels?.
 

  • Improper Grounding
If the signal to be measured is not in reference to the same ground plane as the DAQ device, there is the potential for ground loops. Ground loops can cause offsets and error in measurements.
 

For information on troubleshooting grounding issues see Field Wiring and Noise Considerations for Analog Signals.
 

  • Noise and Crosstalk
Measured signals will almost inevitably include some amount of noise or unwanted signal from the surrounding environment (crosstalk). Proper shielding and connections can reduce the effect of crosstalk between channels and other noise from the environment. The distance of wiring connecting the module to the sensor will also play a factor in how much noise is picked up. 
 

For information on resolving noise and crosstalk issues see Field Wiring and Noise Considerations for Analog Signals.

 

  • Impedance Matching
Another way that impedance can affect a system is transmission line impedance. When two systems are connected, if one system’s impedance is much higher or lower than the other, this can lead to signal reflection.

For example, if multiplexed systems are driven by sources with high source impedance compared to the input impedance of the NI card, you may see voltages from one scanned channel reflected on another scanned channel. In order to eliminate these reflections, the source impedance and input impedance should be matched.

More information on troubleshooting mismatched impedance issues is covered on Impedance and Impedance Matching.
 
  • Overvoltage on Connected Channels
Any overvoltage on any of the channels, whether you are scanning it or not, can lead to crosstalk when signals exceed the range of the channel or when common mode voltages allow the signal to exceed the operating range of the device. In addition, if the channel is scanned, an overvoltage can increase the settling time of measurements and may result in crosstalk-like behavior.
 

For information on troubleshooting overvoltage issues see SCXI Overvoltage Causes Crosstalk and Ghosting.
 

  • Calibration of the DAQ Device
Most DAQ devices should be calibrated by a metrology lab at least once every year. Temperature variations can also affect the accuracy of a measurement. To reduce the effects of temperature variations you should run self-calibration on the DAQ device.
 

For more information on calibration visit Calibration Solutions Overview.

 
  • Improper Measurement Mode

Be sure to use the same Measurement mode in your hardware and software setup.  For example, differential (DIFF) measurement mode takes the value between two analog input channels.  These will be labeled with a positive and negative terminal, such as AI1+ and AI1-.  Referenced Single Ended (RSE) mode measures between a analog input and ground.  Ground is labeled GND, and the analog input will be labeled similar to AI1.
 

Make sure that you use the same measurement style in both your physical setup and in software.  Using DIFF in software, and measuring between AI1 and GND (an RSE wiring) will yield unexpected results.  The Field Wiring and Noise Considerations is a helpful resource for checking proper wiring practices for a given measurement mode.

Make sure the device you're using supports the measurement style you're using. Not all modules support RSE. See the user manual for your device for this information.

  • Damaged DAQ Device
If the DAQ device becomes damaged it is very likely you will receive improper readings. If you have an E or M Series device you can troubleshoot the issue by running the DAQ Diagnostic Utility. Otherwise, you should verify that the behavior is consistent across chassis / systems to confirm that an individual module or card is damaged.
 
  • Other Reasons
    • Blown fuses on your SCXI chassis, PXI(e) chassis, or other signal conditioning.
    • Bent pins on DAQ device or signal conditioning modules - causes symptoms like unconnected channels.
    • Large bias resistors - causes symptoms like high source impedance.
    • Incorrect settings in software - more common when using jumper configurable signal conditioning; software settings should match jumper settings.
    • Software corruption in the driver may cause unexpected values as well. Try force reinstalling your DAQmx driver.
    • Incorrect or unexpected output from the device under test or sensor. Use an oscilloscope to check the output of your device under test or sensor to see if the unexpected behavior is caused by the NI device or the third party hardware.
    • Your NI device may be reading in Vrms while your sensor or device under test is outputting VDC, which can cause measurements to appear incorrect. Consult your device specification sheets to see what the output and input measurement types are. You can do a quick check for this by multiplying your data by √2 to see if it causes the data to fall within expected values.
    • If you are using an adapter break out board with your device, note that the terminal indexing of the adapter may not match with the module channels. Be sure to consult the pinout diagram for your device as well as the adapter indexing to make sure you are observing the right channels.
    • For more detailed information on troubleshooting other potential reasons see Why Do My Analog Inputs Seem to Float?
    • You might be wiring your NI module for a current setup and not a voltage setup or vice versa.

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