There are a number of reasons that you may be seeing unexpected voltages or cross-talk on an analog channel. Some of the more common reasons are:
Sometimes the source an ADC is measuring can have a high impedance. 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.
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 device, 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.
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 detailed information on troubleshooting and resolving unconnected channel issues see Incorrect Readings on Unconnected or Open Channels.
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.
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, and grounding issues see the Field Wiring and Noise Considerations for Analog Signals article. Specifically, section two on Measuring Grounded Signal Sources and section three on Measuring Floating (Nonreferenced) Sources.
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, again see the Field Wiring and Noise Considerations for Analog Signals article. Specifically, section four Minimizing Noise Coupling in the Interconnects, section six Solving Noise Problems in Measurement Setups, and section seven Signal Processing Techniques for Noise Reduction. Analyze the frequency components of the noise with a scope or another instrument. This can help to identify the source of the noise. For example, if we are looking at power line noise (60Hz/50Hz), RF interference, or any other source. Once the source is identified, some actions can be taken, like using ferrites for the power cables or low-pass filters for RF interference (this is common when there is RF equipment in a laboratory and the wires in the terminal block act as antennas).
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.
Most DAQ devices should be calibrated at least once every year. Temperature variations can affect the accuracy of a measurement causing you to see unexpected measurements. To reduce the effects of temperature variations ensure that you have calibrated your DAQ device properly. For more information on how to calibrate your device visit Calibration Services Overview.
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 labelled with a positive and negative terminal (e.g., AI1+ and AI1-). While, Referenced Single Ended (RSE) mode measures between an analog input and ground (e.g., AI1 and GND. Using Differential in software while using RSE wiring, where you are measuring between AI1 and GND, will yield unexpected results. Note: Make sure the device supports the measurement style you are using as not all modules support RSE. See the user manual for your device for this information. The Field Wiring and Noise Considerations article is a helpful resource for checking proper wiring practices for a given measurement mode. Additionally, read about the Difference Between Differential, RSE, and NRSE Terminal Configurations to determine if you are using the correct configuration/mode.
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.
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Eliminate Ghosting on Adjacent Input Channels by Decreasing Source Impedance
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