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.
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.
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?.
For information on troubleshooting grounding issues see Field Wiring and Noise Considerations for Analog Signals.
For information on resolving noise and crosstalk issues see Field Wiring and Noise Considerations for Analog Signals.
For information on troubleshooting overvoltage issues see SCXI Overvoltage Causes Crosstalk and Ghosting.
For more information on calibration visit Calibration Solutions 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. 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.
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