Measuring Abnormal Large Periodic Spikes with My Voltage Input Module

Updated Mar 20, 2018

Reported In

Hardware

  • C Series Voltage Input Module
  • C Series Voltage Input Device

Issue Details


I have a C series voltage input module that measures a floating differential input. In my measurements, I see large periodic spikes that should not be there. How can I get rid of these?

 

Solution

This problem will occur if you do not use bias resistors. Failure to use these resistors will result in erratic or saturated (positive full-scale or negative full-scale) readings.

In an ideal world, the module does not have any current on its Analogue inputs since they are theoretically speaking of infinite resistance. However, in reality, the module will give rise to so-called bias currents (originating from the biasing, or calibration, of the module itself). In addition, whatever device you are reading from may produce currents as well. These currents need somewhere to go. Therefore you should use so-called bias resistors to provide a path for the current to ground so that they do not move the voltage level of the floating source out of the valid range of your module. The figure below displays the schematic of a typical measurement with bias resistors.​
 

 
Typically, the resistors are between 10 kΩ and 100 kΩ to work well with low-impedance sources such as sensors and signal conditioning module outputs. 

Additional Information

For a more extensive explanation regarding bias resistors, please see National Instrument's article on Field Wiring and Noise Considerations for Analog Signals.

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