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AC and DC Coupling for NI DAQ Devices

Updated Dec 11, 2019

Reported In

Hardware

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

Driver

  • NI-DAQmx

Issue Details

  • In the Datasheet for my product, I see a section titled Input Coupling, and it is specifies AC, DC, or "AC or DC." What are do each of these input coupling types mean?
  • Does my module support DC voltage? 

Solution

In the specification sheet for your DAQ device, there is an Analog Input Characteristic called Input Coupling. Some basic information about this specification can be could in Specifications Explained: NI Multifunction I/O (MIO) DAQ - Coupling.

This specifies whether your device supports AC, DC, or both types of input coupling.
  • AC Coupling: AC coupling consists of using a capacitor to filter out the DC signal component from a signal with both AC and DC components. The capacitor must be in series with the signal. AC coupling is useful because the DC component of a signal acts as a voltage offset, and removing it from the signal can increase the resolution of signal measurements. AC coupling is also known as capacitive coupling.
  • DC Coupling: DC coupling allows both AC and DC signals to pass through a connection. When using DC coupling, no additional capacitor is added to filter the signal. The DC-coupled configuration is usually best if the signal source has only small amounts of offset voltage, less than ±100 mV, or if the DC content of the acquired signal is important.
  • Software Select Coupling: Some NI devices are hardware configurable to use either AC coupling or DC coupling on a channel by channel basis. You can configure the input coupling type per channel using the Input Coupling DAQmx Channel Property node.

Note: If your signal is entirely DC content, ensure you use DC coupling. Otherwise your entire signal content will be removed and will look like a grounded 0 V signal.

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