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What Are AC and DC Coupling?

Updated Dec 13, 2018

Reported In

Hardware

  • Multifunction I/O Device
  • C Series Multifunction I/O Module
  • PXI Analog Input Module
  • NI-9229
  • NI-9225
  • NI-9250

Issue Details

  • In the Datasheet for my product, I see a section titled Input Coupling, and it is specifies AC or DC. What are AC and DC coupling?
  • Does my module support DC voltage? 

Solution

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 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.
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.
 

Additional Information

To AC couple a signal with an NI DAQ(Data Acquisition) device, insert a capacitor in series with the signal, between the positive lead of the signal and the positive analog input channel on the DAQ device. Wire the negative lead of the signal directly to the corresponding negative analog input channel. This removes the DC component from the signal, which centers the AC signal at 0 volts. Please see the below figure for more detail.

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