Eliminate Ghosting on Adjacent Input Channels by Decreasing Source Impedance

Updated May 3, 2018

Reported In

Hardware

  • NI-9205
  • cDAQ-9174

Software

  • DAQExpress
  • LabVIEW 2014 Full
  • LabVIEW 2014 Professional

Other

Windows 7

Issue Details

When scanning through multiple, adjacent input channels, I am seeing crosstalk or ghosting between my signals. My source impedance is higher than 1 kΩ. When I measure only one channel or use a multimeter to measure the channel, the measurement is correct. 

Solution

Source impedances greater than 1 kΩ are not recommended with multiplexed data acquisition devices because they increase the amount of settling time required for an accurate measurement.
  • Create a unity gain buffer, or a voltage follower, to decrease the time required for settling.
    • The buffer provides a high input impedance and a low output impedance. This means the multiplexer can receive low-impedance signals from high-impedance sources.
    • To implement the buffer, use an operational amplifier. Construct one by connecting the amplifier's output to its inverting input, and connecting the signal source to the non-inverting input. 
  • You can also try reducing the source impedance directly, if it's safe and possible to do so.
    • For example, if the source impedance is directly due to a resistor that's larger than 10 kΩ, place a resistor in parallel that will decrease the total source impedance to 1 kΩ or less.

Additional Information

More methods exist to eliminate or reduce ghosting. For more information, see this KB on How Do I Eliminate Ghosting From My Measurements?

For more information about settling time and how it can affect multiplexed measurements, see the following white paper: Is Your Data Inaccurate Because of Instrumentation Amplifier Settling Time?

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