This content is not available in your preferred language.

The content is shown in another available language. Your browser may include features that can help translate the text.

Eliminate Ghosting on Adjacent Input Channels by Decreasing Source Impedance

Updated Feb 20, 2019

Reported In

Hardware

  • PXIe-1073

Software

  • DAQExpress
  • LabVIEW

Issue Details

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

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. To fix this:
  • 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?

WAS THIS ARTICLE HELPFUL?

Not Helpful