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Using Low Impedance Sources

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    Last Modified: May 14, 2018

    The NI ELVIS III uses one analog-to-digital converter (ADC) for analog input measurements. The multiplexers (MUX) route one analog input channel at a time to the ADC through the gain selector.

    For the fastest settling time, your signal sources must have an impedance of <1 kΩ. Large source impedances increase the settling time of the gain selector, and therefore decrease the accuracy at fast scanning rates.

    Settling times increase when scanning high-impedance signals due to a phenomenon called charge injection. Multiplexers contain switches, usually made up of switched capacitors. When one of the channels, for example channel 0, is selected in a multiplexer, those capacitors accumulate charge. When the next channel, for example channel 1, is selected, the accumulated charge leaks backward through channel 1. If the output impedance of the source connected to channel 1 is high enough, the resulting reading of channel 1 can be partially affected by the voltage on channel 0. This effect is referred to as ghosting. If your source impedance is high, you can decrease the scan rate to allow the gain selector more time to settle.

    Another option is to use a voltage follower circuit to decrease the impedance seen by the workstation. Refer to the KnowledgeBase document, How Do I Create a Buffer to Decrease the Source Impedance of My Analog Input Signal?, by going to ni.com/info and entering the Info Code rdbbis.


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