Thinking this should be obvious, but having trouble identifying a fix: I use multiple channels on an NI PXI DAQ card for RSE voltage measurements. I need to verify the voltage readings returned by the DAQ card with an oscilloscope. Without the DAQ card input running, the Scope shows similar data. If I run the DAQ acquisition at the same time as the Scope, i see periodic blips that happen at the same rate as the card multiplexes between its input channels.
A) Why would I see these blips on the scope? B) What can I do to eliminate them?
Details: The AI connections and Scope probe connections are made at the same location. A higher DC Coupling impedence results in a higher amplitude of the blips. Switching to AC coupling has no effect. The blips are seen even if the circuit is not being driven by a signal.