Common-Mode Offset on the NI PXIe-5820 VST

Issue Details

Whenever I set a common-mode offset for the PXIe-5820, it will continue to generate even after aborting the generation or closing the instrument session. Why is this and how can I avoid it?

Solution

The common-mode offset on the 5820 is a property of the hardware state, meaning that the NI-RFSG driver will leave the common-mode voltage configured to the user desired setting even after aborting the generation or closing the instrument session. When you stop the generation by calling the niRFSG Abort function, the driver will tell the device to generate 0's, but the hardware is left committed to the requested state. 

The reason for this is because '0' does not mean 0 V in a connection with common-mode voltage. For example, if a user is generating a ±1 V sine tone with a 1 V common-mode offset voltage, the signal changes from 0 V to 2 V. On abort, the device will output a constant 1 V which is the same as a ±0 V sine tone with a 1 V common-mode offset. 

In order to remove the offset, you will need to either call niRFSG Reset or explicitly set the common-mode offset property to 0.0 V. You can also call niRFSG Commit if you would like to only commit the correct settings to the hardware without starting the generation. 

Additional Information

The I and Q Out ports of the PXIe-5820 need to be 50 Ω terminated. If you are wanting to measure the common-mode voltage you will need to terminate the outputs and measure the I and/or Q port output referenced to chassis ground.