What is connected to the other end of the BNC cable? In other words, what is the load the analog output is driving?
I ask this because there is a maximum current the analog outputs on the 6024E can drive. The maximum current the analog outputs can drive is +/- 5 mA. So, if you are over-loading an analog output, the output voltage will be pulled-down because of this maximum current specification. I hope this helps.
Regards,
Todd D.
NI Applications Engineer