How do I limit the common mode voltage for the switch

[AbandoningCausality]

The user manual mentions a maximum of ±12V to ground for my switch (pxi-2536) but I don't see a ground input on the connector pinout. If I have a floating power supply, do I have to connect the ground to the chassis ground on the back of the PXI? Is there a better way to limit the common mode voltage?

[ashley_gee]

Hi AbandoningCausality,

I'm a bit unclear on your setup, so some clarification would help.  When you say "connect the ground to the chassis ground on the back of the PXI", what is the first ground you are referring to?

[AbandoningCausality]

Lets say I have a power supply like this:

https://amzn.com/B007BRO92U

The red terminal is going into the rows of the switch. I want to switch that to one of the columns which connect through a load to the black terminal. Now, in that setup, the red terminal and black terminals are not being referenced to the ground of the switch. So for example even though I set my supply to 1.5 V,  that 1.5V is just the voltage between the red terminal and black terminal. There is no (common mode) voltage defined between the black terminal and the ground of the switch. The supply is "floating".

Now, I want to ensure I'm meeting the spec of the switch so I need some way to get the voltage at the black terminal to be within a certain amount of the switch voltage (limiting the common mode). I wanted advice of how to do that.

[ashley_gee · Accepted solution]

Thanks for clarifying, that makes much more sense!  The best way to go about this it seems will be to ground the negative terminal of your power supply to the safety ground of the chassis, as I think you initially suggested.  I don't know which chassis you have, but you should be able to find a section on "Connecting Safety Ground" in the manual. For example, here is the 1044 chassis manual, with this section is on page 2-4. http://www.ni.com/pdf/manuals/371360a.pdf

This should take care of it and avoid any ground loops!

[AbandoningCausality]

For future designs it might be nice to bring out an isolated ground on one of the unused pins on the row connector. This would help reduce ground noise into the power supply.