RSE measurements with SCXI-1104C+SCXI1300?

[JKSH]

Hello,

I have an SCXI-1104C for monitoring single-ended DC voltages. I know that the SCXI module has differential inputs only, so I presumed that I would need to connect each individual negative terminal to ground.

However, I noticed that if I connect Signal- to the SCXI_ChassisGND, and Signal+ to SCXI_Ch0+, I still get the correct reading. The reading floats when I disconnect Signal- from SCXI_ChassisGND.

What am I observing here? Can I reliably use the SCXI-1104C for RSE measurements, by connecting Signal- to ChassisGND?

Thanks!

[Eric-E · Accepted solution]

Hello JKSH,

Consider the connection for the SCXI-1104 where...

Ch0+ is connected to Signal +

Ch0- is connected to Nothing

This is an unguaranteed configuration where Ch0- is just floating at whatever voltage it was at last. This is probably somewhere around chassis ground because it was last connected to chassis ground. However, if you touched the Ch0- with a random voltage, your measurement would be disrupted and at an incorrect level. 

When Ch0- is instead connected to some reference voltage, we guarantee the level and can acquire valid voltages.

I hope this helps!

-Eric E.

[JKSH]

That makes perfect sense. Thank you!