PXI-6120 multiple connections floating and ground referenced without galvanic separation

[Olli]

Hello, I need to refurbish our measurement set-up for PV-modules. The signal conditioning (LEM voltage and current transducers, galvanic separation) gives very big errors in the low signal regions (of interest). Now I decided to change to measurement resistors (voltage divider and current shunt) in order to minimise this error. But I have a little doubt, if my planed signal connection would cause other problems, because there is no other way than pseudodifferential connection. Could someone please check my set-up? In principle it should work this way. Thanks! Olli

[Olli]

the attachment of course...

[Olli]

sorry. I made a little mistake in the scheme that I posted before. Here is the corrected one

[Daniel-E]

Hey Olli,

Pseduodifferential is preferable to a single-ended measurement scheme; here's some information from our website about it:

https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z0000019YuUSAU&l=en-US

Of course, it is not as desireable as a differential connection, but for your application it looks like it might be the best approach. Note that for pseudodifferential you want to have a small resistance between the negative input and ground; you can see more about that in the link above.

[Olli]

Hello Daniel,

thanks for the reply! When I take a look into the manual for the PXI-6115/6120, the picture 4-3 shows a common mode voltage between AI- and AIGND. Therefore I thought to connect them together without any higher resistance than the cable in order to have a proper reference to GND. In fact the manual says not, how to connect the Low signal point with the AIGND...or maybe I did not see this correctly? Or do I have to interpretate the sign for the common voltage (Vcm) as a small resistor? When I take a look at the non-referenced connection in picture 4-5 instead it looks quite clear for me. This was my original connection with the floating transducer signals

In what range should the ressisor value be in my AI0 and AI1 connections? Are there some rules of thumb or should I do an approximation from higher to lower resistance values with some static measurements?

Greetings!

Olli

[Olli]

ah yes there is a value given in the link, that you sent... 50 ohms.

I'll try this (after reading it more carefuly ;-))

thanks again!