USB-6225- Connector 0 adds offset to GND

[yans123]

Hello,

I encountered odd bhaviour of USB 6225 using mass termination cables which I connected to PCB Icreated.

I have 80 AIs measuring ~0.01V signals at low frequncy. 

When both connector 0 and connector 1 plugged in, UI have  offset of 0.06mV in my measruments.

When I dissconnect Connector 0 from my board, the measruments are OK.

DGND and AGND are dissconnected on my PCB (different plans).

Do you have an idea how to fix this?

[Henrik_Volkers]

Some migth be offset voltage, some migth be due to bias currents, or other current (ground loop) , even thermoelectric voltage, or galvanic elements or hum in AC measurements or capacitive coupled spikes that are integrated...

60µV comes easy ....

Hard to say without information on exact cabling, type of measurement, source impedances, lab enviroment, .....  

(and my crystal ball unforseenable droped on the floor and need recalibration :D)

First thing I usually do is : Hook up a (analog) scope and have a look 🙂

[yans123]

Thanks Henrik,

The actual offset= 30mV .

The question here is why when I connect only "connector 1"- there is low offset but when I connect also "connector 0" there is large offset on AGND?

I think the answer is related to that "Con 0" had DGNDs and AGNDs but I'm not sure how to confirm it. I musl also mention that on my PCB- DGND and AGND are on different plains.

Does this gives you an idea to point to the right direction?

Thanks 🙂

[Henrik_Volkers]

Bias currents are my favorite ....  however I told you what information would be helpful to help you...