SCB-68 AIGND vs AIGND

[hvo123]

Hello,

I appologize for the cryptic title but I am a bit confused on the difference between the screw terminal AIGND pins (67, 32, 64, etc...) and the AIGND on the PCB signal conditioning area on the SCB-68 breakout box. 

I am trying to accurately measure a 0-10V signal on analog channel 1 in differential mode.  I followed the connection guidelines (see link) for differential measurements by adding bias resistors on the (+) and (-) inputs to AIGND pin# 67.  With this configuration I occasionally see saturation in my measurements due to high common mode voltage.  I then removed my bias resistors from AIGND pin# 67 and added bias resistors to the PCB signal conditioning area (RC14 and RC15) as suggested in figure 5-1 and my measurements seems correct and does not saturate.  It seems to me that these two operations should be the same, but I get different results.  Resistance measurements between screw terminal AIGND pins (67, 32 and so on) to the PCB signal conditioning AIGND showed high impedance which suggests they're not the same.  All my switches are set to factory default settings.

What is the difference between the two AIGNDs, and which AIGND should I use? 

SCB-68 68-Pin Shielded Connector Block User Manual:

http://www.msm.cam.ac.uk/mechtest/docs/scb68%20manual.pdf

Hardware:

PCI 6221 M DAQ card

SCB-68 interface

Best Regards,

hv

[PeterGC · Accepted solution]

HV,

Those two AIGND are connected inside the DAQ card, so it will show high impedence unless you have the SCB 68 connected to the DAQ card.  I measured ~35Ohms when connected, open circuit without.  Also, make sure switch S3 is down, through trial and error I found this to cause jumps in impedence. 

Hope this helps,

[hvo123]

Thanks Peter!

Just the answer I was looking for.

best regards

hv.