daq6510 7702 4-wire resistance Overflow errors

 I am experiencing overflow errors on my daq6510 and 7702 card. 


The system is setup to measure 4-wire resistance @ 1NPLC cycle, 10 Ohm range with open lead detection. 

At the moment I have it setup to measure 5 Channels. But I keep getting a lot of overflow errors. 
If I change the Channel delay to 50ms or increase the measurement range to 100 Ohm, I no longer experience overflow errors.
 
But these are not the settings I want.
I also have a second system setup to measure 20 channels, with the same settings and that does not experience any overflow errors.

Any ideas on why this is? 
I assume the system manages its own response and settling times.

Are there any differences in the length of cables for your 20-channel setup vs. this one with only 5 channels?

If longer delay is allowing a good outcome, it implies a difference in the time constant of your DUT.

Does it stop reporting the overflow if you turn off the open lead detection?  The reported overflow could come a saturated measure range or from a detected open lead.  Since more delay fixes the overflow, this implies it is coming from the measurement from after the open lead detection has occurred.

To look into it, you might put a scope on the Card Sense HI and LO terminals.  When doing 4-wire ohms, the current is forced on the HI and LO and the resulting I*R is measured on the Sense HI and LO.
Is that spiking and requiring more settling time vs hte 20-channels setup?
 

If longer delay is allowing a good outcome, it implies a difference in the time constant of your DUT.

• Not necessarily, it could also be related to the relay contact settling time. 

Does it stop reporting the overflow if you turn off the open lead detection? 

• No.
• It's important to note, that it's not a constant overflow error.
• Typically there will be at least 1 overflow error on each scan and the overflow will be on a different channel each time

I had shortened one of set of leads before making the initial post, but it had no impact.


The overflow problem occurs at 1 and 10 Ohm ranges, but goes away at the 100 Ohm range

I have also tried increasing the NPLC window from 1 NPLC to 3 NPLC but again, there was no impact. 

I gave the oscilloscope a shot with a differential probe but I couldn't really see much
- I didn't see any little square wave or pulse, that would look like an ideal measurement
- Front panel: couldn't see anything on the scope
- Rear panel: I did see spiking for all measurements, regardless if there was an overflow or not 

Could noise pickup on the cables be a potential culprit ?