Upgrading from PCI-6014 to PCI-6250 results in signal noise

[CraigMcD]

Hi,

I recently upgraded my DAQ hardware from an NI PCI-6014 DAQ and CB-68LP connector block to an NI PCI-6250 and SCB-68A connector block. However, in doing so, some odd noise seems to have suddenly appeared in my acquired signal.

I have a quadrant photodiode (Hamamatsu G6849) with each of it's quadrants connected to its own transimpedence amplifier. The output of the amplifiers is then connected straight into the connector block which is set to direct feedthrough mode. AI 4+/- (pins 28 and 61), AI 5+/- (pins 60 and 26), AI 6+/- (pins 25 and 58) and AI 7+/- (pins 57 and 23) are being used. I've been sampling at a rate of 50 KHz for 4s. All of this is the same as before upgrading the DAQ and connector block and worked with the old components.

I've tried changing the sampling rate and the sampling time; grounding the source as shown in figures 2-4, 2-5 and 2-6 of the SCB-68A user manual; grounding the chassis via the chassis ground lug; turning off all unnecessary electronics to try and eliminate as much possible interference as I can; and using both my LabVIEW code and the test panels in MAX to try and eliminate the problem.

The output from my LabVIEW code:

They used to be smooth traces, the new DAQ card has caused those periodic looking "blips" in the signal.

The output from a MAX test panel, showing "blips" in signal:

Any ideas/comments/suggestions would be greatly appreciated - I'm pretty new to setting up an NI DAQ card for datalogging. Unfortunately, I can't go back to using the old DAQ card and connector block because I've been told to get both the old and new stuff working on different systems.

Thanks,

Craig