Do I make error wiht my connection between DAQ Card and signal?

[datoudream]

My singal is voltage from preamplifier(I-V conventer) of PMT(photomultiplier tube), one pin of which is GND of DC Power. You know, ±15v is needed to supply to the operation amplifier in the preamplifier. So the negative of signal is the GND of DC power. I use PCI 6281 as differential connection to acquire signal. 

According to fig1, I connect the signal and DAQ Card as the diagram in the red box. 

But when I start measuring, the DAQ Card's output is different from an oscilloscope. What's wrong?

[rui_huang]

I have thefollowing two questions to ask:

1. How do you get your measure output, in MAX tespanl or through labview code?

2. How is the signal you get different from that measured in scope?

Best Regards,

NI Application Engineer

[datoudream]

Thanks.

1.through LabView code. The codes shall have no problem.

2.two different apperance:more noisy and a little lower(10%). But the waveform(frequency and phase) is the same.

I mainly ask about whether I connect the AI GND pin and negative pin(for example, AI 8 of chan1 differential input). If I connect them, it is equivalent to connecting the DC power GND and AI GND--GND of 2 systems. Does it bring error?

[rui_huang]

It is strongly recommended that user connect a resistor bwtween the AI- and the AI GND, especially  when the floating signal source is used.