PCI-6110 transient input noise

[daqnewbie]

I am trying to acquire transient voltage signals with a PCI-6110 board, sampling at 5MHz. When I examine one channel of an input signal (from an optical sensor, with its own internal op amps) using the sensor's BNC cable to high-impedance input of an oscilloscope, the ~200mV signal is clean. When I plug the same signal cable into a BNC-2110 BNC I/O connector block, which feeds the PCI-6110 DAQ card, the resulting signal has added noise in the form of small transient ringing which remains steady and synchronous with the input signal. The total signal duration is of order 10 microseconds. It is somewhat coarsely sampled, but my problem is with the added ringing noise. The BNC cable comes directly from the sensor, is about 2' long, and cannot be shortened. The digitized samples are synchronous with transient signal, since the sensor stimulus is being driven by trigger output from the PCI-6110.

I welcome any suggestions on possible causes or solutions to this noise problem. I hope not to have to build or obtain some custom high-speed buffer input amplifiers.

[ChrisR.]

It is probably the extra cable length (from the BNC-2110 to the PCI-6110) that is causing the problem. It could be reflections causing the ringing, or just the added capacitance being too hard for the opamps in your sensor to drive. In either case, you might help the situation by adding a little series resistance as near to the source as possible. 50-100 ohms or something like that in series with the signal source would absorb reflections and isolate cable capacitance from the driving amplifier.

HTH,
Chris