I discovered excess noise in NRSE input mode in our PCI-5629 DAQ board (see attached .pdf). Maybe someone can explain to me what went wrong?
Signals are connected via a BNC-2110 box to the 5629 card. The signal source is an analog low-pass filter with an output impedance of 50 ohms. Normally, it is connected to various lab electronics with a common ground. Therefore, either differential or NRSE inputs are needed to prevent ground loops (and yes, they do occur when using RSE, I checked...). As the PCI-5629 does not have differential inputs (?), that leaves NRSE. For testing purposes, I can connect the signal source to a power supply with an isolating transformer, creating a floating source.
Results (see .pdf):
For each trace, 5 million datapoints were taken at a sampling frequency of 500kHz. A python script calculated the linear spectral density. Input voltage range was set to +/-0.5V
- daq-rse and daq-nrse: these are traces taken with the DAQ input shorted via a zero ohm BNC connector. Software and BNC box were set to RSE and NRSE mode, respectively. The traces differ, but not by much. No problem here.
- sig-float-rse: this is the signal source connected in RSE mode, with the above mentioned isolating transformer. This works fine and the low-pass behaviour of the source can clearly be seen.
- sig-gnd-nrse: this is the grounded signal source, connected in NRSE mode. Obviously, the noise is much higher than in sig-float-rse, the low-pass behaviour is almost invisible.
So, to restate my question: why is sig-gnd-nrse so much noisier than sig-float-rse, and what did I do wrong?
we talk about the ground loop if the source has a ground and the measurement system is also connected to a ground.
It creates a new signal voltage that will either
A) Cause noise in your signal (commonly 60Hz noise from phase differences in the power reference on the different grounds)
B) Break something (if the size of the voltage (potential) between the two grounds is too large)
For more information please go to Field Wiring and Noise Considerations for Analog Signals
Floating sources do require bias resistor(s) between the AI SENSE input and the measurement system ground (AI GND) in the NRSE input configuration.
I hope this will help
Houssam, thank you very much for your input. However, as you can see from the graph I posted, noise in NRSE mode is mostly white, but significantly higher than in RSE mode. The signal source was grounded and the connection used was the bottom right in your schematic.
Anyhow, the problem was solved today in a different way: after reading the manual (... yeah, I guess we should have done that earlier...) we discovered that differential input _is_ supported on the PCI-6259, and we simply have to connect the BNC-2110 box instead of the -2111. Then, using the top right connection in your schematic, everything works as expected.