Multifunction DAQ
Understanding differences in analog noise between PCIe-6255 and USB-6001
My team is using a DAQ to read a differential analog measurement from a strain gauge amplifier. Initially, we had been using a USB-6001 DAQ to test the system wiring and debug. However, for the actual application, we plan to use a PCIe-6255 installed in a PC with an SCB-68A terminal block to sample the differential signal as well as several other sensors in real-time. In transitioning over to using the PCIe-6255, we noticed that, generally, noise in our signal is more pronounced for the PCIe-6255 than the USB-6001.
Attached is some example data from the strain gauge collected separately at 1000 Hz using each device. The strain gauge bridge is installed on a model turbine blade that is being operated by a servomotor, so oscillatory readings are expected. The underlying signal for the USB-6001 is distinct, outside of semi-frequent larger spikes which could be easily removed with a software filter. Conversely, for the PCIe-6255 the signal is overall fuzzier. In both cases, everything upstream of the DAQs is identical: the DAQs are plugged into the same computer in the same environment, with terminal blocks located in roughly the same physical location. Cabling and shielding of the analog signal is identical between the two cases.
The main difference between these two cases is simply changing is the DAQ that the differential wires are plugged into. However, there are a few other differences at the DAQs that I should note:
- I did find that I needed to include input bias resistors (per this guide) between the analog +/- inputs and analog ground for the PCIe-6255 in order to avoid saturated readings, whereas these resistors were not necessary for the USB-6001 (i.e., it was adequate to simply connect the inputs to AI +/-).
- For the PCIe-6255, other instruments (servomotor encoder, 6-axis load cells) are also wired into the SCB-68A, although I observed similar differences in noise characteristics when these instruments were powered off. There is electrical isolation between the servomotor and the SCB-68A.
What might be the mechanisms for different noise characteristics between these two devices, and how might I reduce the overall noise of the PCIe-6255? Why are input bias resistors necessary for the PCIe-6255, but not for the USB-6001? Is the USB-6001 performing some sort of filtering that the PCIe-6255 is not, or perhaps the PCIe-6255 is more sensitive?
