Noise on Lower Voltage Signals with a 1200 Series DAQ Card Primary Software: Other NI Software>>VirtualBenchPrimary Software Version: 2.6 Primary Software Fixed Version: N/A Secondary Software: N/A
Problem: I have a PCI-1200 that I am using to measure a thermocouple (+10 mV to -10 mV), another 30 mV signal, and two signals that range from +5 V to -5V. When I use VirtualBench-DMM to measure my signals, there is very little noise. However when I use VirtualBench-Logger to read my four signals, the two signals that are +/- 5 V look fine but the thermocouple's temperature is erratic and the 30 mV signal is very noisy and not correct. I made sure that I set the maximum and minimum range for each channel to be as small as possible. I've also examined the signals in NI-DAQ's Test Panels and each one looks fine. Solution: The 1200 Series cards do have multiple gains that can be used to get better accuracy on smaller signals, however all channels on the board have to use the same gain. So if we are examining four signals, the largest voltage range that is selected will be used on all channels. Since the 1200's have 12 bit ADC (analog to digital converters) they have 4096 discrete points that a analog voltage can be represented with. If we are just measuring over a +/- 50 mV range this means each discrete point differs by 24.4 microvolts, but if we are using the +/- 5 V range each discrete step is 2.44 mV. In the above scenario the board must use the largest range for all channels because it can only use one range and this will cause trouble reading small signals since a one step change will result in a roughly 10 percent change in the 30 mV signal and may be a 50 percent to 100 percent change in the thermocouple (which translates to several degrees). In order to circumvent this limitation of the 1200 series, you should amplify and/or attenuate all of your signals so that they are in the same input limit range. This problem does not occur with E-Series Data Acquisition boards which have individual gains for each channel. Proper grounding can also be an issue with signals. Consult your User's Manual or a Wiring Application Note to make sure that you have your signals correctly connected. Related Links: Contact Sales if you have an interest in upgrading your board E-Series Multifunction DAQ Devices Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals Product Manuals: PCI-1200 User Manual Attachments:
Report Date: 07/08/1999 Last Updated: 07/27/2007 Document ID: 1N77KCTS |
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