Why Does My USB-6008/6009 Have Different Resolutions for Differential and Single-Ended Measurements?Hardware: Multifunction DAQ (MIO)>>Portable>>USB-6008, Multifunction DAQ (MIO)>>Portable>>USB-6009
Problem: The user guide for the USB-6008 and USB-6009 says that there are two different input resolutions:
Why does the single-ended configuration have one less bit of resolution than the differential configuration? Solution: In differential mode, the USB-6008/6009 can use the full range of the analog-to-digital converter (ADC) to measure positive and negative differences between the analog inputs. In single-ended mode, the ADC expects a positive input with respect to the common ground, and so the unit can only use the positive range of the ADC. The effective difference between these two modes is a sign bit. While this sign bit is not used in single-ended mode, this does not mean that a single-ended measurement cannot be negative. To read negative voltages in single-ended mode, the USB-6008/6009 uses a resistor network to scale the range -10 V to +10 V to the range 0 V to +10 V at the input of the module. However, since only the upper half of the ADC range is used to represent the full analog input range, the resolution decreases by one bit. Related Links: NI Developer Zone: Field Wiring and Noise Considerations for Analog Signals KnowledgeBase 3PD0S4DI: The Differences Between Accuracy and Bit Resolution Attachments:
Report Date: 01/15/2008 Last Updated: 10/07/2008 Document ID: 4HEGDOMV |
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