Why the USB-6008/6009 Has Different Resolutions Depending on Measurement Type

Issue Details

The user guide for the USB-6008 and USB-6009 has two different input resolutions depending on if the measurement is differential or single ended. 
 

ADC Resolution

NI USB-6008	12 bits differential
	11 bits single-ended
NI USB-6009	14 bits differential
	13 bits single-ended

Why is the single ended resolution 1 bit less than the differential resolution?

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.