National Instruments NI 6014 - Manual (Page 23)

Chapter 3 Hardware Overview
© National Instruments Corporation 3-3 NI 6013/6014 User Manual
Input Range
The NI 6013/6014 has a bipolar input range that changes with the
programmed gain. Each channel may be programmed with a unique gain
of 0.5, 1.0, 10, or 100 to maximize the A/D converter (ADC) resolution.
With the proper gain setting, you can use the full resolution of the ADC
to measure the input signal. Table 3-2 shows the input range and precision
according to the gain used.
Scanning Multiple Channels
The devices can scan multiple channels at the same maximum rate as their
single-channel rate; however, pay careful attention to the settling times for
each device. No extra settling time is necessary between channels as long
as the gain is constant and source impedances are low. Refer to
Appendix A,
Specifications , for a complete listing of settling times for each
of the devices.
When scanning among channels at various gains, the settling times may
increase. When the PGIA switches to a higher gain, the signal on the
previous channel may be well outside the new, smaller range. For instance,
suppose a 4 V signal is connected to channel 0 and a 1 mV signal is
connected to channel 1, and suppose the PGIA is programmed to apply
a gain of one to channel 0 and a gain of 100 to channel 1. When the
multiplexer switches to channel 1 and the PGIA switches to a gain of 100,
the new full-scale range is ±50 mV.
The approximately 4 V step from 4 V to 1 mV is 4,000% of the new
full-scale range. It may take as long as 100
µs for the circuitry to settle to
1 LSB after such a large transition. In general, this extra settling time is not
needed when the PGIA is switching to a lower gain.
Table 3-2. Measurement Precision
Gain Input Range Precision 1
0.5 – 10 to +10 V 305.2
µV
1.0 – 5 to +5 V 152.6
µV
10.0 – 500 to +500 mV 15.3
µV
100.0 – 50 to +50 mV 1.53 µV
1 The value of 1 least significant bit (LSB) of the 16-bit ADC; that is, the voltage increment
corresponding to a change of one count in the ADC 16-bit count.
Note: Refer to Appendix A, Specifications , for absolute maximum ratings.