Chapter 3
Hardware Overview
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National Instruments Corporation
3-3
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.
Multichannel Scanning Considerations
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 of the devices. No extra settling time is necessary between channels
as long as the gain is constant and source impedances are low. Refer to
Appendix A,
, 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.
Settling times can also increase when scanning high-impedance signals
due to a phenomenon called charge injection, where the analog input
multiplexer injects a small amount of charge into each signal source when
that source is selected. If the impedance of the source is not low enough,
Table 3-2.
Measurement Precision
Gain
Input Range
Precision
*
0.5
1.0
10.0
100.0
–
10 to +10 V
–
5 to +5 V
–
500 to +500 mV
–
50 to +50 mV
305.2
µ
V
152.6
µ
V
15.3
µ
V
1.53
µ
V
*The value of 1 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: See Appendix A,
, for absolute maximum ratings.
