NI sbRIO-960x/962x/963x OEM Instructions & Specifications 26 ni.com
The following figure shows the circuitry for one AI channel on the
NI sbRIO-9626/9636.
Figure 22. Single-Ended Analog Input on the NI sbRIO-9626/9636
Analog Input Range
An input range is a set of input voltages that an analog input channel can
digitize with the specified accuracy. NI sbRIO devices with selectable input
ranges have a programmable gain instrumentation amplifier (PGIA), which
amplifies or attenuates the AI signal depending on the input range. You can
program the input range for each AI channel independently on the
NI sbRIO 9626/9636.
The ADC converts analog inputs into discrete digital values. For a 12-bit
ADC there are 2
12
(4,096) possible values, and for a 16-bit ADC there are
2
16
(65,536) possible values. These values are spread evenly across the
input range, and the voltage difference between values is proportional to the
input range selected for the channel. The voltage difference between values
is the size of the least significant bit (LSB size) for the channel. The
following equation shows how to calculate the LSB size for a channel set
to the –10 V to 10 V input range, with a 16-bit ADC.
(1)
The NI sbRIO-9626/9636 uses a scaling method that requires some codes
(typically about 5% of the codes) to lie outside the specified range. This
method improves absolute accuracy, but it increases the LSB size by about
5% over the calculated value.
Choose an input range that matches the expected range of your input
signal. A large input range accommodates large variations in signals but
DIFF, RSE
I/O Connector
AI <0..15>
Mux
AI GND
PGIA
AI Termin
al
Configuration
Selection
Inp
ut Range
Selection
ADC
AI Data
10 V – (–10 V)
65,536
= 305 μ
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