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
sbRIO-9637.
The ADC converts analog inputs into discrete digital values. 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.
10 10
65, 536
= 35
The sbRIO-9637 uses a scaling method that requires some ADC 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 results in a larger LSB and, therefore,
lower resolution. A smaller input range improves the resolution, but large input signals may go
out of range.
For more information about selecting ranges, refer to the LabVIEW Help.
The following table shows the input ranges and resulting LSB sizes for AI channels on each
NI sbRIO device.
Table 15. NI sbRIO Device Input Range and Resolutions
Input Range Bit Resolution LSB Size (includes 5% Overranging)
-10 V to 10 V 16-bit 320 μV
-5 V to 5 V 160 μV
-2 V to 2 V 64 μV
-1 V to 1 V 32 μV
Working Voltage Range
The PGIA on NI sbRIO devices operates normally by amplifying signals of interest while
rejecting common-mode signals under the following three conditions:
• The common-mode voltage (V
cm
), which is equivalent to subtracting AIGND from AI-,
must be less than ±10 V. V
cm
is a constant for all range selections.
• The signal voltage (V
s
), which is equivalent to subtracting AI+ from AI-, must be less
than or equal to the range selection of the given channel. If V
s
is greater than the range
selected, the signal clips and information is lost.
• The total working voltage of the positive input, which is equivalent to (V
cm
+ V
s
), or
subtracting AIGND from AI+, must be less than the maximum working voltage specified
30 | ni.com | NI sbRIO-9637 User Manual
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