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Chapter 4
Analog Input
•
Instrumentation Amplifier (NI-PGIA)
—The NI programmable gain instrumentation
amplifier (NI-PGIA) is a measurement and instrument class amplifier that minimizes
settling times for all input ranges. The NI-PGIA can amplify or attenuate an AI signal to
ensure that you use the maximum resolution of the ADC.
MIO X Series devices use the NI-PGIA to deliver high accuracy even when sampling
multiple channels with small input ranges at fast rates. MIO X Series devices can sample
channels in any order, and you can individually program each channel in a sample with a
different input range.
•
A/D Converter
—The analog-to-digital converter (ADC) digitizes the AI signal by
converting the analog voltage into a digital number.
•
AI FIFO
—MIO X Series devices can perform both single and multiple A/D conversions
of a fixed or infinite number of samples. A large first-in-first-out (FIFO) buffer holds data
during AI acquisitions to ensure that no data is lost. MIO X Series devices can handle
multiple A/D conversion operations with DMA or programmed I/O.
Analog Input Range
Input range refers to the set of input voltages that an analog input channel can digitize with the
specified accuracy. The NI-PGIA amplifies or attenuates the AI signal depending on the input
range. You can individually program the input range of each AI channel on your MIO X Series
device.
The input range affects the resolution of the MIO X Series device for an AI channel. Resolution
refers to the voltage of one ADC code. For example, a 16-bit ADC converts analog inputs into
one of 65,536 (= 2
16
) codes—that is, one of 65,536 possible digital values. These values are
spread fairly evenly across the input range. So, for an input range of -10 V to 10 V, the voltage
of each code of a 16-bit ADC is:
MIO X Series devices use a calibration method that requires some codes (typically about 5% of
the codes) to lie outside of the specified range. This calibration method improves absolute
accuracy, but it increases the nominal resolution of input ranges by about 5% over what the
formula shown above would indicate.
Choose an input range that matches the expected input range of your signal. A large input range
can accommodate a large signal variation, but reduces the voltage resolution. Choosing a smaller
input range improves the voltage resolution, but may result in the input signal going out of range.
For more information about setting ranges, refer to the
NI-DAQmx Help
or the
LabVIEW Help
.
10V
10V
–
–
2
16
---------------------------------
305
V
=
Summary of Contents for 6368
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