Chapter 4 Analog Input
NI USB-621x User Manual 4-2 ni.com
Ground-Reference Settings
The analog input ground-reference settings circuitry selects between
differential, referenced single-ended, and non-referenced single-ended
input modes. Each AI channel can use a different mode.
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.
M Series devices use the NI-PGIA to deliver high accuracy even when
sampling multiple channels with small input ranges at fast rates. M Series
devices can sample channels in any order at the maximum conversion rate,
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
M 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.
M Series devices can handle multiple A/D conversion operations with
DMA, interrupts, or programmed I/O.
Analog Input Range
The input range affects the resolution of the M Series device for an AI
channel. 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. So, for
an input range of –10 V to 10 V, the voltage of each code of a 16-bit ADC
is:
M Series devices use a calibration method that requires some codes
(typically about 5% of the codes) to lie outside of the specified range. This
(10 V – (–10 V))
2
16
= 305 μV
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