National-instruments Data Acquisition Device NI USB-621x Manual de usuario descargar pdf (Pagina 35)

Chapter 4 Analog Input

NI USB-621x User Manual 4-2 ni.com

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.

USB-621x devices use the NI-PGIA to deliver high accuracy even

when sampling multiple channels with small input ranges at fast rates.

USB-621x 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.

• ADC—The analog-to-digital converter (ADC) digitizes the AI signal

by converting the analog voltage into a digital number.

• AI FIFO—USB-621x 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. USB-621x devices can handle multiple A/D

conversion operations with DMA, interrupts, or programmed I/O.

• Isolation Barrier and Digital Isolators—Refer to Chapter 9,

Isolation and Digital Isolators on USB-6215/6216/6218 Devices,

for more information.

Analog Input Range

The input range affects the resolution of the USB-621x device for an

AI channel. For example, a 16-bit ADC converts analog inputs into one

of 65,536 (= 2

) 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:

USB-621x 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.

10 V 10 V–()–()

------------------------------------------- 305 μV=

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 184 185

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National-instruments Data Acquisition Device NI USB-621x Manual de usuario Pagina 35