National Instruments NI USB-621x User Manual (English)

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Chapter 2 DAQ System Overview

Factory-calibration constants are permanently stored in an onboard

EEPROM and cannot be modified. When you self-calibrate the device,

software stores new constants in a user-modifiable section of the EEPROM.

To return a device to its initial factory calibration settings, software can

copy the factory-calibration constants to the user-modifiable section of the

EEPROM. Refer to the NI-DAQmx Help or the LabVIEW 8.x Help for more

information about using calibration constants.

Signal Conditioning

Many sensors and transducers require signal conditioning before a

measurement system can effectively and accurately acquire the signal. The

front-end signal conditioning system can include functions such as signal

amplification, attenuation, filtering, electrical isolation, simultaneous

sampling, and multiplexing. In addition, many transducers require

excitation currents or voltages, bridge completion, linearization, or high

amplification for proper and accurate operation. Therefore, most

computer-based measurement systems include some form of signal

conditioning in addition to plug-in data acquisition DAQ devices.

Sensors and Transducers

Sensors can generate electrical signals to measure physical phenomena,

such as temperature, force, sound, or light. Some commonly used sensors

are strain gauges, thermocouples, thermistors, angular encoders, linear

encoders, and resistance temperature detectors (RTDs).

To measure signals from these various transducers, you must convert them

into a form that a DAQ device can accept. For example, the output voltage

of most thermocouples is very small and susceptible to noise. Therefore,

you may need to amplify or filter the thermocouple output before digitizing

it. The manipulation of signals to prepare them for digitizing is called

signal conditioning.

For more information about sensors, refer to the following documents.

• For general information about sensors, visit

ni.com/sensors.

• If you are using LabVIEW, refer to the LabVIEW Help by selecting

Help»Search the LabVIEW Help in LabVIEW and then navigate to

the Taking Measurements book on the Contents tab.

• If you are using other application software, refer to Common Sensors

in the NI-DAQmx Help or the LabVIEW 8.x Help.

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National Instruments NI USB-621x Specifications

General

Model	NI USB-621x
Category	Computer Hardware
Interface	USB
Analog Input Resolution	16 bits
Analog Input Sample Rate	250 kS/s (aggregate)
Analog Output Channels	2
Analog Output Resolution	16 bits
Digital I/O Channels	8
Counter/Timers	2
Operating System Compatibility	Windows, Linux
Power Supply	USB powered
Device Type	Data Acquisition Device
Analog Input Channels	16 single-ended or 8 differential
Analog Input Range	±10 V
Operating Temperature	0 °C to 50 °C