Chapter 2 DAQ System Overview© National Instruments Corporation 2-3 NI USB-621x User ManualFactory-calibration constants are permanently stored in an onboardEEPROM 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 cancopy the factory-calibration constants to the user-modifiable sectionof the EEPROM. Refer to the NI-DAQmx Help or the LabVIEW Help inversion 8.0 or later for more information about using calibration constants.For a detailed calibration procedure for USB-621x devices, refer to theB/E/M/S Series Calibration Procedure for NI-DAQmx by clickingManual Calibration Procedures on ni.com/calibration.Signal ConditioningMany sensors and transducers require signal conditioning before ameasurement system can effectively and accurately acquire the signal.The front-end signal conditioning system can include functions suchas signal amplification, attenuation, filtering, electrical isolation,simultaneous sampling, and multiplexing. In addition, many transducersrequire excitation currents or voltages, bridge completion, linearization,or high amplification for proper and accurate operation. Therefore, mostcomputer-based measurement systems include some form of signalconditioning in addition to plug-in data acquisition DAQ devices.Sensors and TransducersSensors can generate electrical signals to measure physical phenomena,such as temperature, force, sound, or light. Some commonly used sensorsare strain gauges, thermocouples, thermistors, angular encoders, linearencoders, and resistance temperature detectors (RTDs).To measure signals from these various transducers, you must convert theminto a form that a DAQ device can accept. For example, the output voltageof most thermocouples is very small and susceptible to noise. Therefore,you may need to amplify or filter the thermocouple output before digitizingit. The manipulation of signals to prepare them for digitizing is calledsignal 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 selectingHelp»Search the LabVIEW Help in LabVIEW and then navigate tothe Taking Measurements book on the Contents tab.
