National Instruments Data Acquisition Device NI USB-621x User Manual - Signal Conditioning

Chapter 2

DAQ System Overview

©

 National Instruments Corporation

2-3

NI USB-621x User Manual

Factory-calibration constants are permanently stored in an onboard 

EEPROM and cannot be modified. When yo

u

 self-calibrate the device, 

software stores new constants in a 

u

ser-modifiable section of the EEPROM. 

To ret

u

rn a device to its initial factory calibration settings, software can 

copy the factory-calibration constants to the 

u

ser-modifiable section 

of the EEPROM. Refer to the 

NI-DAQmx Help

 or the 

LabVIEW Help

 in 

version 8.0 or later for more information abo

u

t 

u

sing calibration constants.

For a detailed calibration proced

u

re for USB-621

x

 devices, refer to the 

B/E/M/S Series Calibration Procedure for NI-DAQmx

 by clicking 

Manual Calibration Procedures

 on 

ni.com/calibration

.

Signal Conditioning

Many sensors and transd

u

cers req

u

ire signal conditioning before a 

meas

u

rement system can effectively and acc

u

rately acq

u

ire the signal. 

The front-end signal conditioning system can incl

u

de f

u

nctions s

u

ch 

as signal amplification, atten

u

ation, filtering, electrical isolation, 

sim

u

ltaneo

u

s sampling, and m

u

ltiplexing. In addition, many transd

u

cers 

req

u

ire excitation c

u

rrents or voltages, bridge completion, linearization, 

or high amplification for proper and acc

u

rate operation. Therefore, most 

comp

u

ter-based meas

u

rement systems incl

u

de some form of signal 

conditioning in addition to pl

u

g-in data acq

u

isition DAQ devices.

Sensors and Transducers

Sensors can generate electrical signals to meas

u

re physical phenomena, 

s

u

ch as temperat

u

re, force, so

u

nd, or light. Some commonly 

u

sed sensors 

are strain ga

u

ges, thermoco

u

ples, thermistors, ang

u

lar encoders, linear 

encoders, and resistance temperat

u

re detectors (RTDs).

To meas

u

re signals from these vario

u

s transd

u

cers, yo

u

 m

u

st convert them 

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

u

tp

u

t voltage 

of most thermoco

u

ples is very small and s

u

sceptible to noise. Therefore, 

yo

u

 may need to amplify or filter the thermoco

u

ple o

u

tp

u

t before digitizing 

it. The manip

u

lation of signals to prepare them for digitizing is called 

signal conditioning.

For more information abo

u

t sensors, refer to the following doc

u

ments:

•

For general information abo

u

t sensors, visit 

ni.com/

s

en

s

or

s

.

•

If  yo

u

 are 

u

sing 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.