National Instruments SCB-68 - Manual (Page 52)

Chapter 5 Adding Components for Special Functions
© National Instruments Corporation 5-7 SCB-68 Shielded Connector Block User Manual
Thermocouple wire error is the result of inconsistencies in the
thermocouple manufacturing process. These inconsistencies, or
nonhomogeneities, are the result of defects or impurities in the
thermocouple wire. The errors vary widely depending upon the
thermocouple type and even the gauge of wire used, but an error of ±2 °C
is typical. For more information on thermocouple wire errors and more
specific data, consult the thermocouple manufacturer.
For best results, use the average of many readings (about 100 or so); typical
absolute accuracies should then be about ±2 °C.
Lowpass Filtering
This section discusses lowpass filtering and how to add components for
lowpass filtering.
Theory of Operation
Lowpass filters highly or completely attenuate signals with frequencies
above the cut-off frequency, or high-frequency stopband signals, but
lowpass filters do not attenuate signals with frequencies below the cut-off
frequency, or low-frequency passband signals. Ideally, lowpass filters have
a phase shift that is linear with respect to frequency. This linear phase shift
delays signal components of all frequencies by a constant time,
independent of frequency, thereby preserving the overall shape of the
signal.
In practice, lowpass filters subject input signals to a mathematical transfer
function that approximates the characteristics of an ideal filter. By
analyzing the Bode Plot, or the plot that represents the transfer function,
you can determine the filter characteristics.
Figures 5-6 and 5-7 show the Bode Plots for the ideal filter and the real
filter, respectively, and indicate the attenuation of each transfer function.