Chapter 3
BNC-2081 Board
© National Instruments Corporation
3-9
BNC-208X Series User Manual
A grounded signal source is connected in some way to the building system ground; therefore, the signal
source is already connected to a common ground point with respect to the data acquisition board,
assuming the host computer is plugged into the same power system. The nonisolated outputs of
instruments and devices that plug into the building power system fall into this category.
In the case of grounded signal sources connected to the single-ended inputs of the BNC-2081 board, a
small DC error can be measured because of differences between the ground potentials of the signal
source and the Lab board. This error is unavoidable with the BNC-2081 board because the Lab data
acquisition board is configured only for RSE. No special components are needed to measure signals
from grounded sources connected to the BNC-2081 board. The inputs of the BNC-2081 board can be
left in the factory-original condition, that is, with a jumper in the series position
C
and
later in this chapter, can be used.
Building Lowpass Filters
Simple, R-C lowpass filters are easily installed in the BNC-2081 board on any input channel. The filters
are useful for accurate measurement and noise rejection. By substituting resistance and capacitance
values into the following formula (hereafter referred to as Formula 3-1), you can calculate a simple, one-
pole R-C filter to have a -3-dB point fc:
fc =
1
(2
π
RC)
(Formula
3-1)
The frequency response rolls off at a rate of -20 dB per decade of increase thereafter. A Bode plot of the
amplitude versus normalized frequency is shown in Figure 3-7.
Amplitude
Normalized Frequency
1
dB
| | | | | |
0.1 1 10 100 1,000 10,000
(f )
0.1
0.01
0.001
0.0001
0
-20
-40
-60
-80
c
Figure 3-7. Normalized Frequency Response of Lowpass Filter
