Chapter 2
BNC-2080 Board
© National Instruments Corporation
2-15
BNC-208X Series User Manual
Connecting Ground-Referenced Signal Sources
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.
Differential Inputs
If the MIO-16 data acquisition board is configured for differential inputs, ground-referenced signal
sources connected to the BNC-2080 board need no special components added to the BNC-2080
board. The inputs of the BNC-2080 board can be left in the factory-original condition, that is,
with only jumpers in the two series positions,
E
and
F
(see Table 2-3). Signal conditioning
circuitry, such as filters and attenuators, as described in the sections titled
Building Lowpass
Filters,
and
later in this
chapter, can be built in the open component positions.
Single-Ended Inputs
When measuring ground-referenced signals, the external signal supplies its own reference ground
point and the MIO-16 should not supply one. Therefore, the MIO-16 board should be configured
for the NRSE mode. In this configuration, all of the signal grounds should be tied to AISENSE,
which connects to the negative input of the instrumentation amplifier on the MIO-16 board. The
inputs of the BNC-2080 board can be left in the factory-original condition, that is, with jumpers in
the series position (
E
or
F
, depending on the channel). The open positions that connect the input to
AIGND,
B
and
D
(see Table 2-2 and Figure 2-3), should not be used in this configuration.
Referencing the signal to AIGND can cause inaccurate measurements resulting from an incorrect
ground reference.
Building Lowpass Filters
Simple, R-C lowpass filters are easily installed in the BNC-2080 board on any differential 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 2-1),
you can calculate a simple, one-pole R-C filter to have a -3-dB point cutoff frequency (fc):
fc =
1
(2
/
RC)
(Formula
2-1)
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 2-12.
