BNC-2080 Board
Chapter 2
BNC-208X Series User Manual
2-14
© National Instruments Corporation
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, Building Highpass Filters, and
Building Attenuators (Voltage Dividers) 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
π
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.