SC-2071 Board
Chapter 3
SC-207X Series User Manual
3-8
© National Instruments Corporation
Connecting Floating Signal Sources
A floating signal source is a signal source that is not connected in any way to the building ground
system, but has an isolated ground reference point. If an instrument or device has an isolated
output, that instrument or device falls into the floating signal source category. Some examples of
floating signal sources are outputs for the following: thermocouples, transformers, battery-
powered devices, optical isolators, and isolation amplifiers. The ground reference of a floating
source must be tied to the ground of the DAQ device.
No special components are needed to measure signals from floating sources connected to the
SC-2071 board. Because the negative side of the single-ended inputs is grounded, all floating
sources are referenced to the same ground. The SC-2071 board inputs can be left in the factory-
original condition, that is, with a jumper in the series position, C (see Table 3-1). Signal
conditioning circuitry, such as the filters and attenuators described in Building Lowpass Filters,
Building Highpass Filters, and Building Attenuators later in this chapter, can be used.
Connecting Grounded 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 DAQ device
(assuming the host computer is plugged into the same power system). 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 SC-2071
board, a small DC error can be measured because of differences between the ground potentials of
the signal source and the Lab/1200 device. This error is unavoidable with the SC-2071 board
because the Lab/1200 device is configured only for ground-referenced, single-ended inputs. You
need no special components to measure signals from grounded sources connected to the SC-2071
board. The inputs of the SC-2071 board can be left in the factory-original condition, that is, with
only a jumper in the series position C (see Table 3-1 and Figure 3-2). Signal conditioning
circuitry such as the filters and attenuators described in Building Lowpass Filters, Building
Highpass Filters, and Building Attenuators later in this chapter, can be used.
Building Lowpass Filters
You can easily install simple, RC lowpass filters in the SC-2071 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, you can calculate a simple, one-pole RC filter
to have a -3 dB point (cutoff frequency):
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 3-8.
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