National Instruments Corporation SC-207 Series, Руководство пользователя

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Chapter 2 SC-2070 Board
© National Instruments Corporation 2-7 SC-207X Series User Manual
Channel Configurations
You can configure the analog input channels of an MIO-16 board for one of three input modes:
differential (DIFF) input, referenced single-ended input (RSE), or nonreferenced single-ended (NRSE)
input.
As described in Chapter 2 of the MIO-16 user manual, the input configuration of the MIO-16 depends
on the type of signal source. There are two types of signal sources: nonreferenced or floating signals
and ground-referenced signals. To measure floating signal sources, configure the MIO-16 for
referenced single-ended input or differential input with bias resistors. To measure ground-referenced
signal sources, configure the MIO-16 for nonreferenced single-ended input or differential input. Both
types of signal sources and the recommended methods for MIO-16 board connection are discussed as
follows.
Connecting Nonreferenced or 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: 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 board to establish a local or onboard reference for the signal.
Differential Inputs
To provide a return path for the instrumentation amplifier bias currents, floating sources must have a
10 to 100 k
Ω
resistor to AIGND on one input if DC-coupled, or both inputs if AC-coupled. For more
detailed information on connections to floating signal sources and differential inputs, refer to the
configuration chapter in your MIO-16 board user manual. These bias resistors can be installed in
positions B and D (Table 2-1 and Figure 2-2) of the SC-2070 board. Figure 2-7 shows both the
schematic and the component placement for a single 100-k
Ω
bias return resistor on the negative input
from a floating source connected to channel 1 (the D position in Table 2-1). Additional signal
conditioning circuitry, such as filters and attenuators (described in Building Lowpass Filters, Building
Highpass Filters, and Building Attenuators later in this chapter), can be built in the open component
positions.