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Theory of Operation
Chapter 3
SCXI-1100 User Manual
3-12
© National Instruments Corporation
Note: In general, f
c
may be limited to 20 Hz minimum because of the physical and value
requirements of the package type capacitor (C = 4.7
µ
F).
In addition, each input channel has 1/4 W resistor pads. Figure 3-7 shows the locations of these
pads. You can transform a given channel into a current-loop receiver by placing an appropriate
resistor into these pads. You can purchase SCXI process-current packs (resistor kits) separately
from National Instruments. Refer to Chapter 2, Configuration and Installation, for detailed
installations of these capacitors and resistors.
Pads for
Process-Current
Resistor
Pads for Filter
Capacitor
To Multiplexer
CH#+
CH#-
820
Ω
820
Ω
Figure 3-7. Pad Locations
Refer to the Input Filtering and Current Loop Resistors section in Chapter 2, Configuration and
Installation, for more information.
Calibration
The calibration hardware consists of two single-pole double-throw (SPDT) switches. After you
select the Calibration mode, the input multiplexers are disabled and the PGIA inputs are
connected to the calibration switches. After you select the Calibration mode, clearing the
CV/ZERO* bit of the Configuration Register to 0 will ground the PGIA inputs. This procedure
is referred to as auto-zeroing. When you set the CV/ZERO* bit to 1, the PGIA inputs connect to
AB2
±
of the SCXIbus, on which you can send a precision voltage. This procedure, referred to as
auto-calibration, is reserved for use with future products.
Auto-zeroing and auto-calibration are methods for nullifying error sources that compromise the
quality of measurements. Auto-zeroing determines the amount of offset at the output of the
SCXI-1100 at a given PGIA gain. You should perform auto-zeroing at the start of an experiment