Chapter 4
Theory of Operation
©
National Instruments Corporation
4-7
The SCXI-1521/B input terminals consist of three terminals per channel for
direct connectivity to 3-wire resistive strain gauges. EX+ connects to the
voltage excitation, QTR connects to the SCXI-1521/B quarter-bridge
completion resistor, and AI connects to the SCXI-1521/B AI input
amplifier. Figure 4-4 shows the block diagram of the SCXI-1521/B.
The input amplifier stage provides amplification and presents a high input
impedance to external signals. The offset null compensation circuitry is
used to null offset voltages that result from initial imbalances in the
strain-gauge bridge connection.
The input amplification stage is followed by an additional amplification
and lowpass filter stage before reaching the output multiplexer.
The SCXI-1521/B has a fixed lowpass filter with a 3 dB cutoff of 10 Hz.
The SCXI-1521/B has a fixed gain of 42 V/V.
The signal then passes to the multiplexer stage. The modes of operation are
determined by the multiplexer stage. The signal is routed to the E/M Series
DAQ device channel 0 if the SCXI-1521/B is the cabled module and/or to
the SCXI backplane through the SCXIbus connector.
Note
The SCXI-1521/B does not support parallel mode.
The multiplexer stage is controlled by the digital interface and control
circuitry. The digital interface and control circuitry stores the scan list,
controls the multiplexer, and allows flexible scanning (random scanning).
The shunt calibration circuitry is comprised of a fixed value shunt resistor
in series with a switch. The switch is controlled by the digital interface and
control circuity. When the switch is closed, the shunt calibration resistor is
connected in parallel with the quarter-bridge completion resistor.
For more detailed information about the operation of any of these circuitry
stages, refer to the
sections.
Quarter-Bridge Completion
Quarter-bridge completion resistors are provided on the module. The
SCXI-1521 has 350
Ω
resistors, while the SCXI-1521B has 120
Ω
resistors.