Table 2-10 – National Instruments SCXI-1121 User Manual

Page 41

Chapter 2

Configuration and Installation

2-25

SCXI-1321 Offset-Null and Shunt-Calibration Terminal Block

The SCXI-1321 terminal block operates only with Revision C and later
SCXI-1121 modules.

In addition to the 18 screw terminals, the SCXI-1321 has circuitry for
offset-null adjust of Wheatstone bridges as well as a shunt resistor for
strain-gauge shunt calibration. This terminal block works especially well
with bridge-type transducers such as strain gauges. The SCXI-1321 can
also easily accommodate thermocouples, RTDs, thermistors, millivolt
sources, volt sources, and current-loop receivers.

SCXI-1321 Nulling Circuitry

The nulling circuitry operates with full-bridge, half-bridge, quarter-bridge,
and strain-gauge configurations. Each channel has its own nulling circuitry
and its own trimming potentiometer as listed in Table 2-10.

To null the static offset voltage of the bridge, use the following procedure:

Configure your bridge to the selected channel.

Select and read the channel output.

While monitoring the output, rotate the trimmer wiper with a flathead
screwdriver until you reach 0 V.

You have nulled your bridge and are ready for a measurement.

The nulling range for your terminal block is ±2.5 mV, assuming that you
have a 120

Ω

strain gauge and 3.333 V excitation voltage. You can change

this range by replacing the nulling resistor with a resistor of another value.
Each channel has an independent nulling resistor. You can therefore mix
your ranges to accommodate each channel requirement. Table 2-11 lists the
nulling resistors and their corresponding channels.

Table 2-10. Trimmer Potentiometer and Corresponding Channel

Channel Number

Trimmer Potentiometer

R14

R15