Table 2. Channel Densities for Various Transducer Configurations

SCXI Isolated Universal Input Modules

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Cold-Junction Compensation

Both the SCXI-1121 and SCXI-1122 can read the cold-junction

sensor from a compatible terminal block that offers a cold-junction

sensor. In multiplexed mode, both modules must read the sensor as

a separate analog input operation. This is usually done before the start

of a continuous acquisition. For thermocouple measurements, the

SCXI-1102, 1112, and 1125 an sample the cold-junction sensor in the

same scan as the thermocouples, to provide better accuracy.

Transducer Conditioning

SCXI-1121

The SCXI-1121 offers a jumper-configurable, isolated excitation

source for each analog input channel. You can independently configure

each excitation source for 3.33 V, 10 V, 0.15 mA, or 0.45 mA with

jumpers for each setting. With this architecture, you can select from

120 or 350

bridge-based transducers, 100

RTDs, or up to 10 k

thermistors on a per-channel basis. Each channel also includes a

jumper-configurable internal half-bridge completion circuitry for

half and quarter-bridge measurements.

With the SCXI-1321, you can manually null the offset of bridges with

potentiometers and perform shunt calibration of strain gauges. The

SCXI-1321 includes four software programmable switches that connect

a socketed 301 k

shunt resistor across the transducer elements.

SCXI-1122

The SCXI-1122 offers only one voltage and one current excitation

channel for transducers such as RTDs, thermistors, and bridge-based

transducers. The isolated voltage excitation channel is a 3.333 V source

with internal half-bridge completion and a single remote sensing

terminal to help reduce lead resistance effects. The SCXI-1122 also

includes software-controlled shunt calibration capabilities for bridge

transducers. When activated, the module switches a 301 k

shunt

The isolated current excitation channel is a 1.0 mA source.

This source can drive fifteen 100

devices, such as RTDs, connected

in series. You can also configure the SCXI-1122 for 4-wire scanning

mode. When configured for 4-wire scanning mode, the current source

can drive up to eight 5 k

devices, such as thermistors, and still

maintain electrical isolation between transducers. The SCXI-1122 is

intended for use in low-speed, static (DC) measurements, not

dynamic measurements.

Calibration

Each channel of the SCXI-1121 has offset potentiometers so you can

calibrate each channel manually. The SCXI-1122 contains calibration

hardware to null-out error sources. With automatic input zeroing,

software programmable analog switches ground the inputs of the

instrumentation amplifier for offset error calibration. An onboard

EEPROM stores the calibration constants for each channel for each

input range in a user-defined area. The EEPROM also stores a set of

factory calibration constants in permanent memory, and cannot be

modified. NI-DAQ driver software transparently uses the calibration

constants to correct for gain, offset, and excitation source errors.

RTDs

Thermistors

SCXI Module

1

1

Full

1

1

Full

1

1

Full

100 Ω

< 5 kΩ

< 10 kΩ

SCXI-1121

44

4

4

–

44

4

4

4

4

SCXI-1122

16

16

8

–

–

–

16

16

16

16 (series), 8 (4-wire)

8 (4-wire)

–

*Bridge Transducer