Chapter 9 SCXI—Signal Conditioning

Refer to the examples in the appropriate library for the module in examples\daq\scxi if you are measuring temperature with the SCXI-1120 and SCXI-1121 modules. This VI is similar to the VI used to measure temperature on the SCXI-1100. Both VIs average and linearize temperature data using the Intermediate analog input VIs. The main differences between the VIs are that the SCXI-1120/1121 VI does not measure the amplifier offset, and the input limits for the module and the temperature sensor are different from the input limits for the SCXI-1100.

The SCXI-1120 and SCXI-1121 modules do not have the internal switch used to programmatically ground the amplifiers as in the SCXI-1100 for the amplifier offset measurement. To determine the amplifier offset, you must manually wire the amplifier terminals to ground and use a separate VI to read the offset voltage. You also can manually calibrate the SCXI-1120 and SCXI-1121 to remove any amplifier offset on a channel-by-channel basis. Refer to the SCXI-1120 or SCXI-1121 user manuals for specific instructions.

Refer to the examples in the appropriate library for the module in examples\daq\scxi if you are measuring thermocouples with the SCXI-1125 or the SCXI-1112. These examples demonstrate how to scan the CJC channel (cjtemp) while scanning the thermocouple channels. By scanning the cold-junction sensor with the thermocouple channels, these examples are better suited to take temperature measurements over longer periods of time by accounting for temperature changes at the thermocouple junction inside the terminal block. The SCXI-1125 thermocouple example also demonstrates the ability to shunt the inputs and take an offset reading before collecting temperature data. This allows you to compensate for any offset drift due to operation at elevated temperatures or for offset produced by the system along the signal path.

Measuring Temperature with RTDs

Resistance-temperature detectors (RTDs) are temperature-sensing devices whose resistance increases with temperature. They are known for their accuracy over a wide temperature range. RTDs require current excitation to produce a measurable voltage. RTDs are available in 2-wire, 3-wire, or 4-wire configuration. The lead wires in the 4-wire configuration are resistance-matched. If you use a 2-wire or 3-wire RTD, they are unmatched. Resistance in the lead wires that connect your RTD to the measuring system will add error to your readings. If you are using lead lengths greater than 10 feet, you need to compensate for this lead resistance. RTDs also are classified by the type of metal they use. The most common metal is platinum.

Chapter 9 SCXI—Signal Conditioning

Refer to Application Note 046, Measuring Temperature with RTDs—A Tutorial for more information about how the lead wires affect RTD measurements as well as general RTD information. You can find this note on the National Instruments Developer Zone, zone.ni.com

Signal conditioning is needed to interface an RTD to a DAQ device or an SCXI-1200 module. Signal conditioning required for RTDs include current excitation for the RTD, amplification of the measured signal, filtering of the signal to remove unwanted noise, and isolation of the RTD and monitored system from the host computer. Typically, you would use the SCXI-1121 module with RTDs because it easily performs all the signal conditioning listed previously. You must set up the excitation level, gain, and filter settings on the SCXI-1121 module with jumpers as well as in the configuration utility of your system.

The SC-2042 RTD is a signal conditioning device designed specifically for RTD measurement, and you can use it as an alternative to SCXI modules. Refer to the National Instruments catalog for more information.

You do not have to worry about CJC with RTDs as you do when measuring thermocouples. To build an application in LabVIEW, you can use the Easy I/O analog input VIs. If you are measuring multiple transducers on several different channels, you need to scan the necessary channels with little overhead. Because the Easy I/O VIs reconfigure your SCXI module every time your application performs an acquisition, it is recommended that you use the Intermediate analog input VIs instead.

Using the DAQ Channel Wizard to configure your channels can simplify the programming needed to measure your signal, as shown in Figure 9-9. LabVIEW configures the hardware with appropriate input limits and gain, measures the RTD, and scales the measurement for you. Enter the name of your configured channel in the channels input parameter. The acquired data is in the physical units you specify in the DAQ Channel Wizard.