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Using the SCXI-1520's Excitation Source for Two Sensors

Hardware: SCXI>>Modules>>SCXI-1520, SCXI>>Modules>>SCXI-1102, SCXI>>Modules>>SCXI-1531, SCXI>>Modules>>SCXI-1122

Problem: I have a SCXI-1520 with 8 Strain Gages and a SCXI-1102 measuring an RTD that requires excitation but I don't have any additional excitation sources. How can I measure my RTD?

Solution: There are several solutions:

Solution 1
Add a SCXI-1581 which is a 32-channel module specifically designed to provide excitation for RTDs to be used in conjunction with the SCXI-1102

Solution 2
Replace the SCXI-1102 with a SCXI-1122 which is a 16-channel isolated multiplexer module that has a built-in single voltage excitation (VEX) or current excitation (IEX) which are available on the front connector.

Solution 3
A third option would be to connect your RTD in Parallel (See Figure 1). Effectively, by connecting the strain gage and RTD in parallel, the same voltage excitation is applied to both circuits. This wouldn't in theory affect either measurement if your voltage source is ideal. However, it does affect the current and overall effective resistance of the circuit. For example, if you have a 120 Ohm RTD and a 120 Ohm balanced bridge, then the effective resistance of the entire circuit is 60 Ohm. Therefore, the more parallel sensors you have, the lower the overall resistance. Because the voltage source on the SCXI-1520 is not an ideal source you may saturate the voltage source. The minimum resistance of the entire circuit can be calculated by Rmin=Vex(min)/I(max) where Vex(min) is the minimum voltage that the SCXI-1520 can provide and I(max) is the maximum current that the source can provide. For the SCXI-1520 this is 0.625 V and 29 mA respectively. Thus, the total effective resistance for the circuit must always exceed 21.55 Ohms. Since both RTDs and Strain Gages both change in resistance in their measurement, you must calculate the minimum effective resistance for the entire circuit that will occur throughout the ranges of their measurements.



Related Links: How Do I Connect an RTD to an SCXI-1581, SCXI-1121 or SCXI-1122 Module? SCXI-1520 User Manual>

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Report Date: 10/04/2005
Last Updated: 03/03/2006
Document ID: 3Q38LBTJ

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