measurement application
response allows for faster settling when measuring higher levels of current.
Figure 13 illustrates how a triaxial cable works with the SMU instrument’s driven guard to prevent the leakage resistance of the cable from degrading the low current measurements. In the circuit on the top, the leakage resistance of the coaxial cable is in parallel with the device under test, creating an unwanted leakage current. This leakage current will degrade low current measurements.
Figure
13.Cable and connection considerations
accuracy. Obviously, although both the Keithley and non-Keithley SMU instruments can appear similar when looking at the “headline” specs, the Model 2636A actually has 400 times better offset accuracy, so it has much better sensitivity, and is capable of far more accurate low current measurements. Cabling. Using triaxial cables rather than the more common coaxial cables is essential to achieving optimal low current measurement performance. Triaxial cables have an extra shield that coaxial ones don’t, which ensures lower current leakage, better R-C time constant response, and greater noise immunity. In addition, the better R-C
In the circuit on the bottom, the inside shield of the triaxial cable is connected to the guard terminal of the SMU instrument. Now this shield is driven by the SMU’s unity-gain, low impedance amplifier (Guard). The difference in potential between the Force/Output Hi terminal and the Guard terminal is nearly 0V, so the leakage current is eliminated.
Due to their high level of performance, triaxial cables can be expensive, so when specifying your final test configuration or comparing price quotations from various manufacturers, make certain they are included with the SMU instrument.
If they are considered an optional accessory instead, you could be in for a costly surprise. In addition, some SMU instruments require optional adapters to convert more common input connectors, like banana jacks, to use triaxial cables.
Again, be sure to understand and specify your cabling carefully, because it can easily add more than $2000 to the total cost of an SMU instrument.
Conclusion
The integrity of the measurements an SMU instrument produces must always be a primary selection consideration. Poor measurement integrity can produce costly errors in both R&D and production test applications, leading to expensive rework, time-to-market delays, poor yields, customer dissatisfaction, and other problems.
A careful evaluation of an SMU’s accuracy, repeatability, resolution, sensitivity, and integration time is critical. Other key considerations when selecting an SMU instrument include system-level throughput, source stability, measurement settling time, offset error, and noise, and finally, cabling and connection issues.
© 2012 Angel Business Communications. Permission required.
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www.siliconsemiconductor.net Issue 2 2012
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