Derek MacLachlan, Keithley Instruments
Test & Measurement World
This step addresses building the test system and verifying its performance, including a number of techniques that can be used to improve measurement quality.
Once a system builder has picked appropriate equipment, cables, and fixtures, and established that the equipment’s specifications can meet the requirements, it’s time to assemble it and verify its performance once step at a time. It is essential to check that each piece of test equipment has been calibrated within its specified calibration period, which is usually one year. If the instrument will be used for making voltage measurements, placing a short across the inputs of the meter will provide an indication of any offset errors. This can be directly compared to the specifications from the data sheet. If the instrument will be used for current measurements, then checking to see the current level with the ammeter open circuit will give an indication of offset current. Again, this can be directly compared to the specifications from the data sheet. Next, include the system cabling and repeat the tests, followed by the test fixture, then the device under test (DUT), repeating the tests after each addition. If the performance of the system does not meet the application’s requirements, this «one step at a time» approach should help identify what is causing the problems.
Then, check the system timing to ensure there are sufficient delays to allow for settling time, and reassess it to make sure it satisfies the application’s speed goals. Insufficient delay times between measurements can often create accuracy and repeatability problems. In fact, this is among the most common sources of error in test systems, and it’s especially evident when running the test at speed produces a different result than when performing the test step by step or manually.
Although inductance can affect settling times, capacitance in the system is a more common problem. In a manual system, a delay of 0.25 to 0.5 seconds will seem to be instantaneous. But in an automated test system, steps are typically executed in a millisecond or less, and even the simplest systems may require delays of five to ten milliseconds after a change in stimulus to get accurate results.
Large systems with lots of cabling (and therefore, lots of cable capacitance, and/or those that measure high impedances ( = RC) may require even longer delays or special techniques like guarding. Coaxial cable typically has capacitance in the range of 30 pF per foot.
The common solution is to provide sufficient delays in the measurement process to allow for settling. Delays of several milliseconds are commonly needed, but some applications may require even longer delays. To address this need, most Keithley instruments include a programmable trigger delay.
Guarding is one technique for dealing with capacitance issues, reducing leakage errors and decreasing response time. Guarding consists of a conductor driven by a low impedance source surrounding the lead of a high impedance signal. The guard voltage is kept at or near the potential of the signal voltage.
Leading Sources of Measurement Error
Although all systems are unique, the following sources of error are among the most common:
Table 1. Thermoelectric Potentials
This offset-compensated ohms technique is built into many Keithley instruments. When this feature is enabled, the measurement cycle now consists of two parts (Figure 4): the first part is measuring the voltage with stimulus current switched on, the second part is to measure it with the stimulus current switched off. Subtracting the latter from the former will subtract out the errors due to thermoelectric EMFs. Therefore, this technique will effectively eliminate accuracy issues due to temperature drift.
Once the test system has been built and verified, it’s ready to begin making measurements in which users can have confidence. However, it’s important to recheck the performance of any test setup on a regular basis. Because of component drift, the accuracy of an instrument will vary over time, so ensure that the instrumentation is calibrated regularly.
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