Agilent Technologies 4294A Specifications download pdf (Page 116)

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Terminal connection method:

In order to make short repeatability small, there are test fixtures which utilize the 4-terminal con-

nection method (for example 16044A). By employing this technique, the effect of contact resistance

is reduced and short repeatability is drastically improved. As a result, the range of accurate low

impedance measurements is vastly expanded.

In the figure below, the difference between the 2-terminal connection and the 4-terminal connection

is shown. In a 2-terminal connection, the contact resistance, which exists between the fixture’s con-

tact and the DUT, is measured together with the DUT’s impedance. Contact resistance cannot be

eliminated by compensation because the value changes each time the DUT is contacted.

2 terminal and 4 terminal connection techniques

In a 4-terminal connection, the voltage and current terminals are separate. Since the input impedance

of the voltmeter is large, no current flows to the voltage terminals. Hence, the voltage that is applied

across the DUT can be accurately detected without being affected by the contact resistance. Also, the

current that flows through the DUT flows directly into the current terminal and is accurately detected

without being affected by the contact resistance. Due to the reasons stated above, it is possible to elimi-

nate the effect of contact resistance and realize a small short repeatability.

Test fixture’s adaptability for a particular measurement:

In order to make use of what has been discussed previously, the test fixture’s adaptability for a par-

ticular measurement will be focused upon. To see whether a test fixture is adaptable, it is important

to think about the test fixture’s additional error (proportional error, short and open repeatability),

measurement impedance and the test frequency range.

If the measurement impedance is in the 1 to 10 kΩ range, use only proportional error to calculate

the additional error of the test fixture. It is fine to assume that this is a close approximation to the

fixture’s additional error.

If the measurement impedance is not in this range, use proportional error, short and open offset

errors to calculate the test fixture’s additional error. Recent test fixtures have all three terms speci-

fied in their operational manual, so use these values for the calculation.

Some of the recent test fixtures (16044A), due to their structure, have different performance charac-

teristics with different measurement instruments. For these test fixtures refer to their operational

manual for more details about the specifications.

So, how are test fixtures, which are not specified with short and open repeatability, assessed

whether they are adaptable or not? This assessment is made possible by using the following method

to approximate short and open repeatability. To measure a test fixture’s short repeatability, mea-

sure the impedance of the short condition after performing short compensation. Take the shorting

plate out of the fixture and then insert it back in. Measure the short condition again. By repeating

this process at least 50 times, it will show the variations in the short condition (See figure below).

The final step to determine an approximation of short repeatability is to add a margin to the values

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Agilent Technologies 4294A Specifications Page 116