Specifications

4.3.4 What should be used as the load?
The key point in open/short/load compensation is to select a load whose impedance value is accu-
rately known. The criteria is as follows.
Use a stable resistor or capacitor as the load device.
The load device’s impedance value must be stable under conditions of varying temperature, magnetic
flux, and other component dependency factors. So, avoid using inductors that are relatively sensi-
tive to measurement conditions for the load.
Use a load of the same size and measure it in the same way as the DUT will be measured.
As shown in Figure 4-7, if the load is measured under different electrode conditions, its measured
data will not effectively compensate for the residuals. It is a good idea to use one of the actual DUTs
as a working standard. If the load is a different type from the DUT (e.g. load is C and the DUT is R),
at least keep the same distance between the electrodes.
Use a load that is close in value to the DUT.
Whatever the load value is, the load compensation is effective over the entire measurement range if
the measurement circuit has a linear characteristic. In practice, the circuit between the UNKNOWN
terminals and the DUT may have a non-linear factor, especially when an additional circuit includes
a non-linear component such as an inductor, active switch, amplifier, etc. As shown in Figure 4-8,
additional measurement error will be added when the measured DUT value is far from the load
value used for the compensation. So, the impedance value of the load should be as close as possible
to that of the DUT to be measured. If various impedances are to be measured, select a load that is
nearly the center value of the DUT’s impedance range. In addition, the load value should not be near
the open or short impedance. Otherwise, the load compensation will not be effective and the result
of the open/short/load compensation will be much the same as (or even worse than) that of the
open/short compensation.
Use an accurately known load value.
The impedance value of the load must be known before performing the open/short/load compensa-
tion. To measure the load value, it is practical to use the same measurement instrument, but under
the best possible measurement conditions. Set the measurement time, averaging, and test signal
level so that the instrument can measure the load with maximum accuracy. Also, use a test fixture
that mounts directly to the instrument. Figure 4-9 shows an example of such a measurement.
4-7