Agilent Technologies 4294A Specifications download pdf (Page 59)

Figure 4-8. Actual open/short load measurement example

4-2-6. Application limit for open, short and load compensations

When the residuals are too significant compared to the DUT’s impedance value, compensation may

not work properly. For example, if the measured short impedance Zsm is about the same as DUT’s

impedance, total measurement error will be doubled. The following are typical criteria for this limi-

tation:

(1) Measured open impedance Zom must be more than 100 times that of the DUT’s impedance.

(2) Measured short impedance Zsm should be less than 1/100 that of the DUT’s impedance.

4-3. Error caused by contact resistance

Any contact resistance existing between the DUT electrodes and the contact electrodes of the test

fixture or test station will result in measurement error. The effects of the contact resistance are dif-

ferent for the DUT connection methods, 2-terminal and 4-terminal. In the case of a 2-terminal con-

nection, the contact resistance is added to the DUT impedance in series and produces a positive

error in D (dissipation factor) reading (Figure 4-9 (a)). In the case of a 4-terminal connection, con-

tact resistances Rhc, Rhp, Rlc and Rlp exist as shown in Figure 4-9 (b). The effects of the contact

resistance differ depending on the terminals. Rhc decreases the test signal level applied to the DUT,

but it does not directly produce measurement error. Rlp may cause the auto balancing bridge to be

unstable, but generally its effect is negligible. Rhp and Chp (distributed capacitance of the coaxial

test lead) form a low-pass filter, which causes attenuation and phase shift of the Hp input signal,

producing measurement error. Rlc and Clc also form a low-pass filter and cause an error in mea-

sured DUT current and phase angle. Since the resultant dissipation factor error is proportional to -

ωRhp×Chp and -ωRlc×Clc, the D error is a negative value and increases with frequency. This error

becomes significant when the 4-terminal connection method is used in high frequency measure-

ments. The 4-terminal connection gives a constant D error (that is determined by the contact resis-

tance and test lead capacitance only) while the error of the 2-terminal connection varies depending

on the DUT’s value (Figure 4-9 (c)). The 4-terminal connection provides minimal error only when

the effects of contact resistance and test lead capacitance are negligible (mainly at low frequencies).

4-7

Step 1: Using a direct-connected test fix-

ture, measure the load.

Step 2: Measure load compensation data using fix-

ture to be compensated.

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Agilent Technologies 4294A Specifications Page 59