Agilent Technologies 4294A Specifications download pdf (Page 27)

The induced errors are dependent upon test frequency, test fixture, test leads, DUT connection con-

figuration, and surrounding conditions of the DUT. Hence, the procedure to perform compensation

with actual measurement setup is a key technique to obtain accurate measurement results. The

compensation theory and practice are discussed comprehensively in Section 4.

2-4-7. Guarding

When in-circuit measurements are being performed or when one parameter of a three terminal

device is to be measured, for the targeted component, as shown in Figure 2-11 (a), the effects of par-

alleled impedance can be reduced by using guarding techniques. The guarding techniques can also

be utilized to reduce the outcome of stray capacitance when the measurements are affected by the

strays present between the measurement terminals or between the DUT terminals and a closely

located conductor. (Refer to paragraph 3-5 for the methods of eliminating the stray capacitance

effects.)

The guard terminal is the circuit common of the auto balancing bridge and is connected to the

shields of the 4-terminal pair connectors. The guard terminal is electrically different from the

ground terminal which is connected directly to the chassis (Figure 2-11 (b)). When the guard is

properly connected as shown in Figure 2-11 (c), it reduces the test signal current but does not affect

the measurement of the DUT’s impedance Zx, because Zx is calculated using current Ix.

The details of the guard effects are described as follows. The current I

, which flows through Z

does not flow into the ammeter. As long as I

does not cause a significant voltage drop of the

applied test signal, it scarcely influences on measurements. The current I

, which is supposed to

flow through Z

, is small and negligible compared to Ix, because the internal resistance of the amme-

ter (equivalent input impedance of the auto balancing bridge circuit) is very low in comparison to

. In addition, the potential at the Low terminal of the bridge circuit, in the balanced condition, is

zero (virtual ground). However, if Z

is too low, the measurement will become unstable because

ammeter noise increases.

Note: In order to avoid possible bridge unbalance and not cause significant measurement errors, Z

should not be lower

than certain impedance. Minimum allowable value of Z

depends on Zx, test cable length, test frequency and other

measurement conditions.

The actual guard connection is shown in Figure 2-11 (d). The guard lead impedance Zg should be as

small as possible. If Zg is not low enough, an error current will flow through the series circuit of Z

and Z

and, it is parallel with Ix.

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Agilent Technologies 4294A Specifications Page 27