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Volts and Ohms Measurements 3-3

Input cable leakage and capacitance

In a similar manner to leakage in the test circuit, leakage in the input cable could also corrupt

high-impedance measurements. In the unguarded mode, leakage in a triax cable occurs between

the center conductor (HI) and the inner shield (LO).

Inherently, an input cable has capacitance that is formed by the center conductor (HI), inner

shield (LO) and the insulator between them. For high-impedance measurements, the RC time

constant can signiﬁcantly slow down measurement response.

To minimize the effects of cable leakage, and input capacitance, keep the input cable as short

as possible and use guard. With guard enabled, the same potential is applied to both the center

conductor and inner shield of the cable. This eliminates leakage current and capacitor charging/

discharging.

NOTE Detailed information on "Cable Leakage Resistance”, “Input Capacitance (Settling

Time)” and “Guarding Input Cables” is provided in “Volts and Ohms Measurement

Considerations” (located in this section).

GRD

COMMON

GRD Disabled

GRD Enabled

6514

Insulator

(one of two)

Insulator

(one of two)

RL1

RL2

Leakage Path

DUT

Metal Mounting Plate

A. Unguarded

B. Guarded

Figure 3-1

High-impedance

voltage measurements

Keithley 6514 User Manual

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