Keithley SourceMeter 2600 Series Reference Manual page 236

8-30 Source-Measure Concepts Series 2600 System SourceMeters Reference Manual
A coaxial cable can be used if the guard potential does not exceed 30Vrms (42.4V
peak). The center conductor is used for In/Out HI, and the outer shield is used for
guard. For higher guard potentials, use a triaxial cable as previously explained.
Figure 8-15 shows how cable guard can eliminate leakage current through the
insulators in a test fixture. In Figure 8-15A, leakage current (I ) flows through the
L
insulators (R and R ) to In/Out LO, adversely affecting the low-current (or high-
L1 L2
resistance) measurement of the DUT.
In Figure 8-15B, the driven guard is connected to the cable shield and extended to
the metal guard plate for the insulators. Since the voltage on either end of R is
L1
the same (0V drop), no current can flow through the leakage resistance path.
Thus, the SourceMeter only measures the current through the DUT.
2600S-901-01 Rev. A / May 2006 Return to Section 8 topics