Low-Noise Input Cables; Shielding And Guarding - Keithley 6517B Reference Manual

Model 6517B Electrometer Reference Manual

Low-noise input cables

Triaxial cables can generate enough triboelectric currents to corrupt the measurement. These
currents are caused by friction between the center conductor and the inner shield when the cable is
flexed or allowed to move around. The use of low-noise cables help minimize these triboelectric
currents.
The following low-noise cables are recommended for use with the Model 6517B:

Model 237-ALG-2: This 2-meter low noise triaxial cable is terminated with a 3-slot male triaxial
connector on one end and three alligator clips on the other end. The alligator clip with the red
boot is connected to the center conductor (input high). The black booted clip is connected to the
inner shield (input low or guard). The green booted clip is connected to the outer shield (chassis
ground).

Model 7078-TRX-3: This 3-foot low noise triaxial cable is terminated with a 3-slot male triaxial
connector at each end.

Model 7078-TRX-10: This is the same as the Model 7078-TRX-3 except that it is 10 feet in length.

Model 7078-TRX-20: This is the same as the Model 7078-TRX-3 except that it is 20 feet in length.
When using cables, keep the following points in mind:

For voltage measurements, the increased input capacitance caused by a long input cable can
significantly slow down the reading response. To minimize this problem, always use the shortest
possible triaxial input cable and use guarding.

For current and resistance measurements, the increased input capacitance caused by a long
input cable can result in noisy readings. To minimize this problem, always use the shortest
possible triaxial input cable and enable damping. Damping reduces the noise but it also slows
down the response time of the measurement.

Shielding and guarding

The following information covers the basics on using noise shields, guard shields, and safety shields.
Noise shield
A noise shield is used to prevent unwanted signals from being induced on the electrometer input.
Effective shielding encloses the circuit or device under test (DUT) and extends to the electrometer
input through a triaxial cable. The generic connection for the noise shield is shown in the figure below.
6517B-901-01 Rev. C / August 2015
Figure 18: Noise shield
Section 3: Connections
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