Low Noise Cables, Shielding, And Guarding; Figure 2-11 Noise Shield - Keithley 6517A User Manual

Front Panel Operation
V-source probes and cables
The following probe and cable sets are available from Kei-
thley as options:
• Model 8606 High Performance Probe Tip Kit: Con-
sists of two spade lugs, two alligator clips, and two
spring hook test probes. (The spade lugs and alligator
clips are rated at 30V RMS, 42.4V peak; the test probes
are rated at 1000V.) These components are designed to
be used with high performance test leads terminated
with banana plugs, such as the Model 8607 Perfor-
mance Banana Cables.
• Model 8607 High Performance Banana Cables: Con-
sists of two high voltage (1000V) banana cables. The
cables are terminated with banana plugs that have re-
tractable sheaths.

2.4.4 Low noise cables, shielding, and guarding

When making precision measurements, you should always
use low noise cables and, when feasible, utilize proper
shielding and guarding techniques.
Low noise input cables
Triax cables can generate enough triboelectric currents to
corrupt the measurement. These currents are caused by fric-
tion 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. See
paragraph 2.21.2 for more information on minimizing tri-
boelectric currents.
The following low noise cables are recommended for use
with the Model 6517:
• Model 237-ALG-2 — This 2-meter low noise triax ca-
ble is terminated with a 3-slot male triax 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 con-
nected to the inner shield (input low or guard). The
green booted clip is connected to the outer shield (chas-
sis ground).
• Model 7078-TRX-3 — This 3-foot low noise triax ca-
ble is terminated with a 3-slot male triax connector on
either 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.
2-12
Notes:
1. For voltage measurements, the increased input capaci-
tance caused by a long input cable can significantly slow
down the reading response. To minimize this problem,
always use the shortest possible triax input cable and/or
use guarding.
2. 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 triax input cable and/or enable
damping (see paragraphs 2.6.2 and 2.7.2). Damping will
reduce the noise but it will also slow 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. Effec-
tive shielding encloses the device or circuit under test and ex-
tends to the electrometer input via a triax cable. The generic
connection for the noise shield is shown in Figure 2-11
which also summarizes the measurements that may benefit
from it.
Metal Noise Shield
Device or
Circuit
Under
Use Noise shield for:
Test
1) Unguarded voltage measurements
2) Unguarded current measurements
3) Low level charge measurements
Figure 2-11
Noise shield
Typically, the noise shield is connected to electrometer input
LO. However, sometimes better noise performance can be
achieved by instead connecting the noise shield to both elec-
trometer LO and chassis ground. Electrometer LO can be
connected to chassis ground at the rear panel of the Model
6517 by installing the ground link between the COMMON
binding post and the chassis ground binding post. You may
have to experiment to determine which method provides the
best noise performance.
Connect to 6517A LO, chassis ground
or both (via triax cable)
(below 1µA)