Lake Shore Model 370 AC Resistance Bridge User’s Manual
3.4.3 Driven
Guards
A configuration of guarded and shielded cables is shown below and is recommended for long cables or when measuring
large resistances. Variations may be necessary depending on the configuration of available tri-axial cable. Guards are
tied only to the guard pins on the input connector and should never be tied together, tied to shields or terminated in any
other way. They should always be left unattached at the resistor end of the cable. The cable shield is tied to the shield
pins on the input connectors. Paragraph 3.4.4 explains how the shield can be extended to the experimental Dewar
without creating ground loops. The guards themselves provide some shielding but they are not as effective as shields at
reducing induced noise.
Pin
4
5
6
Name
G+
+
Shield
Pin
2
1
3
Name
G
Shield
Driven_Guards.eps
Figure 3-5. Guarded Shielded Cables (Typical for Current and Voltage Leads)
3.4.4 Isolation
and
Grounding
The Model 370 has two distinct circuit areas that have different grounding needs, the chassis grounded circuits and the
isolated measurement circuits. (The control heater output is a third isolated circuit area that should be considered when
controlling temperature). Chassis grounded circuits include the instrument chassis, digital control, computer interface
and analog voltage outputs. Measurement circuits include the current and voltage leads with their associated guards and
shield, the monitor voltage, the reference voltage and scanner control lines. Proper installation of the instrument can
enhance the benefits of both isolation and shielding. Improper installation can defeat both. Some trial and error may be
required to determine the best configuration. A typical grounding scheme is shown in Figure 3-6.
Chassis grounded circuits should have a low impedance path to Earth ground for both performance and safety. This is
achieved by using a 3-conductor power cord in a properly grounded outlet. All instrumentation that is connected to the
analog outputs or computer interface (or use Earth ground as measurement common) should have similar low impedance
to the same ground. Good grounding of all chassis in a group of instruments prevents voltage differences between their
chassis (usually at line frequency) from inducing noise on the measurement. If good quality Earth ground is not
available, the chassis of line powered instruments should be strapped together with heavy gauge copper wire or ground
strap. Signal leads in communication cables should not be relied on for ground strapping.
Measurement circuits should be as well isolated from Earth ground as possible with one important exception.
Measurement leads outside the Dewar should be shielded and in general protected from contact with Earth ground.
Inside, care must be taken to prevent electrical shorts between leads and conductive parts of the cooling system. During
installation it is important to check for these shorts as well as shorts between the leads themselves. Guards should be
terminated at the instrument end of cables only and not at the resistor end. Monitor and reference voltage outputs and
scanner control lines should be treated like part of the measurement and connected to isolated or differential
measurement inputs. The only unanswered question is: Where should cable shields be terminated?
Shield termination is the exception to the rule that isolated measurement circuits should be isolated from Earth ground.
In an ideal installation the shield of the instrument can be joined with the natural shielding of a metal test Dewar to shield
all internal measurement leads from external noise. (Obviously this will not shield noise generated within the Dewar.) In
many installations the best noise performance of the Model 370 can be achieved when the cable shield is tied directly to
the Dewar or other large conductive mass surrounding the experimental space. The problem is that most test Dewars are
connected to Earth ground either directly or through the many vacuum and transfer lines running in and out. It would be
very difficult to isolate a Dewar that was not specifically designed that way, therefore, the isolation of the instrument
must be relied upon to prevent ground loops in this configuration. When the cable shield is connected to a Dewar and
therefore Earth ground, that should be the
only
connection between the measurement circuits and Earth.
Installation
3-5
