Cryo-con Model 24C
System Shielding and Grounding Issues
System Shielding and Grounding Issues
The Model 24C supports a single-point grounding scheme to prevent ground loops
and low frequency power-line noise pickup.
A single-point-ground scheme starts with the establishment of a good quality ground
point somewhere in your system. All components of the system, including the cryostat
and connected instruments, should have a direct low impedance connection to this
point.
In many systems, the ground point can be the third-wire-ground connection of the AC
power outlet. If your facility does not provide a good quality ground in it's AC power
distribution scheme, it is strongly recommended that one be fabricated. Noise pickup
and ground loop problems are usually traced to how this connection is made.
To minimize RFI noise pickup, The Model 24C implements a shielding scheme that
allows the construction of a complete shield around the instrument and cryostat. The
instrument's enclosure is all Aluminum with wide conductive overlaps on all mating
surfaces. Connecting cable shields to this enclosure on one end and to the cryostat
on the other completes a continuous shield, thus forming a Faraday Cage RFI shield
around the entire system.
In order for the instrument’s grounding and shielding scheme is working effectively:
1. All sensors and heaters must be electrically floating with respect to ground.
2. The instrument side of all sensor cable shields must be connected to their
connector’s metal back-shell. Heater cables should have their shields
connected to the chassis ground lug.
3. At least one cable must have it's shield connected to the connector's back-
shell on the cryostat end.
4. A good quality earth-ground point must be established. All instruments and
the cryostat should have a direct connection to this point.
i
Note:
There is some possibility that a ground-loop will be
formed when a sensor cable shield is connected at both the
cryostat and instrument end. If this happens, it is recommended
that the ground-loop first be fixed and then the connection be
made. Ground-loops are usually fixed by properly implementing a
single-point-ground scheme.
i
Note:
The Ethernet LAN interface is electrically isolated and
cannot introduce ground loops.
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