Lake Shore 372, Руководство пользователя, Страница: 44 / 214

32 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
If the shield or any other part of the measurement circuit is connected to earth
ground, it is necessary to prevent ground loops that couple line frequency noise into
the measurement. A ground loop is created when measurement common is tied to
earth ground at more than one point. The isolation of the instrument breaks the loop
between the instrument power cord and the measurement common. No loop is
formed if measurement common is tied to earth ground at only one point.
Loops are also the primary means of coupling H-field noise. Conductive shields do not
reduce the effects of H-fields. Two ways to minimize H-field coupling are to reduce
the loop area or break the loop (as described with ground loops). Loop area can be
minimized by tightly twisting lead pairs, both voltage and current, within the
shielded cables. This not only reduces the loop area, but also guarantees that both
leads are exposed to the same common mode effects which are rejected by the Model
372 measurement input.
2.5.6.2 Shields
Shields provide a low impedance path to measurement common to block E-field
noise from getting to the signal leads. Shield pins are provided in the input connectors
as attachment points for the shield conductors in lead cables. Shielding individual
leads inside a cryostat is difficult because of limited space and the potential for heat
leak. Cable shields are usually connected to the experimental Dewar to create a fara-
day shield around the measurement.
2.5.6.3 Electrical Isolation
The precision analog front end of the Model 372 is electrically isolated from digital
circuitry and chassis of the instrument by opto-couplers. This can improve measure-
ments in two ways. First, isolation makes it more difficult for digital noise from con-
trol circuits and computer interfaces to affect the measurement leads. Second,
isolation breaks ground loops. It is often desirable to extend shielding from the mea-
surement leads to the experimental Dewar. If the instrument shield and the Dewar
both have low impedance to earth ground, a ground loop will be created. Isolation
separates the instrument measurement from earth ground to break the loop.
2.5.6.4 Driven Guards—Measurement Input Only
For installations where lead length is greater than 3m (10 ft) or measured resistance
is greater than 100 k
)
, four separate driven guards are provided. The driven guards
follow the signal voltage on each lead to maintain the lowest possible voltage differ-
ence between the signal lead and its immediate shield. This technique reduces the
undesired effect of cable capacitance. Without the guards, cable capacitance and
measured resistance act together as a low-pass filter and can attenuate the desired
measurement signal. Cables between the instrument and cryostat can be guarded
even if it is impractical to continue guarding inside the cryostat. Driven guards are not
available for scanned inputs, but they are recommended for cables that run between
the Model 372 and optional scanners.
2.5.6.5 Active Common Mode Reduction—Measurement Input Only
It is common for precision measurement equipment to use a differential input as the
first stage of signal conditioning. The differential input offers high impedance on both
input voltage terminals to reduce the effect of common mode voltage (voltage that
acts on both leads at the same time). Common mode voltage can come from many
sources including external noise coupling into the lead wires and the instrument’s
own current source.