Lake Shore Model 421, Руководство пользователя

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Lake Shore Model 421 Gaussmeter User’s Manual
C-4 Hall Generator
Model 120CS
Current Source
Digital Voltmeter
+I
C
–I
C
+V
H
–V
H
+
–
I
C
V
H
R
L
Hall Generator
Load resistor required
for optimum linearity
(if specified)
C-421-C-3.eps
Figure C-3. Typical Hall Generator Hookup
C4.0 USING A HALL GENERATOR WITH THE MODEL 421
To hookup a Hall generator you must use a Lake Shore Model MCBL-6 Cable Assembly. The cable is
200 cm (79 inches) long with a DA-15 connector on one end and four leads on the other. The Hall
generator is a 4-lead device. The 4 leads are lIc (Red), –Ic (Black or Green), +VH (Blue), and
–VH (Yellow), corresponding to the 4 leads on all the Hall generators.
The Model 421 has an input impedance of 420
Ω
. Therefore, the actual sensitivity at the gaussmeter
input will be less than the value given with the Hall generator due to drop in the leads and cable. This
fact is important because a sensitivity value is supposed to be loaded into the cable PROM to set
calibration. We recommend that the customer always check accuracy against a reference field rather
than use the sensitivity value sent with the bare Hall generator. Because Lake Shore has no control
of the conditions beyond the cable, the customer must accept responsibility for accuracy and
compatibility.
Finally, Manganin wire is not usually acceptable for cryogenic installations. The resistance of
Manganin wire is often too high. In cryogenic applications, Hall generators are normally connected
using twisted pairs of copper wire such as 34 gauge, Teflon insulated. There are two reasons for this:
1. The gaussmeter current source is normally limited in compliance voltage. The Model 421 should
not drive a load (Hall generator, Wires in cryostat, and probe cable) greater than 50
Ω
. In fact, for
best performance, the load should be less than 30
Ω
.
2. Because the Model 421 input impedance is 420
Ω
, there is a voltage drop due to lead resistance
in series with the gaussmeter input. The Lake Shore Hall generator sensitivity given on the data
sheet is basically with no lead resistance. See Figure C-4.
Gaussmeter
Input
420
Ω
Sensitivity at Gaussmeter input is
reduced by the lead/input voltage divider
R
cable
R
cable
R
cust
R
cust
The gaussmeter needs input sensitivity
between 0.5 to 1.5 mV/kG (HST) or
5.0 and 15 mV/kG (HSE) at its input
V
H
R
cable
= Lake Shore Model
MCBL-6 Cable Assembly
R
cust
= Customer
Supplied Leads
Hall Generator
Open Circuit
Sensitivity
C-421-C-4.eps
Figure C-4. Hall Generator Input Impedance