Lake Shore 372, Руководство пользователя, Страница: 21 / 214

1.5.1 Lower Input Voltage Noise 9
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1.5.1 Lower Input
Voltage Noise The limiting factor for making extremely low-impedance measurements directly
with the Model 372 is the input voltage noise figure of 10 nV/
x
Hz. The preamp in the
3708 reduces this by a factor of 5 to an impressive 2 nV/
x
Hz. By reducing the amount
of input noise, even smaller return signals can be recovered with excellent accuracy.
When combined with the ability of the Model 372 to smooth measurement values
with user-settable filters ranging from 1 to 200 s, the 3708 preamp and scanner pro-
vides the best solution to measuring low impedance devices at cryogenic tempera-
tures. The 3708 scanner and preamp allows up to eight simultaneous connections to
be made, with the scanner feature enabling measurement to be switched between
those connections. Unlike the 3726 scanner, all connections that are not actively
being measured are left open, allowing the 3708 to be connected to Hall bar devices.
1.5.2 Overcoming Cable
Length With such small resultant voltages needing to be measured, it can be very helpful to
have these signals amplified slightly as close as possible to the source of these signals.
The compact size of the 3708 scanner and preamp allows it be mounted close to the
device or sample being measured, thereby maintaining signal-to-noise ratio for the
measurement signal between the sample and the Model 372 that will ultimately per-
form the measurements. Cable lengths of up to 10 m are supported by the 3708,
allowing the Model 372 to be located away from the experiment area if needed.
FIGURE 1-11 A model 372 and 3708 used in a Hall measurement application