Lake Shore 370, Руководство пользователя

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Lake Shore Model 370 AC Resistance Bridge User’s Manual
1.1 DESCRIPTION
The Model 370 AC Resistance Bridge is designed for precise, accurate, low noise, low excitation power AC resistance
measurement. Its primary application is the measurement of resistance materials in cryogenic environments from 20 mK to 1
K. Fully integrated, the Model 370 includes features to reduce and control noise at every step of the resistance measurement
process. A unique, patented, matched impedance current source and active common mode reduction circuitry minimize noise
and self-heating errors. With 16 channels, IEEE-488 and serial interfaces, and closed loop temperature control, the Model
370 offers seamless integration with existing cryogenic systems and is the most complete package on the market today. Used
with Lake Shore calibrated subkelvin resistance temperature sensors, it not only measures and displays but also controls
temperature for dilution refrigerators and other cryogenic systems.
Resistance Measurement
With the same attention to precision and detail that helped Lake Shore become the world leader in subkelvin temperature
sensors, the Model 370 AC Resistance Bridge combines a full range of design strategies which optimize resolution and
minimize measurement uncertainty in low power resistance measurement.
The Model 370 uses four-lead AC measurement for the best possible accuracy with the lowest possible excitation current.
AC coupling at each amplifier stage reduces offsets for higher gain and greater sensitivity than DC techniques allow. Phase
sensitive detection, an AC filtering technique used in lock-in amplifiers, reclaims small measurement signals from
environmental noise. A low excitation frequency at 13.7 Hz reduces the effect of lead capacitance on measurement. These
features, in conjunction with innovative lead shielding and active noise reduction circuitry, significantly reduce measurement
noise and resistor self-heating.
The Model 370 current excitation source provides stable, reliable, low power excitation current. Twenty-one AC current
levels from 3.16 pA to 31.6 mA RMS offer low noise with no significant DC component to contribute to sensor self-heating.
Two operating modes provide excitation control options to meet user and application needs.
Performance Enhancement
Innovative features enhance the performance of the Model 370 from excitation to output. Common mode voltage can come
from many sources, including external noise coupling into the lead wires. The Model 370 provides a unique, patented,
matched impedance current source as its first defense against common mode noise. Just as voltage input terminals for a
differential input have the same input impedance, the two current source output terminals of the Model 370 have the same
source impedance. This matched impedance ensures that common mode voltages do not become normal mode voltages. With
this strategy, the differential input remains truly differential for accurate resistance measurement.
To further reduce the effect of common mode voltage, the Model 370 includes an active common mode reduction circuit.
This circuit dynamically adjusts the current source output operation point to minimize common mode voltage at the
measurement input. Active common mode reduction allows the Model 370 to operate in environments that would otherwise
saturate the differential input amplifiers.
Optocouplers isolate the analog front end of the Model 370 from digital circuitry and the instrument chassis. Optical isolation
minimizes the effect of digital noise on the measurement and breaks ground loops.
For applications where lead length is greater than 10 ft, or resistance is greater than 100 k
Ω , the Model 370 also includes four
separate driven guards that follow the voltage on each lead to provide the lowest possible voltage difference between the lead
and its shield. Driven guards reduce the effect of cable capacitance and provide the best possible shielding. Driven guards are
not available for scanned inputs.
To accommodate conversion of changing output from phase sensitive detection to a stable signal, the Model 370 operates
with a 200 ms minimum filter time constant. While this is adequate for measurement of small resistance values with large
excitation, the Model 370 software provides additional filtering to ensure good resolution as resistance increases and
excitation decreases. Linear filtering or averaging offers the best possible settling time, with selections from 1 s to 200 s.
Excitation Modes
The Model 370 provides full scale resistance ranges from 2 m
Ω
to 2 M
Ω
. The selected resistance range is continuously
displayed; excitation power dissipated in the resistor is also continuously calculated and displayed. The Model 370 includes
both a current excitation mode and a voltage mode for resistor excitation. Current excitation is the instrument’s primary mode
of operation. In current mode, the Model 370 provides the appropriate voltage gain when the resistance range and a fixed
excitation current are selected. The instrument changes gain when the resistance range is changed. In current mode, the actual
excitation current is continuously displayed.
1-2 Introduction