Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.8.1 Scanner
Performance
Up to 16 resistors with up to 64 leads can be attached to the Model 3716/L Scanner. Up to 8 resistors with up to 32 leads
can be attached to the Model 3708 Scanner. The large number and potential length of lead wires provide an excellent
path for noise to couple into the measurement system. Proper installation combined with the noise reduction features
built into both the Model 370 and Model 3716/L and 3708 can minimize the effects but a full loaded scanner may never
be as quiet as a single guarded and shielded resistance input. A measurable increase in noise is most likely when
measuring large resistance of more than 100 k
Ω
. Large resistances pose an additional problem because the guards
available for a single resistance input end at the scanner enclosure. Scanner and lead capacitance after that point can
attenuate measurement signals for large resistors.
On the other hand, the times 100 amplifier in the scanner can actually reduce measurement noise in installations that
require the Model 370 to be a significant distance from the test Dewar. The built-in amplifier in the scanner may improve
measurements any time long input leads are required, even if only one channel is being used.
Accuracy is not as big a concern as noise when using the Model 3716/L and 3708 Scanners. The Model 370 AC
measurement eliminates any voltage offsets that may be present in the scanners relays or amplifier. The amplifier gain is
calibrated at the factory and can be recalibrated in the field to keep scanner gain error well below overall measurement
uncertainty.
Settling time is a concern any time multiple inputs are multiplexed and must be scanned. The channel change settling
time of the Model 3716L and 3708 Scanners are virtually the same as the range change settling time of the Model 370.
Additional time may be required for the instrument to autorange if that feature is enabled and the resistance on the new
channel changes significantly between scans.
2.8.2
Controlling Temperature While Scanning
The close integration of the Model 370 and Model 3716 Scanner permits fast efficient scan channel changes making it
possible to both control temperature and take data with the same instrument. Firmware in the Model 370 manages
automatic switching between the control channel and scan channel and it will freeze the control heater output when the
control channel is not active. Even with these features, scanning during closed loop temperature control will never be as
stable as a dedicated control instrument.
The biggest causes of control instability are the subtle changes in cooling power that are present in low temperature
cooling systems. These changes are offset by changes in heater output during closed loop control to keep temperature
constant. When the instrument is scanning other channels it cannot read the control feedback sensor for several seconds
or tens of seconds at a time. When the control channel is inactive the control output freezes (creating open loop control)
and changes in cooling power result in changing temperature. The amount of instability depends on speed and mass of
the cooled load. Scanning while controlling should be reserved for the least critical temperature control applications. All
instrument specifications for temperature control are for continuous closed loop control without scanning.
2.9 TEMPERATURE
MEASUREMENT
Temperature measurement is a popular use of resistance bridges because most of the temperature sensors that can
operate at low temperatures are negative temperature coefficient resistive temperature detectors (NTC RTDs) and need
the flexibility of a bridge. Resistive temperature sensors must be treated with the same care as any other resistor being
measured at low temperatures. The Model 370 is well equipped to measure and control temperature using these devices.
2.9.1 Sensor
Selection
Lake Shore offers a full line of cryogenic temperature sensors including several models that are well suited for low
temperature work to 50 mK and below. Two of the most popular are Germanium and Ruthenium Oxide RTDs. The Lake
Shore Temperature Measurement and Control catalog gives detailed specifications of these sensors and should be
consulted as a reference when choosing temperature sensors. The Model 370 will work with resistive temperature
sensors from Lake Shore or any other manufacture. A few important selection criteria are discussed below.
2.9.1.1 Sensitivity
Sensitivity is often the most important criteria when selecting a temperature sensor because it dictates many of the other
specifications including range, resolution and accuracy. Many NTC resistive materials like germanium have a beneficial
characteristic of temperature sensitivity that makes them particularly useful at low temperatures. The resistance and
sensitivity of these materials increase exponentially as temperature decreases. The sensitivity of the sensors is best at the
2-14
Theory of Operation
