Lake Shore 350, User Manual

Page: 33 / 192

2.4.2 Excitation Modes 19
|
www.lakeshore.com
2.4.2 Excitation Modes The Model 350 uses a low noise current source excitation for sensor measurements.
The instrument firmware is also able to simulate voltage excitation of the measured
sensor to create an autorange mode. The autorange mode is usually preferred
because it conveniently limits the excitation power of NTC sensors as the tempera-
ture decreases. There are two autorange modes available (1 mV and 10 mV). Resis-
tance range selection is different for these two voltages. Autorange is available for
both voltage excitations.
2.4.2.1 Autorange Excitation Mode
The user specifies an input voltage range that is a “not-to-exceed” setting for the
instrument. The instrument selects the appropriate current setting that keeps the
voltage across the sensor as high as possible without exceeding the voltage range set-
ting. Current is changed when the resistance range changes to maintain the voltage
limit. The autorange mode is preferred when measuring negative temperature coeffi-
cient materials common in cryogenic temperature sensors. With voltage limited,
excitation power in the sensor reduces with lowering temperature (P = V
2 /R) limiting
the effect of self-heating. In autorange mode the sensor excitation setting is the max-
imum not the actual voltage.
The autorange algorithm includes a hysteresis band to prevent instability when the
sensor resistance is at a range boundary. It is possible for the instrument to choose a
different excitation current when approaching a value from below than it chooses
when approaching from above. Autorange should not be used when a specific exci-
tation current is critical.
2.4.2.2 Manual Excitation Mode
The user specifies a fixed resistance range, which defines the excitation current. The
instrument selects the appropriate voltage gain for the selected resistance range. The
instrument keeps the current and resistance range constant when a manual range is
selected. In this mode it is easier to calculate the actual power in the sensor (P = I
2 × R)
because the true current is displayed alongside the resistance range in the menu.
2.4.3 Resistance
Ranges The Model 350 has full-scale resistance ranges from 10
)
to 300 k
)
. Most ranges are
available with either a 10 mV or 1 mV excitation. The goal is to use enough excitation
to maintain reasonable accuracy, resolution and settling time without putting too
much heat into the sensor. Usually the 10 mV autorange excitation mode results in
the best performance, but as temperatures decrease, the 1 mV autorange excitation
mode may result in better performance by limiting the self-heating of the sensor. As
temperatures further decrease, the manual excitation mode may be required to
achieve the optimal performance.
The autorange feature of the Model 350 will increase the resistance range any time
the sensor resistance is greater than full scale for the range, and it will decrease the
range any time the measured resistance is below 20% of full scale. The manual exci-
tation mode allows the user to choose the excitation current, and the instrument
does not change ranges automatically.
2.4.4 Measurement
Circuit Small signal measurement instruments require special consideration during installa-
tion. The following sections illustrate how to treat the instrument and its environ-
ment as a measurement circuit and in general how to integrate instruments like the
Model 350. The circuit analogy is appropriate because almost anything electrical or
electronic in a laboratory has the potential to induce electric field (E-field) or mag-
netic field (H-field) noise on the measurement.