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38 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
load. In this case, the reactive component of interest is parasitic capacitance. For an
introduction to the reactance reading, refer to section 2.5.3.
It is possible to calculate the parallel resistance value based on the measured
impedance, and therefore arrive at a corrected resistance value that removes the
effect of the parasitic capacitance. Given an impedance measurement of:
R
measured
+ jX
measured
, the following equation can be used:
As an example, if the Model 372 is displaying the a complex impedance reading of:
66.50 – j15.30 k
)
Because of the large negative reactive component, it is concluded that there is a large
parasitic capacitance in the test setup. The equivalent parallel resistance can be
calculated to determine the approximate value of the actual DUT (assuming it is
purely resistive):
Lead inductance can create similar effects, but typical installation techniques will not
produce large enough inductance to cause measurable attenuation. Inductance
should be considered if inductive noise reduction filters are placed in series with
leads.
2.7.6.2 Parasitic Capacitance Temperature Compensation
When making temperature measurements with a resistive sensor, the effects of para-
sitic capacitance that create an offset in measured resistance will result in a corre-
sponding offset in measured temperature. This temperature reading can also be
corrected using the measured real and imaginary components of the measurement:
Keep in mind that most cryogenic temperature sensors are NTC (negative tempera-
ture coefficient) devices, which means sensor sensitivity will be a negative value.
Therefore, the corrected temperature value will always be less than the measured
temperature value. This temperature calculation may be useful in scenarios where it
is not feasible to entirely remove the parasitic cable capacitance.
As an example, consider the previous case, where the impedance measurement was
66.50 – j15.30 k
)
. If this was a Rox RX-102A sensors and the temperature reading
was being displayed as 55 mK, then the corrected temperature would be (assuming
sensitivity is 5.09 k
)
/mK):
This corrected calculation can be very useful in determining whether the reactance
measured by the Model 372 is affecting the reported temperature in either a signifi-
cant or tolerable way.