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34 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
Range change requires additional settling because the internal circuits need time to
reestablish their operating point. The hardware settles in 2 to 3 s after range change.
Settling may take a little longer especially if resistance is very large or excitation is
very low. A change pause can be programmed into the instrument to allow for settling
without disturbing other instrument firmware features. During the pause period, fea-
tures including filter, max/min capture, analog voltage output calculations and tem-
perature control and error testing are suspended. The pause time can be set from 3 to
60 s and must be added to the filter settling time to approximate overall settling after
range change. Scan channel change is similar to range change except the new chan-
nel may autorange after a channel change creating a further delay.
The instruments A/D sampling (update) rate is 10 readings/s and all readings are
available over computer interface with an efficiently written program. This rate
allows the A/D to capture any resistance changes that are able to pass through the
input hardware. The temperature control loop, analog voltage outputs and max/min
capture are updated with each A/D reading. The display is updated at 2 readings/s.
2.6 Noise Sources The effect of electrical noise is usually small and can be ignored when making routine
measurements, but all noise matters when signals fall to the microvolt level. The fol-
lowing paragraphs describe the most common noise source encountered with small
signal measurements and some techniques that can help reduce their effect. More
specific installation instructions are given in Chapter 3. The Model 372 offers a diag-
nostic monitor voltage output that can be helpful in diagnosing noise related prob-
lems. The instrument will also display a message if noise overloads the measurement
electronics. Refer to Chapter 7 for more information on diagnosing noise problems.
2.6.1 Induced Electrical
Noise Noise from the environment is typically classified as either electric field (E-field) or
magnetic field (H-field). E-field noise capacitively couples into measurement leads or
the resistor being measured. Voltage is induced on the leads as if the noise source and
lead are two halves of a capacitor. The amount of coupling is related to the voltage
change at the source, the common area between source and receiver (length of lead)
and dielectric between them (distance in air). A cable shield is the first line of defense
against E-field noise. No measurement cable of any length should be run from the
Model 372 without a shield. The shield provides a low impedance path to measure-
ment common that prevents the voltage change at the source from being seen at the
measurement leads.
H-field noise inductively couples into measurement leads or other conductive loops.
Current is induced in the lead as if the noise source and lead are two halves of a trans-
former. The amount of coupling is related to the field change from the source and the
loop area. Tightly twisted leads inside the shielded cable reduce the total loop area
and minimize the effect of H-field noise. It is also effective to move the instrument
and leads away from noise sources because field strength reduces by the inverse cube
of distance.
Even with proper installation it is difficult to reduce induced noise below 1 to 10 µV.
High resistance ranges are affected more because the leads create a more efficient
antenna when they are terminated at a high resistance.
2.6.2 Ground Loop
Noise Ground loops produce noise that acts similar to induced noise but the cause of the
noise is different. Current is generated in any conductive circuit or loop when that
loop contains changing current or magnetic field. In measurements this problem is
commonly referred to as a ground loop and generally results when improperly
grounded instruments are exposed to AC line current. Isolation reduces ground loop