Lake Shore 370, Руководство пользователя

Страница: 32 / 184

Lake Shore Model 370 AC Resistance Bridge User’s Manual
2.5.5 Resistor Thermal Noise
Thermal noise or Johnson noise is the most common noise associated with resistors. This noise is present in all resistors,
is independent of excitation and has a flat frequency spectrum. The equation for Johnson noise is often expressed as
Vnoise = (4kTR
Δ
f)
1/2 , where Vnoise is the RMS noise voltage, k is Boltzmann’s constant (1.38 × 10
–23
J/K), T is
temperature in Kelvin and Δ f is the noise bandwidth in Hz. Some considerations when using the equation, peak to peak
noise will be within 5 times RMS noise more than 99% of the time,
Δ
f can be calculated several ways and a common
method is (
π
/2) f
3db
where f
3db
is the 3 dB bandwidth of the dominant low-pass filter in the measuring circuit. As a
practical example, the noise voltage of a 100 k
Ω
resistor at room temperature using a 1 second filter is approximately
16 nV RMS or 80 nV PP which is 8% of a measurement made at 1 µV. When the resistor is cooled the thermal noise
drops accordingly.
Please note that thermal noise is normally the biggest noise source in resistor being measured but not the only one.
Metallic conductors approach the theoretical limit for thermal noise but other materials have additional noise, sometimes
referred to as flicker noise, that has a 1/f spectrum. Flicker noise is different for every material, is proportional to current
and can change with construction techniques including lead attachment. Shot noise, which is a product of the discrete
nature of electron flowing through resistive materials, is insignificant in most measurement but can become measurable
when current drops into the picoamp range.
2.5.6 Interference Between Two Model 370s
The phase sensitive detector in the Model 370 effectively cancels noise from external sources but it does have some
difficulty canceling noise at or very near its own measurement frequency. An odd operating frequency was intentionally
chosen to minimize the possibility that outside noise sources would be the same frequency as the instrument. One source
of noise that can be very close to the measurement frequency of a Model 370 is another Model 370. When two or more
instruments are operated with leads closely coupled in the same Dewar it is possible for them to beat against each other
and cause small oscillations. If this happens, the measurement frequency of one instrument can be changed slightly to
avoid interference.
2.5.7 Vibration Noise
Noise can be generated in lead cables when they are moved or vibrated. This noise can be related to the triboelectric
effect or changes in capacitance between conductors in the cable. A sudden movement of measurement lead cables can
overload the instrument momentarily when it is on the most sensitive ranges. Cables carrying small signals should be
fastened down without putting strain on the connections.
2.6 MEASUREMENT ERROR
2.6.1 Warm Up and Temperature Drift
When the Model 370 is first turned on it should be allowed to warm up for at least 30 minutes before use to allow its
internal temperature to stabilize. Warm up is an exaggerated form of temperature drift because the temperature change
inside the enclosure is larger and faster than would be experienced in a laboratory setting. Temperature drift, change in
reading with change in ambient temperature, is small in the Model 370 and seldom a significant factor in absolute
accuracy. Measurement repeatability is often dominated by temperature drift and temperature change should be
considered if the instrument is being installed in a rack. If possible, precision measurement instruments should not be
placed in the same rack with large power supplies or other equipment that can change the rack temperature quickly.
Instrument racks should be well ventilated to prevent the build up of heat.
2.6.2 Resistor Installation
Installation is an important part of resistor measurements at cryogenic temperatures. The measured resistor must be at the
temperature of its surroundings for the measurement to be valid. Some installation suggestions for temperature sensors
are given in Paragraph 2.9.3. These suggestions are applicable to any resistor.
2-10 Theory of Operation