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These grounding methods can act as a big loop wire. They pick up noises from the environment and generate voltages in
the grounding system. The 50 Hz magnetic field of the AC power is a normal noise source that the ground loop always
pickup. For distributed grounding systems, the ground voltage can cause the ground current flow in the ground loop. Since
the ground is with low impedance, noise current is always very high.
Cures for ground loop problems include:
1) Connect all grounds to the same physical point.
2) Use a heavy ground bus to reduce the resistance of ground connections.
3) Remove sources of large ground currents from the ground bus.
Microphonics
Not all noise source are electrical in origin. According to microphonic effects, mechanical noise can be transformed into
electrical noise. Physical changes in the device or cables (due to vibrations for example) will cause electrical noise over
the whole bandwidth of the lock-in.
Solutions to minimize microphonic signals:
1) Eliminate mechanical vibrations near the experiment.
2) Tie down cables carrying sensitive signals.
3) Use a low noise cable that is designed to reduce microphonic effects.
Thermocouple Effects
When two dissimilar metals contact, there will be potential difference between them. The reason for potential difference
includes: (1) different electronic work function of two metals. (2) different electron concentration of two metals.
Suppose that metal A and metal B have work functions Va and Vb respectively. The electromotive force (emf) between A
and B is:
)
ln(
b
a
b
a
ab
N
N
q
kT
V
V
V
Where K=1.38
×
10
-23
J/K is Boltzmann's constant. T is the temperature in
°
Kelvin;q is the elementary charge of 1.60
×
10
-19
Coulomb. It's found that from the formula, the contact potential depends on the nature of the metal and the temperature of
the contact surface.
If two dissimilar metals contact, the electromotive force at the contact point will plus a slowly changing millivolt level.
This kind of noise is sensitive to temperature. This noise is typically at very low frequency because the temperature of the
detector and experiment generally changes slowly. This effect is large on the scale of many detector outputs and can be a
problem for low frequency measurements, especially in the mHz range.
Solutions to minimize thermocouple effects:
1) Hold the temperature of the detector or experiment constant.
