Based on Origin

1. External noise
   1. Atmospheric
   2. Extraterrestrial
      1. solar
      2. Cosmic
   3. Industrial
2. Internal noise
   1. Thermal Agitation Noise
   2. Shot Noise
   3. Transit Time Noise
   4. Flicker Noise
   5. Miscellaneous Sources

Thermal noise

Thermal Agitation Noise

Also known as Johnson noise or White noise.

${\displaystyle P_{n}\propto \ T\;\delta \!f=k\,T\;\delta \!f}$

where k = Boltzmann's constant = 1.38x10^-23J/K

T = absolute temperature, K = 273 + °C

δ f = bandwidth of interest

P_n = maximum noise power output of a resistor

${\displaystyle P_{n}={\frac {V^{2}}{R_{L}}}={\frac {\left({\frac {V_{n}}{2}}\right)^{2}}{R}}={\frac {V_{n}^{2}}{4R}}}$
${\displaystyle V_{n}^{2}=4RP_{n}=4RkT\;\delta \!f}$
${\displaystyle V_{n}={\sqrt {4kT\;\delta \!f\;R}}}$

Shot Noise

${\displaystyle i_{n}={\sqrt {2ei_{p}\;\delta \!f}}}$

where i_n = r.m.s. shot-noise current

e = charge of an electron = 1.6x10^-19C

i_p = direct diode current

δ f = bandwidth of system

Addition due to several sources

noise voltages:

${\displaystyle V_{n}1={\sqrt {4kT\,\delta \!f\,R_{1}}}}$, ${\displaystyle V_{n}2={\sqrt {4kT\,\delta \!f\,R_{2}}}}$...and so on, then

${\displaystyle V_{n,tot}={\sqrt {{V_{n}1^{2}}+{V_{n}2^{2}}+...}}={\sqrt {4kT\,\delta \!f\,R_{tot}}}}$

where R_tot = R₁+R₂+...

Addition due to Cascaded Amplifier stages

R_eq = R₁+R'₂
${\displaystyle R_{eq}=R_{1}+{\frac {R_{2}}{A_{1}^{2}}}+{\frac {R_{3}}{{A_{1}^{2}}{A_{2}^{2}}}}}$

CMOS

BJT

Differential signaling

Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analogue signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, PCI Express and USB.

Good grounding

An ideal signal ground maintains zero voltage regardless of how much electrical current flows into ground or out of ground.

When low-level signals travel near high currents, their return currents shouldn't be allowed to flow in the same conductor. Otherwise, noise such as AC ripple on the high current will modulate the low-level signal.