Noise guarding and shielding Tadeusz Stepinski, Signaler och system Noise Physics of noise Noise calculations Guarding and shielding Sources of interference Shielding Guarding Symmetric-ended signals Physics of Noise Thermal noise (Johnson noise) Noise power P = 4kTB P = 4kT = 4.4 0 B [ WHz] Equivalents e ktb [ ] i [ ] 4 = ktb V = 4 A Thermal noise has Gaussian p.d.f. and an uniform spectrum
Physics of Noise Shot noise Flows of charge carriers crossing potential barriers independently of each other Encountered in p-n junction and vacuum tubes Diode equation kt 6 I D = I0[ exp( qvd / kt ) ] rd = qi D I D Equivalent noise source 4kTB e = ktbrd i = rd Shut noise is non-gaussian, it is defined by a given function h(t) and a set of Poisson points t i s( t) = h( t t ) i i 3 Physics of Noise Flicker (/f) noise Flows of carriers in a discontinuous medium Encountered in diodes, transistors, thin-film devices and light sources Flicker noise dominates the thermal at frequencies < 00 hz e n = KI a D (/ f ) α B K - constant ID - direct current 0.5 < a < α =, or.7 depending on source mechanism 4
Noise calculation The noise voltage e n Equivalent short-circuit input MS noise voltage Definition: noise voltage that should appear at the input of the noiseless amplifier if the input was shorted Units: nv per Hz or nv in a given frequency band The noise current i n Equivalent short-circuit input MS noise current Definition: noise current that should appear at the input of the noiseless amplifier due only to noise currents Units: na per Hz or na in a given frequency band 5 Noise calculation The noise figure (NF) Definition ( S / N) NF =0log ( S / N) in out elationship between noise voltage, noise current and NF en + e + in NF = 0log e gen 6
Interference The nature of interference Interference - any kind of physical influence on a given system which reduces quality and performance of that system Example - EEG measurement 7 Guarding and shielding Sources of interference temperature coefficient of system elements digital equipment (GSM) frequency converters of AC motors ignition mechanism of cars, motorcycle mechanical shocks welding equipment, micro-oven high voltage discharge GUADING - measures and precautions to prevent stray currents entering sensitive electronic parts. SHIELDING technique of placing electronic systems in a metal casing to prevent electrostatic and/or magnetic fields entering sensitive components. 8
Shielding Capacitive and inductive coupling of interference Capacitive coupling of interference Current flows to ground Inductive coupling of interference Emf is induced in conductor 9 Shielding Capacitive coupling of interference Shield is grounded at one end only Interference short connected to ground Proper shielding with capacitive coupling of interference 0
Shielding Inductive coupling of interference (concentric cable) Principle of shielding with inductive coupling of interference A compensating emf is induced in shield Proper shielding with inductive coupling of interference Shield is grounded at both ends Guarding and shielding Interference caused by a magnetic interference Solution twisted pair opposite fields are generated by twisted conductors, producing zero net field.
Shielding Aspects of shielding Example - a shielded amplifier Wrong shielding parasitic feedback Correct shielding eliminates parasitic feedback 3 Aspects of Guarding Several ground points in a system create ground loops A ground loop components will be added to the measured difference voltage A common mode voltage will be converted to an output differential voltage Due to the presence of Z currents I and I will be different 4
Guarding A system grounded at both ends Example - a thermocouple Common mode voltage V cm appears due to current I tr V cm 5 Guarding and Shielding Differential-ended signals Symmetrical signals are ground-independent Principle Practical solutions 6
Guarding and Shielding More about CM Example: CM in shielding circumstances 7