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CAD For Noise Analysis • SPICE contains built-in models for passive elements, semiconductor devices …etc. by including control lines in an input file, SPICE can be made to perform various analyses of a circuit including noise analysis. • Noise analysis is performed in SPICE by using .NOISE statement in the input control file. • SPICE calculates shot noise and thermal noise for the circuit. Amplifier Noise Sources • For modeling purposes it is advantageous to lump all the noise produced by an amplifier or other functional block into 2 noise sources En and In as shown: • There are several ways to implement this type of an amplifier model in SPICE. The easiest way is to use 2 resistors as noise sources and reflect their noises to the appropriate nodes using dependent voltage and current sources. • Diodes can also be used as sources of shot noise and flicker noise and reflect their noises to the appropriated nodes in the circuit. Resistor Modeling Approach • • • • • SPICE calculates the thermal noise current in a resistor according to I t2 4kT / R Example: Create En and In of 3nV / Hz and 10 pA / Hz respectively We create 2 thermal noise current standards of 1 pA / Hz by using 2 resistors of value 23 Rn 4(1.38 10 )(300) 16.56k 24 10 • 2 separate sources are necessary since it is normally assumed that the En and In amplifier noise sources are uncorrelated. • Polarity does not matter for any of the sources because noise is a squared quantity. The multipliers needed for the H and F sources are respectively 3nV / Hz 10 pA / Hz 3k and 10 1 pA / Hz 1 pA / Hz • • • • The SPICE codes are: RNE 100 0 16.5k; VESEN 100 0 DC 0; HEN 1 3 VESEN 3k; RNI 200 0 16.56 k; VISEN 200 0 DC 0; FIN 2 3 VISEN 10 • If we desire to include the amplifier’s input impedance into our noise model, an extra noise will be unintentionally introduced through the resistive component of the input impedance. Modeling 1/f Noise • SPICE calculates the noise current in a diode according to KF ( I dc ) AF 2 Id 2qI dc f • Example: Model En noise of 20 nV/Hz1/2 with a noise corner frequency fnce = 200 Hz and In noise of 0.5 pA/Hz1/2 with a noise corner frequency fnci = 500 Hz in each diode 1024 • The dc diode current is calculated according to I dc 2 1.602 10 19 3.122A • Next the flicker noise term is set equal to the 1-pA/Hz1/2 reference noise current at the fnc frequencies. For the En source with AF =1, KF 17 two right-hand terms. Thus isKF calculated 2qf by 6equating .408 10the A nce • Similarly KF for In is 1.602 10 16 A