CAD For Noise Analysis

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CAD For Noise Analysis
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• 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.

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Resistor Modeling Approach

• SPICE calculates the thermal noise current in a
  resistor according to
• Example
• Create En and In of and 10
  respectively
• We create 2 thermal noise current standards of 1
  by using 2 resistors of value
• 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
• 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

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• If we desire to include the amplifiers 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
• 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
• The dc diode current is calculated according to
• 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 is
  calculated by equating the two right-hand terms.
  Thus
• Similarly KF for In is