User`s guide

10
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10 AC Analyses
What is Noise Anal
y
sis?
When running a noise analysis, PSpice A/D calculates and
reports the following for each frequency specified for the AC
sweep analysis:
• device noise, which is the noise contribution propagated to
the specified output net from every resistor and
semiconductor device in the circuit; for semiconductor
devices, the device noise is also broken down into
constituent noise contributions where applicable
• total output and equivalent input noise
How PSpice A/D calculates total output
and input noise
To calculate total noise at an output net, PSpice A/D computes
the RMS sum of the noise propagated to the net by all noise-
generating devices in the circuit.
To calculate the equivalent input noise, PSpice A/D then divides
total output noise by the gain from the input source to the output
net. This results in the amount of noise which, if injected at the
input source into a noiseless circuit, would produce the total
noise originally calculated for the output net.
This value... Means this...
Output noise RMS sum of all the device contributions
propagated to a specified output net
Input noise equivalent noise that would be needed at the
input source to generate the calculated
output noise in an ideal (noiseless) circuit
Example: Diodes have separate
noise contributions from thermal,
shot, and flicker noise.