User Guide

Chapter 9 AC analyses
242
What is noise analysis?
When running a noise analysis, PSpice calculates and
reports the following for each frequency specified for the
AC Sweep/Noise 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 calculates total output
and input noise
To calculate total noise at an output net, PSpice 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 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.
Table 7
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
Examp
l
e: Dio
d
es
h
ave separate noise
contributions from thermal, shot, and
flicker noise.
Pspug.book Page 242 Wednesday, November 11, 1998 1:14 PM