Data Sheet

Proteus VSM
239
schematic. It is really only output noise we are interested in, so tag the OUT voltage probe and
drag it onto the graph. As before, the default values for the simulation are fine for our needs, but
you need to set the input reference to the input generator IN. The Edit Noise Graph dialogue
form has the check box for displaying the results in dBs. If you use this option, then be aware
that 0dB is considered to be 1 volt r.m.s. Click on Cancel to close the dialogue form.
Simulate the graph as before. When the graph is maximised, you can see that the values that
result from this form of analysis are typically extremely small (pV in our case) as you might
expect from a noise analysis of this type. But how do you track down sources of noise in your
circuit? The answer lies in the simulation log. View the simulation log now, by clicking on the
simulation advisor at the bottom of the schematic frame.
Use the down arrow icon to move down past the operating point printout, and you should see a
line of text that starts
'Total Noise Contributions at ...'