Bad Circuit
Design 8 - Canceling Wideband Thermal Noise
Thermal
noise can’t be “canceled” since it’s wideband noise. Thermal noise can be
reduced
by averaging, see Fig. 8.26, and by filtering (actually, averaging is also
filtering,
see the bottom of page 126 in the mixed-signal book).
A
couple of comments, often a designer will focus on reducing the noise in a
circuit.
Well,
shorting the output of an amplifier to ground reduces the amplifier’s output
noise
to
zero but it’s bad design. It’s better to focus on increasing the
signal-to-noise ratio
and
reducing the input-referred noise as discussed in the book. Simply reducing the
noise
in a circuit doesn’t mean much by itself ;-)
Low
frequency thermal and Flicker noise as well as offset can be "canceled"
(the signal
and
noise can be high-pass filtered to remove the low-frequency components). Since
these
noise signals are slow moving they can be sampled and then subtracted from the
output of a circuit, that is, differentiated, 1 - z-1 (again, high-pass filtering). For
additional information see the correlated double sampling, CDS, discussion in the
mixed-signal book.