Bad Circuit Design 8 - Canceling Wideband Thermal Noise
Thermal noise canít be ďcanceledĒ since itís 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 amplifierís output noise
to zero but itís bad design. Itís 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 doesnít 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