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 

mixed-signal book.