Digital Signal Processing Reference
In-Depth Information
nVdd
cmfb
cmfb
r
out
r
out
r
out
r
out
v
inp
v
inn
3
rd
order
beat products
i
diff
i
common mode
common-mode
+ differential
+ DC-offset
I
bias
r
bias
C
bias
Second-order intermodulation
Common-mode suppression
determined by r
out
/r
bias
.
between differential signal and
common-mode/DC components
causes third-order products.
Bias source impedance is
reduced by parasitic capacitor.
Figure A.20.
The common-mode distortion suppression quality in a differential
amplifier relies on the impedance of the current source. At high
frequencies, the parasitic capacitance causes increasing common
mode output components, which are converted to the differential
domain by beat products in the next stage.
consequence, the common-mode suppression performance deteriorates for in-
creasing frequencies. At first sight, it may seem to the reader that the decrease
of the common-mode rejection capabilities can be safely ignored for as long
as the common-mode distortion remains strictly separated from the differential
domain.
The real problem in fact, emerges in the subsequent gain stage. The increased
common-mode noise power can cause unexpected third-order intermodula-
tion products in the nonlinear transistors of the subsequent amplifier. In other
words, even-order distortion, noise or power supply ripple begins to leak into
the differential domain. It is emphasized that the
mix products
between both
domains have to interpreted in a very wide sense of the word. Also random
(but relatively stable) mismatch errors in, for example, the threshold voltage
or a difference in mobility between the transistors of a differential amplifier
cause small differences in the dc biasing. The mixing products originating
from this dc operating point offset and a common-mode interferer will end up
in differential intermodulation byproducts corrupting the output spectrum. The
latter case, where only the dc offset is taken into account, is also known as the
so-called
common-mode to differential conversion gain
. The ratio of the differ-
ential gain to the common-mode conversion gain of the amplifier is commonly
