IEEE Solid-State Circuits Magazine - Spring 2016 - 9

accomplished during phase 2 of the
clock ( { 2: blue switches), where the
two phases are assumed nonoverlapping. To see this better, Figure 3(b)
depicts an equivalent circuit during
each of the two phases. This clearly
shows that, in both phases of the
clock, the input is essentially multiplied by A, as either there is no swap
of wires (as in { 1 ) or there are two
swaps (as in { 2 ), which is equivalent
to no swapping. The offset voltage,
however, is amplified by A during { 1
and by -A during { 2 ; that is, the offset is chopped once.
A transistor-level implementation
of a chopper simply replaces the ideal
switches with transmission gates. These
transmission gates add extra parasitic

capacitances to the input and output
nodes of the core amplifier, in addition to possible parasitic capacitances
across the switches, causing charge
sharing between nodes as the switches
are turned on and off. Furthermore,
the chopping operation puts extra
constraints on bandwidth and power
consumption of the core amplifier.
A detailed discussion of these effects
goes beyond the scope of this article.
Interested readers are referred to a
recent tutorial [1] and two key papers
on this topic [2], [3].
In summary, the chopper stabilization technique chops the input
voltage twice but chops the input offset voltage only once. This results in
the input voltage preserving its wave-

A SSoc iATe eDiTor'S vie W

the magnetic field. What will happen? Here engineers with physics
insight are separated from the MATLAB cowboys.
This candidate needs a technical follow-up question. How much
energy is stored on an uncharged
capacitor C? (Answer: no energy is
stored.) And how much on a second
capacitor of the same size C, charged
to V? [Answer: (1/2)CV2.] If the capacitors are shorted together and as
charge must be conserved, the final
voltage over both capacitors is V/2.
The total energy has dropped to (1/4)
CV2. So, knowing a capacitor doesn't
dissipate power, why is the energy
stored by the capacitors before connecting the capacitors together not
equal to the energy stored by the parallel combination after the capacitors
are shorted? Where is power lost in
the system?

form as it reaches the output while
delivering a chopped (up-converted)
version of the offset voltage to the
output. A simple low-pass filter can
then attenuate (or ideally eliminate)
the chopped offset voltage.

References

[1] A. Thomsen, "Managing offset and flicker
noise," presented at the IEEE Int. SolidState Circuits Conf., Feb. 2012. [Online].
Available: http://sscs.ieee.org/education/
tutorials-on-line
[2] K. C. Hsieh, P. R. Gray, D. Senderowicz,
and D. G. Messerschmitt, "A low-noise
chopper-stabilized differential switchedcapacitor filtering technique," IEEE J. SolidState Circuits, vol. SC-16, no. 6, pp. 708-715,
Dec. 1981.
[3] C. C. Enz, E. A. Vittoz, and F. Krummenacher, "A CMOS chopper amplifier," IEEE J.
Solid-State Circuits, vol. SC-22, no. 3, pp.
335-342, June 1987.

(continued from p. 6)

Actually, I missed that last one
and that there are no signs of erduring my interview and still got the
ratic behavior.
job. These questions do not check
It is always a challenge to get a canwhether the candidate remembers
didate to the point where he asks a
the right answer but
question on a topic relhow he approaches
evant to my company.
a problem.
Sometimes that hapIt is always a
The technical part
pens when we move
challenge to
of the interview is
on to the next interget a candidate
over. The conversaviewer. I tend to drop
to the point
tion moves to the can into a laboratory. Techwhere he asks
didate's plans and ideas
nically driven people
a question on a
for his future. Although
want to know what is
topic relevant to
one cannot expect a
happening there, and
my company.
five-year road map, it
the interaction with
is always usef ul to
the researchers reveals
know what is certainly excluded.
more aspects of his personality. In
Some interviewers appreciate a broad
the office of the next interviewer, I
scope of interests: three sports, two
say goodbye to that gangly guy. One
societal engagements, and backday he will work here and has to make
packing around the entire world. For
sure I get my pension. Good education
me, it is more important that peoand sharp selection is, for me, a matple make some reasonable choices
ter of self-interest!

IEEE SOLID-STATE CIRCUITS MAGAZINE

s p r i n g 2 0 16

http://sscs.ieee.org/education/

Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Spring 2016