IEEE Solid-States Circuits Magazine - Summer 2021 - 13

CIRCUIT INTUITIONS
Ali Sheikholeslami
AC Coupling and Baseline Wander
W
Welcome to the 29th article in the
" Circuit Intuitions " column series.
As the title suggests, each article
provides insights and intuitions into
circuit design and analysis. These
articles are aimed at undergraduate
students but may serve the interests
of other readers. If you read
this article, I would appreciate your
comments and feedback as well as
your requests and suggestions for
future installments in this series.
Please email me your comments at
ali@ece.utoronto.ca.
A signal that travels from one
microchip to another through a cable
or a printed circuit board trace often
contains a dc component at the transmitter
(Tx) that needs to be adjusted
before entering the receiver (Rx) to
satisfy its dc requirements. This is
simply because the output stage of
the Tx may not necessarily use the
same bias voltage as the input stage
of the Rx. As displayed in Figure 1,
the task of adjusting the dc level of
a signal can be easily done by first
dropping its initial dc level (say, Vdc1
)
by placing a coupling capacitor (C)
on the signal path and then adding
a new dc level (say, Vdc2
) through a
resistor (R). Since the coupling capacitor
exhibits infinite impedance for
any dc signal and a finite impedance
for any ac signal, it blocks the dc signal
altogether yet allows the ac signal
to pass through. This is indeed
the act of ac coupling between two
blocks. The coupling capacitor, no
matter how small, will be able to
Digital Object Identifier 10.1109/MSSC.2021.3088964
Date of current version: 25 August 2021
v1
vac1
Vdc1
+
-
C
R
Vdc2
Time
(a)
(b)
FIGURE 1: (a) The coupling capacitor is used to couple ac signals between two nodes while
isolating their dc signals. (b) Example waveforms showing when the dc level of a signal is
dropped while its ac level is preserved.
IEEE SOLID-STATE CIRCUITS MAGAZINE
SUMMER 2021
13
Vdc2
Time
v2
v1
Vdc1
v2
completely block the dc signal. However,
to do perfect coupling (that
is, to completely pass an ac signal
from one terminal to the other), the
capacitance must be infinitely large.
In all practical cases, when we use
a finite capacitance, the coupling
results in signal attenuation for the
low-frequency components of the ac
signal. In this article, we explain this
in more detail and build a case for
an elegant solution in wireline communication
where we can restore the
low-frequency components that may
have been attenuated or lost in the
process of ac coupling.
The series combination of C
and R, as displayed in Figure 1(a),
forms a high-pass filter with a dB3cutoff
frequency of /.
fRC123dB
=
r
That is, the signal components with
high frequencies (ffdB3
& ) will pass
through the capacitor with little
attenuation, and those with low frequencies
(ffdB3
attenuated. This is, of course, okay
if the ac portion of the input signal
does not have any components
at low frequencies, i.e., if all of its
% ) will be heavily
components are far higher than f dB3
.
But, in some applications, such as in
wireline communication, our transmit
data are usually in the form of a
random binary sequence with nonnegligible
components near dc. A
simplified circuit diagram for this
application is presented in Figure 2(a),
where vTx
represents a single-ended
equivalent of the Tx signal (assuming
no frequency-dependent attenuation
by the channel) with voltage
levels of V1+
and V1v
,Tx
, and v PH
represents the high-pass-filtered
version of
coupled to the Rx
through the coupling capacitor. The
Rx is represented by an ideal comparator
producing V1 when the
input signal is positive and V1when
negative.
Figure 2(b) presents an example
of the transmit signal that includes
a long consecutive identical digits
(CID) of 1s and a long CID of
- 1s. The CIDs contribute to the
low-frequency (slow-changing)
components of the signal that are
lower than the high-pass filter's
f ,dB3
unable to fully pass through

IEEE Solid-States Circuits Magazine - Summer 2021

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