IEEE Solid-State Circuits Magazine - Fall 2017 - 15

the scaling stage and causes a change
at the summing node. This node
has a finite time constant, introducing its own delay, TFB = t 3 - t 2 . When
CK goes high, the master latch enters
the sense mode and must change its
output according to the new value of
D sum before CK goes low again. The
necessary time for this change is the
setup time of the FF, Tsetup = t 5 - t 4 .
Thus, TCK - Q + TFB + Tsetup must not
exceed one clock cycle and hence one
bit period:

Din

D

+

Q

D

Q

Dout

-

-

CK

CK

h1

h2

FIGURE 5: A two-tap DFE architecture.

(1)

Similar speed limitations exist in other
variants of this architecture as well (see
the "DFE Variants" section).
Three other nonidealities affect
the performance of the DFE. First,
the data input port of the summer in
Figure 6 cannot be arbitrarily nonlinear because the dispersed data's
amplitude carries information about
the channel and must not experience
significant limiting. We can see intuitively that, if the D in waveform in
Figure 4(d) is greatly amplified and
sliced, then all of the bits exhibit a
full swing but the jitter introduced
by the channel remains. As a guideline, we choose the 1-dB compression
point of this port to be greater than
the main cursor amplitude [2] so that
the nonlinearity negligibly increases
the ISI.
Second, the input offset of the
FF, VOS, shifts the net voltage sensed
at the summing node, D sum, equivalently deg rading the volt age
m a r gin for the negative or positive data values sampled by the FF.
Third, the total noise in D sum, Vn,
yields a finite bit error rate. This
noise includes that produced by
the summer and the stages preceding the DFE and the input-referred
noise of the FF. As a rule of thumb,
w e e n s u r e t h a t 8 (4VOS + Vn, rms)
r em a in s less t h a n the peak data
swing D sum. The factor of eight is
chosen to ensure error rates on the
order of 10 -12 and the factor of four
represents the four-sigma variance
of the offset.

Dsum
Din

Master
Latch
DM
D

+

Slave
Latch

Q

D

Q

Dout

-
CK

CK

h1
DF
CK
Dout
Dout
Dsum
Dsum
DM
DM
t1

t2 t3 t4

t5

t

FIGURE 6: A DFE with differential signal paths.

+

-
+h1

Din

MUX

TCK - Q + TFB + Tsetup # Tb .

-h1

+

-

D

Select

Q

Dout

CK

FIGURE 7: An unrolled DFE architecture.

IEEE SOLID-STATE CIRCUITS MAGAZINE

FA L L 2 0 17

15



Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2017

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