MAX9234/MAX9236/MAX9238
Hot-Swappable, 21-Bit, DC-Balanced LVDS
Deserializers
7
Maxim Integrated
Detailed Description
The MAX9234/MAX9236 operate at a parallel clock fre-
quency of 8MHz to 34MHz. The MAX9238 operates at a
parallel clock frequency of 16MHz to 66MHz. The tran-
sition times of the single-ended outputs are increased
on the MAX9234/MAX9236 for reduced EMI.
DC Balance
Data coding by the MAX9209/MAX9211/MAX9213/
MAX9215 serializers (which are companion devices to
the MAX9234/MAX9236/MAX9238 deserializers) limits
the imbalance of ones and zeros transmitted on each
channel. If +1 is assigned to each binary 1 transmitted
and -1 is assigned to each binary 0 transmitted, the varia-
tion in the running sum of assigned values is called the
digital sum variation (DSV). The maximum DSV for the
data channels is 10. At most, 10 more zeros than ones,
or 10 more ones than zeros, are transmitted. The maxi-
mum DSV for the clock channel is five. Limiting the DSV
and choosing the correct coupling capacitors maintains
differential signal amplitude and reduces jitter due to
droop on AC-coupled links.
To obtain DC balance on the data channels, the serial-
izer parallel data is inverted or not inverted, depending
on the sign of the digital sum at the word boundary.
Two complementary bits are appended to each group
of 7 parallel input data bits to indicate to the MAX9234/
MAX9236/MAX9238 deserializers whether the data bits
are inverted (see Figure 9). The deserializer restores
the original state of the parallel data. The LVDS clock
signal alternates duty cycles of 4/9 and 5/9, which
maintain DC balance.
AC-Coupling Benefits
Bit errors experienced with DC-coupling can be elimi-
nated by increasing the receiver common-mode voltage
range by AC-coupling. AC-coupling increases the com-
mon-mode voltage range of an LVDS receiver to nearly
the voltage rating of the capacitor. The typical LVDS dri-
ver output is 350mV centered on an offset voltage of
1.25V, making single-ended output voltages of 1.425V
and 1.075V. An LVDS receiver accepts signals from 0 to
2.4V, allowing approximately ±1V common-mode differ-
ence between the driver and receiver on a DC-coupled
link (2.4V - 1.425V = 0.975V and 1.075V - 0V = 1.075V).
Common-mode voltage differences may be due to
ground potential variation or common-mode noise. If
there is more than ±1V of difference, the receiver is not
guaranteed to read the input signal correctly and may
cause bit errors. AC-coupling filters low-frequency
ground shifts and common-mode noise and passes
high-frequency data. A common-mode voltage differ-
ence up to the voltage rating of the coupling capacitor
(minus half the differential swing) is tolerated. DC-bal-
anced coding of the data is required to maintain the dif-
ferential signal amplitude and limit jitter on an
AC-coupled link. A capacitor in series with each output
of the LVDS driver is sufficient for AC-coupling.
However, two capacitors—one at the serializer output
and one at the deserializer input—provide protection in
case either end of the cable is shorted to a high voltage.
RISING-EDGE STROBE SHOWN.
