Rainbow Electronics MAX3942 - Manual (Page 8)

MAX3942
10Gbps Modulator Driver
8 _______________________________________________________________________________________
The input data is retimed on the rising edge of CLK+. If
RTEN is connected to ground, the retiming function is dis-
abled and the input data is directly connected to the out-
put stage. Leave CLK+ and CLK- open when retiming is
disabled.
Pulse-Width Control
The pulse-width control circuit can be used to compen-
sate for pulse-width distortion introduced by the modu-
lator. The differential voltage between PWC+ and PWC-
adjusts the pulse-width compensation. The adjustment
range is typically ±50ps. Optional single-ended opera-
tion is possible by forcing a voltage on the PWC+ pin
while leaving the PWC- pin unconnected. When PWC-
is connected to ground, the pulse-width control circuit
is automatically disabled.
Modulation Output Enable
The MAX3942 incorporates a modulation current-
enable input. When MODEN is low or floating, the mod-
ulation outputs OUT+ and OUT- are enabled. When
MODEN is high, the drive current is switched to OUT+.
The typical enable time is 2ns and the typical disable
time is 2ns.
Design Procedure
Programming the Modulation Voltage
The modulation voltage results from I MOD passing
through the load impedance (Z L ) in parallel with the
internal 50 Ω termination resistor (R
OUT ):
To program the desired modulation current, force a
voltage at the MODSET pin (see the Typical Application
Circuit). The resulting I
MOD
current can be calculated
by the following equation:
An internal, independent current source drives a constant
37mA to the modulation circuitry and any voltage above
V
EE on the MODSET pin adds to this. The input imped-
ance of the MODSET pin is typically 20k
Ω. Note that the
minimum output voltage is V
EE
+ 1.9V.
Programming the Pulse-Width Control
Three methods of control are possible when pulse predis-
tortion is desired to minimize distortion at the receiver.
The pulse width may be set with a 2k
Ω potentiometer with
the center tapped to V EE (or equivalent fixed resistors), or
by applying a voltage to the PWC+ pin, or by applying a
differential voltage across the PWC+ and PWC- pins. See
Table 1 for the desired effect of the pulse-width setting.
Pulse width is defined as (positive pulse width)/((positive
pulse width + negative pulse width)/2).
Input Termination Requirement
The MAX3942 data and clock inputs are CML compati-
ble. However, it is not necessary to drive the IC with a
standard CML signal. As long as the specified input volt-
age swings are met, the MAX3942 operates properly.
Applications Information
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3942 output and the modulator as
short as possible. Use good high-frequency layout
techniques and multilayer boards with an uninterrupted
ground plane to minimize EMI and crosstalk. Circuit
boards should be made using low-loss dielectrics. Use
controlled-impedance lines for the clock and data
inputs, as well as for the data output.
I
V
11.1
MOD
MODSET ≈ +
Ω
37mA
V I
Z R
Z R
MOD MOD
L OUT
L OUT
≈ Ч
Ч
+
Table 1. Pulse-Width Control
PULSE
WIDTH
(%)
R PWC+ , R PWC- FOR
R PWC+ + R PWC- = 2k Ω
V
PWC+
(PWC- OPEN)
(V)
V
PWC+
-
V PWC-
(V)
100 R
PWC+
= R
PWC-
V
EE
+ 1 0
>100 R PWC+ > R PWC- > V EE + 1 >0
<100 R PWC+ < R PWC- < V EE + 1 <0