Rainbow Electronics MAX3941 - Manual (Page 9)

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 EAM.
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 MAX3941 incorporates a modulation current-
enable input. When MODEN is low or floating, the mod-
ulation/bias output (OUT) is enabled. When MODEN is
high, the output is switched to the logic 0 state. The
typical enable time is 2ns and the typical disable time
is 2ns.
Design Procedure
Programming the Modulation Voltage
The EAM modulation voltage results from I MOD passing
through the EAM 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 (Figure 5).
Programming the Bias Voltage
As in the case of modulation, the EAM bias voltage
results from I
BIAS passing through the EAM impedance
(Z L ) in parallel with the internal 50
Ω termination resistor
(R
OUT
):
To program the desired bias current, force a voltage at
the BIASSET pin (see the Typical Application Circuit).
The resulting I
BIAS
current can be calculated by the fol-
lowing equation:
The input impedance of the BIASSET pin is typically
20k
Ω. Note that the minimum output voltage is V
EE +
1.9V (Figure 5).
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 can be set with a 2k
Ω potentiometer with
the center tapped to V EE (or equivalent fixed resistors),
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 MAX3941 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 MAX3941 operates properly.
Applications Information
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3941 output and the EAM module 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. Be sure to filter the
power supply with capacitors placed close to the IC.
I
V
3
BIAS
BIASSET ≈
6 4 .
Ω
V I
Z R
Z R
BIAS BIAS
L OUT
L OUT
≈ Ч
Ч
+
I
V
11.1
MOD
MODSET ≈ +
Ω
37mA
V I
Z R
Z R
MOD MOD
L OUT
L OUT
≈ Ч
Ч
+
MAX3941
10Gbps EAM Driver with Integrated
Bias Network
_______________________________________________________________________________________ 9
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